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The Mystery of Iridescence in Glass

AnnaPokorska20 May 2019

This is the second part on a series on ‘Iridescence’. You can read the first part here, or return and read an introduction to colours, as well as individually about the colours blue, red, yellow, and green

If you’ve ever wandered through a museum displaying ancient artefacts, chances are you were amazed at the quality and artistry displayed in glass objects of that time. The   has some incredible pieces shining with iridescent colours:

Left: glass weight from the Fatimid period; Middle: glass fragment from the Roman period, possibly part of an eye amulet; Right: glass fragment from the late Roman period (Petrie Museum: UC13298, UC22744, UC67914).

However, despite the undeniable talents of ancient glassmakers, this particular effect was not intentional or even achieved during production. In fact, iridescence found in ancient glass is a result of weathering of its surface caused by burial.  The weathering process itself depends largely on the burial conditions such as heat, humidity and type of soil, although the chemistry of the glass, determined by the purity of raw materials and their compositional ratio, also plays a part. The iridescence is produced when alkalis, or soluble salts, are leached from the buried glass by slightly acidic water present in the soil. This in turn causes the formation of very fine layers which can delaminate or even flake off creating a prism effect.

But it wasn’t until the very end of the 19th century that the iridescence of ancient glass was replicated by Louis Comfort Tiffany (1848-1933), the son of Charles Tiffany – the New York jeweller. He began his career as an aspiring painter but soon realised that his true potential was in interior decoration. It is generally thought that during his extensive travels Tiffany became inspired by the glasswork and mosaics of antiquity and devoted to the idea of restoring stained glass to its former glory by striving to achieve the same standards of beauty as the ones present in antique masterpieces[1]. Prior to the twelfth century, stained glass works were executed with differently coloured glass pieces as opposed to the later technique of painting on clear glass, which dulled it considerably and created a flat two-dimensional effect. Tiffany’s experiments with glass during the 1880s completely revolutionized the look of the medium and in 1894 he patented favrile glass[2]. By adding different or same shades of colour into the hot mixture Tiffany created a material different from other iridescent glasses as the effect was not just confined to the surface but part of the glass itself.

Tiffany Glass and Decorating Company was established in 1892 in New York and began producing its first favrile glass objects 1896, examples of which can be found in the Victoria and Albert Museum collection as well as other major museums, particularly in America.

Favrile glass objects produced by the Tiffany Glass and Decorating Company between 1896 and 1902 (Image: © Victoria and Albert Museum, London).

Left: “The Flight of Souls”, Tiffany stained glass window which won first prize at the 1900 Paris Exposition, now at the Wade Memorial Chapel, Cleveland, Ohio (Image: CoffeeDoc03); Right: Hanging Head Dragonfly Tiffany lamp from the Art Institute of Chicago collection (Image: mark6mauno).

Tiffany won first prize for the above stained-glass window using his new material at the 1900 Paris Exposition and continued to use favrile for other products, including his famous lamps. Being the innovator that he was, he also carried on experimenting with the medium, eventually developing many other, equally impressive, types of glass such as opalescent, streamer, fracture, ring-mottle, ripple and drapery. But that’s for another time!

[1]     Bing, S Louis C. Tiffany’s Coloured Glass Work, in Artistic America, Tiffany glass and Art Nouveau, Cambridge (Mass.); London: M.I.T. Press, 1970

[2]     The original trade name was actually fabrile, which was derived from an Old English word meaning ‘handcrafted’.

Iridescence, a natural superpower

AnnaPokorska1 May 2019

So far, in my previous blog posts I’ve talked about individual colours and how they were created and used in Ancient Egypt (see the beginning of the series here). But let us now explore a fascinating property which brings them all together – iridescence. It’s a phenomenon whereby surface colour appears to change with the angle of viewing or illumination and is caused by an optical effect rather than pigmentation. The word itself derives from the Greek goddess of the rainbow – Iris, while the Latin suffix ‘-escent’ means having a tendency towards something. A perhaps less glamorous term for iridescence, goniochromism, can also be traced back to Greek words ‘gonia’ meaning angle, and ‘chroma’ meaning colour.

Iris Carrying the Water of the River Styx to Olympus for the Gods to Swear By, Guy Head, c. 1793 – Nelson-Atkins Museum of Art (Photo: Daderot).

Iridescence is a type of structural colouration and occurs in the natural world (e.g. insects, birds) as well as in man-made materials (glass, soap bubbles, playing surface of a CD).

Blue Morpho butterfly showing off its glorious colour (Photo: Derkarts).

A brilliant example of the use of iridescence in nature can be found in the Blue Morpho butterfly (Morpho menelaus) whose upper wings appear to be bright blue. It is one of the largest butterflies in the world and can be found in South American rainforests. Those beautiful and rare butterflies use iridescence to evade predators by becoming briefly invisible! As they fly, the colour of their wings shifts between brilliant blue and brown, so against the background of the forest and sky they seem to disappear for a flash just to reappear a little further away, confusing anyone who might be trying to catch them.

Perhaps a more familiar example of iridescent colouring is mother-of-pearl, or nacre, which has long been admired and used for many decorative purposes, from jewellery to furniture, artwork to cutlery. Some specimens can even be found in the Petrie Museum collection. In nature, nacre occurs on the inner shell of some molluscs (such as abalone sea snails) or on the surface of pearls. Its purpose is once again defensive as the molluscs secrete layers of nacre on the inner surface of their shells to protect the soft layers beneath from parasites and debris. As a material, nacre is made up of tiny hexagonal platelets of aragonite, a form of calcium carbonate. The thickness of the platelets (between 300 and 1500 nm) allows them  to interfere with different wavelengths of visible light at various viewing angles, creating an iridescent effect. However, studies using Scanning Electron Microscopy (SEM) have shown that the effect is also partially caused by diffraction resulting from a high groove density of the surface.

Inside of an abalone shell (Photo: Marac).

Some plants have also evolved to use thin layers of photosynthetic structures, called iridoplasts, to bend and absorb more light in dark environments such as the lower levels of tropical forests. This causes the surface of their leaves to appear iridescent and almost glowing in the dark. For instance, peacock begonia (Begonia pavonina) from South East Asia shows a beautifully intense metallic blue as it amplifies  the small amount of visible light it receives. The iridoplasts bend the light repeatedly thus making very efficient use of long red and green wavelengths while reflecting the blue ones.

Peacock begonia (Photo: Shyamal).

Many more examples of iridescence exist in nature and this blog post could easily become a very long article if I attempted to include them all. I guess it’s very easy to assume that this phenomenon is mainly decorative and meant to create attraction, like peacock’s feathers for example. But, as we can see, there are plenty of instances where the effect serves a purpose very different to what we might originally have imagined or is an almost accidental by-product of a completely unrelated function . In my next post I will explore how one man managed to replicate natural iridescence for purely ornamental purposes, so stay tuned for Part 2!

Colours of Ancient Egypt – Green

AnnaPokorska6 March 2019

This is the fifth post in the Colours of Ancient Egypt series; you can read the introduction here, or all about the colour blue, red, and yellow.

In Ancient Egypt, perhaps unsurprisingly, the colour green was associated with life and vegetation. However, it was also linked with the ideas of death. In fact, Osiris, the Egyptian god of fertility, death and afterlife, was commonly portrayed as having green skin. Even scarabs, popular amulets and seals, were often green due the beetle’s symbolic connotation to rebirth and immortality.

Painted wooden stela of Neskhons, wife of the High Priest of Amun Pinedjem (II) making an offering to Osiris, identifiable by his green skin (Petrie Museum, UC14226).

Green faience scarab amulet from Amarna (Petrie Museum, UC1196).

By far the most prevalent, and likely the oldest, green pigment was made from a mineral called malachite. It is a copper carbonate and a relatively stable colourant, although sensitive to excessive heat and acid exposure. It was popular in Egyptian tomb painting from the 4th Dynasty (c. 2613 to 2494 BC) onwards but didn’t find much use in European painting until the 15th and 16th centuries.

Cross-section of malachite (Image: Rob Lavinsky).

A copper acetate, called verdigris, has also been found on Egyptian art. It gives a slightly transparent bluish green, often applied over a ground of lead white or lead-tin yellow. It’s artificially produced by exposing copper plates to acetic acid, a by-product of wine-making. The reaction that follows produces a blue-green deposit, which can then be scraped off, ground, and used as a pigment. Unfortunately, verdigris is very reactive and can become dark brown or even black with ageing. However, it was identified as the primary green pigment on the headband of Queen Nefertiti’s bust, where it retains its hue.

In addition to its instability, verdigris is also moderately toxic due to its copper content. Therefore, its use gradually declined through history, to be mostly replaced by a new pigment, viridian, developed and patented in France in 1859. Viridian is both permanent and non-toxic which immediately made it a great substitute for the older green pigments.

The famous bust of Queen Nefertiti on display at the Neues Museum, Berlin (Image: Philip Pikart).

Other sources of green colour included an artificial green frit (produced the same way as blue frit or Egyptian blue, except that the lime content has to be higher than the copper content) as well as mixing Egyptian blue with yellow ochre. The latter method was occasionally used during the 12th Dynasty (1991-1786 BC) but became popular during the Amarna period (1370-1352). For faience, copper and iron oxides were mostly used, until the discovery of yellow lead antimonate gave Egyptian artisans many more choices of hue.

Green was certainly a colour of great importance to Egyptians, although nowadays it appears overshadowed by the significance and properties of Egyptian blue. However, we can still find and admire green pigments in great condition amongst ancient Egyptian artefacts. Next time you’re visiting the Petrie Museum, check out the wall block fragment from the pyramid of King Pepy I with instructions on his ascent into heaven! Guess what colour the inscription is…;)

 

Colours of Ancient Egypt – Yellow

AnnaPokorska20 February 2019

This is the fourth post in the Colours of Ancient Egypt series; here you can read the introduction, here all about the colour blue, and here about the colour red.

Due to its availability in several different forms and shades, yellow was present in many aspects of ancient Egyptian art and decoration, from painting to pottery.

Fragment of a vessel (Petrie Museum, UC25325; Photo: Anna Pokorska).

Pottery vessel containing rough pieces of pale and deep yellow pigment (Petrie Museum, UC59746).

Just as men’s skin was painted red in Egyptian painting, women’s can be distinguished by its yellow colouring, which we can see in a fragment of a statue made out of yellow jasper possibly depicting Queen Nefertiti or Queen Kiya and dated ca.1353–1336 BC.

Fragmentary head of a Queen in yellow jasper, from the 18th Dynasty (Metropolitan Museum of Art, NY).

Yellow was also used to mimic gold in works where the use of the precious metal wasn’t possible. The most prevalent yellow pigments in ancient Egypt were derived from natural ochres and had the same properties as their red equivalents — but they were by no means the only source of the colour.

Painted linen mummy shroud painted with red lead, carbon black, orpiment and Egyptian blue pigments (Petrie Museum, UC38058).

Orpiment was a common yellow pigment with a rich lemon or canary yellow shade. It is an arsenic sulphide and occurs naturally in small deposits as a product of hydrothermal veins, hot spring deposits and volcanic sublimation, although nowadays it can be easily obtained artificially. The arsenic content makes it highly toxic and the sulphur will darken lead-based pigments if used together in a mixture.

Closely related to — although not as widespread as — orpiment is an orange pigment called realgar which can often be found in the same deposits. Despite its toxicity, it was the only orange pigment available until chrome yellows and oranges were introduced in the beginning of the 19th century. An interesting feature of realgar is that prolonged light exposure turns it into a yellow compound called pararealgar without changing its elemental composition.

In addition, Egyptians were able to synthetically produce a highly toxic lead (II) antimonate, also known as Naples yellow. It was often used as an enamel colour from about 1500BC, although it didn’t appear in painting until the Renaissance. As one of the oldest produced artificial pigments it was highly toxic and provided a warm orange shade of yellow. Interestingly, a mineral of the same chemical composition, called bindheimite, exists in nature but wasn’t used to create the pigment. Instead it was made by a calcination of a lead compound (such as lead white) with an antimony compound (e.g. potassium antimonate). A 19th century recipe recommends mixing the ingredients, placing them over a gentle heat and then gradually increasing the temperature. After approx. 5 hrs the calcination is complete, and the resulting product can be ground in water with an ivory spatula (because iron can react with the powder and change its colour). The shade of the pigment could also be manipulated by changing the proportions of the ingredients. Lead antimonate is very stable to light exposure but due to the lead content will turn black on contact with hydrogen sulphide (e.g. in air).

Why were so many dangerous substances used as pigments for so long, especially as harmless clays were so abundant? Although their toxic effects were known, the depth and brilliance of the lead and arsenic compounds made the natural iron oxides appear rather dull and brownish in colour by comparison. In fact, even the pigments that strove to replace them — cadmium, chromium and cobalt yellows which appeared during the 19th century — are all harmful to some extent, and it wasn’t until the development of organic pigments (based on carbon and hydrogen) that we overcame this issue!

 

Myths in the Museum: Horseshoe Crabs, Blue Blood, and Modern Medicine

JenDatiles7 December 2018

This is the third segment in the Myths in the Museum series; you can go back and read about the dugong and mermaid, and the narwhal and unicorn.

 

With Halloween now behind us and the golden days of autumn getting shorter and shorter, a new time of year is fast coming upon us…one filled with tissues, stuffy noses, and general misery. Flu season.

Yes, it’s that time again, when the cold frost that heralds winter comes nipping at our toes at night to suck the warmth from our bodies like the vampire that it is. Feverishly we brew our teas, cling to those hankies and wrap ourselves in our best woollies and Jon Snow faux furs in an attempt to fend off illness. Yet we ourselves are guilty of our own vampiric methods in this War of the Wheezing. Our flu shots, and basically most drugs and medical injections today, are possible because we harvest another species’ blood: Horseshoe crab blood.

 

Still from the PBS Documentary Crash (Source: The Atlantic, 2014)

 

The horseshoe crab, Limulus Polyphemus, is actually more closely related to scorpions, spiders, and mites than to crabs. Its common name is obvious; its exoskeleton is a large shell shaped like—you guessed it—a horseshoe. These strange-looking creatures have 10 eyes distributed around the shell to help them navigate their way. Don’t be fooled by the tail that looks like a stinger; it serves as a rudder while swimming, and can help the crab reorient itself when it gets flipped over. The horseshoe crab is the only species within its family, Merostomata, which means “legs attached to mouth”. Take a look at the 6 pairs of appendages on its underside, and you’ll see why.

 

Horseshoe crabs, our ‘living fossils’ (Source: PBS)

 

The blood of horseshoe crabs produces limulus amebocyte lysate (LAL), a protein that can detect the presence of endotoxins, bacteria, and other sources of contamination, which we use to render our medicines safe. This protein is found nowhere else on earth. It’s no wonder that this marvellous miracle protein would be found in the blood of horseshoe crabs; they’ve have remained virtually unchanged in the 450 million years they’ve existed. They’re literally living fossils, and yet another example of the strange mysteries of ocean life.

In the 1960s humans discovered the amazing LAL and soon after put it to use in pharmaceutical laboratories around the world. Horseshoe crabs were gathered from their native Atlantic habitats, taken to facilities, drained of up to 40% of their blood, and returned to the ocean. The problem, however, is that this method does little to track what happens to the crabs after they’ve returned to the wild, starved and injured. It is estimated that 50,000 die in the process each year; this, sadly, may be a gross underestimation.

 

Crabs collected from Delaware Bay, 1928 (Source: Delaware Public Archives)

 

Since the 1850s, Atlantic fishermen have harvested about 1.1-2 million horseshoe crabs annually to use as eel and fish bait. Once the medical industry got involved, however, horseshoe crab populations have drastically reduced, and by 2016 the species was added to the IUCN Red List.

A recent publication in June 2018 claims to have found a synthetic alternative to LAL; if true, this could mean a total turnaround for the species. And, possibly, humans may not have to rely on draining these ocean species’ blood and threaten their existence to protect ours.

 

Colours of Ancient Egypt – Red

AnnaPokorska4 December 2018

This is the third post in the Colours of Ancient Egypt series; here you can read the introduction, and here all about the colour blue.

Red was an easy colour to obtain in ancient Egypt as naturally red minerals, or clays, were abundant. In fact, they were already used as pigments for painting in pre-historic times. Of the earth pigments, as they are often called, ochre was used for red colouring. Like others, it is an iron oxide but gets its red shade from a mineral hematite, which can be naturally present in varying quantities. Another way of obtaining the pigment is by heating the more common yellow clay to produce what is called ‘burnt ochre’.

Painted wooden stela showing man Ihefy adoring hawk-headed Horus (Petrie Museum, UC14695).

In ancient Egyptian painting we find the red colour often used to distinguish gender, as men’s skin was often painted red[1]. We can see an example of that in this painted wooden stela from the Petrie Museum.

Less obviously, red ochre was also popular in cosmetics such as rouge and lip colour. In fact, those pigments are still found in beauty products today due to their ready availability, stability and non-toxicity. However, perhaps the most surprising application of these materials is actually medicinal. The Ebers Papyrus, one of the oldest and most important medical texts from ancient Egypt (dated 1550 BC), prescribes ochre clays as a cure for any intestinal or eye problems.

However, minerals were not the only source of red colourants. Ancient Egyptians were also able to tint their textiles using madder or kermes carmine dyes. The former was derived from the root of a madder plant, rubia tinctorum (see below).

Madder plant (Image: Franz Eugen Köhler).

It was one of the most widely used natural red dyes until the development of synthetic equivalents in the 19th and 20th century. In fact, some madder-dyed cloth was even found in Tutankhamun’s tomb. On the other hand, kermes carmine was made from wingless insects found on certain species of European oak trees. Like madder it was used both as a textile dye and a lake, which is a pale pigment obtained by precipitating a dye onto an inert colourless substrate such as chalk. Kermes’ deep crimson shade made it a very popular colourant for centuries.

So far, I’ve mainly talked about pigments and dyes used for decoration, but I would be remiss if I didn’t mention at this point one of my favourite objects in the Petrie collection:

Fragment of a composite statue from Amarna: right ankle and heel, in red jasper (Petrie Museum, UC150; Photo: Anna Pokorska).

This is a right ankle and heel in red jasper, part of a full-size composite statue from Amarna, dated to the 18th Dynasty. I’ve often stopped in front of it imagining what the statue would have looked like whole. I have to admit that I previously assumed the sculpture to have been entirely made of red jasper, which, in my mind, looked incredible. However, that was not the case; only the exposed flesh would have been carved from red jasper (thus depicting a male figure), while the rest of the statue was likely made from Egyptian alabaster, limestone or wood. The Metropolitan Museum of Art in New York has fragments of a king’s head made of the same material and dated to the same period. In fact, some of the fragments come from the Petrie collection which makes me wonder if they were perhaps part of the same statue.

Fragmentary head of a king in red jasper, from the 18th Dynasty (Metropolitan Museum of Art, NY).

We may never know. But one thing is certain: even though we’ve since been able to create many synthetic red colourants of various shades, natural red pigments used by the ancients remain as popular as ever.

 

[1] Lorelei Corcoran, Color Symbolism, in ‘The Encyclopedia of Ancient History’, Edited by Roger S. Bagnall, Kai Brodersen, Craige B. Champion, Andrew Erskine, and Sabine R. Huebner, Blackwell Publishing Ltd. (2013), pp. 1673–1674

Colours of Ancient Egypt – Blue

AnnaPokorska16 October 2018

This is the second in the Colours of Ancient Egypt series; if you want to start at the beginning, click here

The colour blue has already featured in a couple of posts in this blog (e.g. check out Cerys Jones’ post on why the Common Kingfisher looks blue) but it seems impossible to me to discuss colour, especially in Ancient Egypt, and not start with blue. Arguably, blue has the most interesting history of all the colours, which can be attributed to the fact that it is not a colour that appears much in nature – that is, if you exclude large bodies of water and the sky, obviously. Naturally occurring materials which can be made into blue colourants are rare and the process of production is often very time-consuming. In Ancient Egypt, pigments for painting and ceramics were ground from precious minerals such as azurite and lapis lazuli; indigo, a textile dye now famous for its use in colouring jeans, was extracted from plants.

 

Left: two pieces of azurite (Petrie Museum, UC43790); Right: lapis lazuli (Image: Hannes Grobe)

However, all the above-mentioned colourants presented issues which limited their use. Azurite pigment is unstable in air and would eventually be transformed into its green counterpart, malachite. Lapis lazuli had to be imported from north-east Afghanistan (still the major source of the precious stone) and the extraction process would produce only small amounts of the purest colourant powder called ultramarine. Finally, indigo dyes can fade quickly when exposed to sunlight.

And yet it seems that the Ancient Egyptians attributed important meaning to the colour blue and it was used in many amulets and jewellery pieces such as the blue faience ring, lapis lazuli and gold bracelet or the serpent amulet from the Petrie Museum collection (below).

From left to right: blue faience ring with openwork bezel in form of uadjat eye (Petrie Museum, UC24520); lapis lazuli serpent amulet (UC38655); fragment of bracelet with alternative zig-zag lapis lazuli and gold beads (UC25970).

Therefore, the race to artificially produce a stable blue colourant began rather early. In fact, the earliest evidence of the first-known synthetic pigment, Egyptian blue, has been dated to the pre-dynastic period (ca. 3250 BC)[1]. It was a calcium copper silicate (or cuprorivaite) and – although the exact method of manufacture has been lost since the fall of the Roman Empire – we now know that it was made by heating a mixture of quartz sand, a copper compound, calcium carbonate and a small amount of an alkali such as natron, to temperatures over 800°C.

 

 

 

 

 

 

 

 

Fragment of fused Egyptian blue (Petrie Museum, UC25037).

This resulted in a bright blue pigment that proved very stable to the elements and was thus widely used well beyond Egypt. In fact, its presence has recently been discovered on the Parthenon Marbles in the British Museum due to its unusually strong photoluminescence, i.e. when the pigment is illuminated with red light (wavelengths around 630 nm) it emits near infrared radiation (with a max emission at 910 nm).

After its disappearance, artists and artisans had to make do with natural pigments and, being the most stable and brilliant, ultramarine became the coveted colourant once again. In fact, during the Renaissance, it is reputed to have been more expensive than gold and, as a result, often reserved for the pictorial representations of the Madonna and Christ. And so, the search for another replacement was back on. But it wasn’t until the early 1700s that another synthetic blue pigment was discovered, this time accidentally, by a paint maker from Berlin who, while attempting to make a red dye, unintentionally used blood-tainted potash in his recipe. The iron from the blood reacted with the other ingredients creating a distinctly blue compound, iron ferrocyanide, which would later be named Prussian blue. Naturally, other man-made blue pigments and dyes followed, including artificial ultramarine, indigo and phthalocyanine blues.

However, it wasn’t quite the end of the line for Egyptian blue, which was rediscovered and extensively studied in the 19th century by such great people as Sir Humphry Davy. And not only are we now able to reproduce the compound for artistic purposes, scientists are finding more and more surprising applications for its luminescence properties, such as biomedical analysis, telecommunications and (my personal favourite) security and crime detection[2].

References:

[1]  Lorelei H. Corcoran, “The Color Blue as an ‘Animator’ in Ancient Egyptian Art,” in Rachael B.Goldman, (Ed.), Essays in Global Color History, Interpreting the Ancient Spectrum (NJ, Gorgias Press, 2016), pp. 59-82.

[2] Benjamin Errington, Glen Lawson, Simon W. Lewis, Gregory D. Smith, ‘Micronised Egyptian blue pigment: A novel near-infrared luminescent fingerprint dusting powder’, Dyes and Pigments, vol 132, (2016), pp 310-315.

Myths in the Museum: The Unicorn Horn of UCL

JenDatiles18 September 2018

It’s there, just across the main UCL campus on Gower Street. A mystical power of unknown proportions coveted by monarchs and conquerors of golden ages past. Quiet and unassuming, mounted on a museum cabinet crammed with jars of preserved worms and spiders bobbing about in 70% ethanol for eternity, this long, white, spiraled object that looks suspiciously like a wizard’s wand or sorcerer’s staff, sought after by the most powerful dynasties to walk the earth…

No, it’s not a unicorn horn. It’s the Grant Museum of Zoology’s narwhal tusk.

 

The Narwhal Tusk of UCL. (Grant Museum, Z2168)

 

Don’t feel bad for mistaking it for a unicorn horn, though. For centuries the Vikings harvested these tusks—which can be up to 10 feet long—from the ocean creatures off the arctic coast of Greenland and used, gifted, and traded them. They were brought to northern Europe via the major trade routes across the Atlantic linking Greenland and Iceland with the British Isles, Scandinavia, and ultimately the Baltic. Since the unicorn symbolized immortality, power, and protection against poison, narwhal tusks were rare and highly sought after to adorn royal objects in Europe and into Asia. They also served as magico-medical material in the cabinets of wealthy physics and apothecaries (whether their unicorn horn powder was ‘authentic’ is another story).

 

Five types of unicorn, described by Pierre Pomet in his 1694 natural history treatise. (Credit: New York Academy of Medicine)

 

Unicorns feature heavily in myths and tales as a symbol of both power and pure magic. (Screenshot from Disney/Walden’s Chronicles of Narnia: Lion, the Witch, and the Wardrobe; 2005)

 

La Dame à la licorne: À mon seul désir. The famous 16th-century Flemish tapestry, one of six in a series, depicting a noblewoman with her lion and unicorn. It now hangs in Musée de Cluny, Paris.

Perhaps the most famous example of European monarchies’ obsession with owning unicorn horn bling is the Danish throne in Rosenborg Castle. It was commissioned in 1662 to symbolize the ‘absolute monarch’, and was inspired by the throne of Solomon—so naturally its surface was almost entirely covered with precious ‘unicorn horn’. Narwhal tusks were procured by Danish traders, since during this time the Danish monarchs claimed Iceland and the Faroe Islands.

IMPOSING: Rosenborg Castle’s Coronation Throne, used for the Danish coronations between 1671-1840. (Credit: Danish Royal Collections)

So what are these ‘unicorns of the sea’? Narwhals, Monodon monoceros (Greek for ‘one-tooth’ ‘one-horn’) are mid-sized porpoises native to the arctic. Narwhals and beluga whales are the only members of the family Monodontidae, and our knowledge of their daily habits remains elusive. Though they usually don’t share a habitat, just this week a juvenile narwhal male was seen by Quebec researchers playing with a beluga pod over 1000 km south of its usual Arctic range, apparently adopted by its cousins!

Now for the million-dollar question: what is the tusk, besides a magnet for power-crazy monarchs and mystical medicine hunters? The ‘horn’ or ‘tusk’ of a narwhal is actually… a tooth. Unlike many other debunked myths from the Middle Ages, the potency of this unicorn horn’s still relatively shrouded in mystery. For years scientists have debated and theorized about its actual use, from weapons to ‘joust’ for dominance with other males as part of mating rituals, to sensory tools to detect water temperature, pressure and salinity. It wasn’t until last year that drone footage captured footage of narwhals using their tusks to hunt codfish, suggesting the complicated nerve systems within these tusks may have stunning capabilities.

[above and below] Narwhals, narwhals, swimming in the ocean. (Credit: World Wildlife Fund)

So do unicorns exist? We’d have to say no. But until technology catches up to human curiosity and scientific research, these sea unicorns remain as elusive as the myth that surrounds their magical tusks.

 

Colours of Ancient Egypt – Introduction

AnnaPokorska18 August 2018

When viewing exhibitions of objects from ancient Egypt (or any ancient civilisation for that matter) we are used to seeing the beige and grey appearance of bare stone. Indeed, we have come to appreciate the simplicity and purity of ancient sculptures, reliefs and carvings, perpetuated by the numerous plaster casts made and distributed both for research or as works of art in their own right (case in point – the Plaster Court at the Victoria and Albert Museum).

However, this is quite far from the truth. In fact, colour was not only common but of great symbolic importance in Egypt. This is hardly surprising as we use colour to communicate every day even in the modern era (with the most obvious and striking example of the traffic light system, or the wearing of black in many cultures to signal mourning). Although some traditional meanings will have changed over the centuries and varied between cultures, the principle still remains and is widely studied and exploited in a fascinating way in such fields as psychology, marketing and advertising. But I digress…

Let us return to ancient Egypt. To date, many attempts have been made to restore the original colours of artefacts. One such example is the virtual restoration of the Temple of Dendur at the Metropolitan Museum of Art in New York where experts have a created a colour projection to be overlaid on top of the damaged hieroglyphs. An article on the whole project, called Color the Temple, can be read here.

Some people object to these types of intervention, sceptical of how well they recreate and represent the work of the artist, especially if little physical evidence of the original colours in a particular artefact exists. And indeed, we must always be careful when it comes to any type of restoration to take it only for what it is – someone else’s idea of what the object would have originally looked like (often dependent on the restorer’s skill). Although they might still have a way to go, I personally find these virtual restoration techniques intriguing and full of potential. They certainly help my imagination and understanding of the ancient Egyptian civilisation.

But we can find authentic and undamaged examples of colour even in the Petrie Museum collection. One of the first objects one sees when entering the main exhibition is a limestone wall block fragment from the pyramid of King Pepy I at Saqqara, its beautiful hieroglyphs tinted in green (below).

Wall block fragment from the pyramid of King Pepy I at Saqqara. (Petrie Museum, UC14540)

Painted wooden stela of Neskhons, wife of the High Priest of Amun Pinedjem (II) making an offering to Osiris. (Petrie Museum, UC14226)

 

While on the other side of the display is a painted, rather than carved, wooden stela of Neskhons, wife of the High Priest of Amun Pinedjem (II) making an offering to Osiris (above).

Egyptian artists would have had at their disposal mostly pigments made from grinding common (as well as some not-so-common) minerals and earths. Hidden away in the Petrie Museum storage is a drawer full of exactly those kinds of pigments (below).

Pigment drawer in storage at the Petrie Museum. (Photo: Anna Pokorska)

 

The yellowed typed note reads:

‘The pigments used by the ancient Egyptians for their paintings have been analysed and are mostly made from naturally occurring minerals, finely ground, or from natural substances.

Black – some form of carbon, usually soot.

Blue – originally azurite, a blue carbonate of copper found locally. From the IVth Dynasty on artificial frit was used composed of a crystalline compound of silica, copper and calcium.

Brown – generally ochre, a natural oxide of iron.

Green – powdered malachite (a natural ore of copper), and an artificial frit analogous to the blue frit described above.

Pink – an oxide of iron.

Red – red ochre, a natural oxide of iron.

White – either calcium carbonate (whiting) or calcium sulphate (gypsum).

Yellow – yellow ochre, an oxide of iron and less often orpiment a natural sulphide of arsenic.

The pigments were pounded in to a fine powder, mixed with water to which a little size, gum or albumen was added to make the whole adhesive.’

Unfortunately (or perhaps fortunately), this subject is too broad and interesting to fit into a single blog post and I’ve decided to explore it further, perhaps expanding beyond Egypt and the ancient times. We shall see where this journey takes me, but I hope you will join me as I investigate individual colours in my future posts.

 You can now read about the colours blue and red.

Adventures in Eighteenth-century Papermaking

Hannah LWills21 July 2017

By Hannah Wills

 

 

Earlier this summer, I gave a talk with some of the other engagers at our ‘Materials & Objects’ event at the UCL Art Museum. In preparing for the event, we were all challenged to think about the objects, materials, and physical ‘stuff’ that we work with on a daily basis. As I’ve written about before, my research focuses on the notebooks and diaries of a late eighteenth-century physician and natural philosopher, Charles Blagden (1748-1820), who served as secretary to the Royal Society. One of the things I’m interested in is how Blagden used his notebooks and diaries to keep track of his day-to-day activities, as well as the business of one of London’s major learned societies. Record keeping and note taking was a central part of Blagden’s life, and it’s owing to his impressive record keeping habit that there’s one material I handle in my research more than any other: eighteenth-century paper.

A selection of Blagden’s many notebooks, held at the Wellcome Library. (Image credit: Charles Blagden, L0068242 Lectures on chemistry, Wellcome Library, London. Wellcome Images, MSS 1219 - MSS1227. CC BY 4.0)

A selection of Blagden’s many notebooks, held at the Wellcome Library. (Image credit: Charles Blagden, L0068242 Lectures on chemistry, Wellcome Library, London. Wellcome Images, MSS 1219 – MSS1227. CC BY 4.0)

 

When I began preparing my talk for ‘Materials & Objects’, I started to think about how I might bring paper, a relatively mundane material, to life. My initial reading on the craft of papermaking told me that despite it being a 2000-year old process, making paper by hand has changed relatively little between then and now.[i] Out of curiosity, I decided to do an experiment, and to see if I could replicate some of the processes of eighteenth-century papermaking at home, in my kitchen.

The first stage in the papermaking process is to select the material from which the paper is going to be made. In the eighteenth century, this would typically have been cotton and linen rags. Towards the end of the century, shortages of rags, in part caused by an increased use of paper for printing, meant that makers began to experiment with other materials. In 1801, the very first book printed on recycled paper was published in London—that is, paper that had been printed on once before already.[ii]

Having selected the material, the next step is to break it down, making it into a pulp. When papermaking was first introduced in Europe in the twelfth century, rags were wetted, pressed into balls, and then left to ferment. After this, the rags were macerated in large water-powered stamping mills.[iii] In the eighteenth century, a beating engine, or a Hollander, was used to tear up the material, creating a wet pulp by circulating rags around a large tub with a cylinder fitted with cutting bars (see below).[iv] For my purposes, I found a kitchen blender worked well to break up scraps of used paper from my recycling bin at home, ready to make into new blank sheets.

(Left) Eighteenth-century illustration of a beating engine, from Diderot and d’Alembert’s Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers, vol. 5, Paris, 1767. (Right) A kitchen blender achieves roughly the same effect, breaking up old used paper soaked in water to create a pulp. (Image credits: Left “Papermaking. Plate VIII" The Encyclopedia of Diderot & d'Alembert Collaborative Translation Project. Translated by Abigail Wendler Bainbridge. Ann Arbor: Michigan Publishing, University of Michigan Library, 2013. CC BY-NC-ND 3.0. Right Hannah Wills)

(Left) Eighteenth-century illustration of a beating engine, from Diderot and d’Alembert’s Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers, vol. 5, Paris, 1767. (Right) A kitchen blender achieves roughly the same effect, breaking up old used paper soaked in water to create a pulp. (Image credits: Left “Papermaking. Plate VIII” The Encyclopedia of Diderot & d’Alembert Collaborative Translation Project. Translated by Abigail Wendler Bainbridge. Ann Arbor: Michigan Publishing, University of Michigan Library, 2013. CC BY-NC-ND 3.0. Right Hannah Wills)

 

Having been broken down, the liquid pulp mixture is then transferred to a container. In the eighteenth century, someone known as the ‘vatman’ would have stood over this container and dipped a mould into the solution at a near-perpendicular angle. Turning the mould face upwards in the solution before lifting it out horizontally, the vatman would have pulled out the mould to find an even covering of macerated fibres assembled across its surface. It is these fibres that would later form the finished sheet of paper.[v]

An eighteenth-century vatman dipping the mould into the vat. (Image credit: Detail “Papermaking. Plate X" The Encyclopedia of Diderot & d'Alembert Collaborative Translation Project. CC BY-NC-ND 3.0)

An eighteenth-century vatman dipping the mould into the vat. (Image credit: Detail “Papermaking. Plate X” The Encyclopedia of Diderot & d’Alembert Collaborative Translation Project. CC BY-NC-ND 3.0)

 

The moulds used in papermaking determine several features of the finished sheets of paper, including shape, texture and appearance. The type of mould first used in European papermaking was known as a ‘laid’ mould. This mould typically featured wires laced horizontally into vertical wooden ribs, meaning that when the mould was pulled out of the vat, the pulp would lie heavier on either size of the wooden ribs, giving vertical dark patches and the characteristic markings of ‘laid’ paper.[vi]

Screenshot 2017-07-20 11.16.04

A laid mould, with vertical wooden ribs and horizontal wires. A design and marker’s name are visible sewn into the mould, and will leave what is known as the ‘watermark’ on individual sheets of paper. (Image credit: Laid mold and deckle, Denmark – Robert C. Williams Paper Museum, CC0 1.0)

Screenshot 2017-07-20 15.10.07

Characteristic ‘link and chain’ pattern found on laid paper, left by the ribs and wires. This piece is a modern imitation of antique laid paper. (Image credit: Hannah Wills)

 

In mid-eighteenth century Britain, a new type of mould became widely used, developed by the Whatman papermakers based in Kent. This mould was known as a ‘wove’ mould, and had a much smoother surface, consisting of a fine brass screening that was woven like cloth. These moulds imparted a more uniform and fabric-like texture to individual sheets.[vii]

A wove mould, featuring two large watermark designs. Between the watermarks the smooth surface of the woven screening is visible, which leaves the paper with a fabric-like textured appearance, without the prominent horizontal and vertical lines of laid paper. (Image credit: Wove mould made by J. Brewer, London, England - Robert C. Williams Paper Museum, CC0 1.0)

A wove mould, featuring two large watermark designs. Between the watermarks the smooth surface of the woven screening is visible, which leaves the paper with a fabric-like textured appearance, without the prominent horizontal and vertical lines of laid paper. (Image credit: Wove mould made by J. Brewer, London, England – Robert C. Williams Paper Museum, CC0 1.0)

 

For my own papermaking, I chose to dip a piece of fine sieve-like material into my makeshift vat, aiming to replicate partially the texture and appearance of a ‘wove’ mould. The implement I chose for this was a small kitchen pan splatter guard, made up of fine mesh that when pulled out of the vat would hold a layer of fibres on its surface.

My chosen mould, a kitchen pan splatter guard, made from fine sieve-like material. (Image credit: Hannah Wills)

My chosen mould, a kitchen pan splatter guard, made from fine sieve-like material. (Image credit: Hannah Wills)

Dipping the mould into the vat and removing slowly, fibres are left on the surface of the mould. (Image credit: Hannah Wills)

Dipping the mould into the vat and removing slowly, fibres are left on the surface of the mould. (Image credit: Hannah Wills)

 

After the mould was pulled from the vat, the eighteenth-century vatman would pass it on to a coucher who would remove the sheet from the mould, before pressing it between felts to remove the water.[viii]

On the left, the vatman pulls the mould from the vat, before passing it to the coucher on the right hand side of the image, who removes the sheet from the mould before pressing a number of sheets at the same time in a large press. (Image credit: “Papermaking. Plate X" The Encyclopedia of Diderot & d'Alembert Collaborative Translation Project. CC BY-NC-ND 3.0)

On the left, the vatman pulls the mould from the vat, before passing it to the coucher on the right hand side of the image, who removes the sheet from the mould before pressing a number of sheets at the same time in a large press. (Image credit: “Papermaking. Plate X” The Encyclopedia of Diderot & d’Alembert Collaborative Translation Project. CC BY-NC-ND 3.0)

 

In order to remove my sheet of paper from the mould, I placed another sieve-material implement over the top of the fibres and pressed down with a sponge. With a tea towel placed underneath, this worked to remove much of the water without the need for a proper press. Pulling the top piece of sieve away from the bottom, I was left with a drier surface of fibres, which could be carefully lifted off the mould, and set aside to dry.

(Left) Pressing the sheet of fibres between two splatter guards. (Right) After the top guard is removed, the pressed sheet of paper is revealed. The circular shape is due to the shape of the mould. (Image credits: Both Hannah Wills)

(Left) Pressing the sheet of fibres between two splatter guards. (Right) After the top guard is removed, the pressed sheet of paper is revealed. The circular shape is due to the shape of the mould. (Image credits: Both Hannah Wills)

 

At this point in the eighteenth-century process, sheets were ‘sized’—dipped into a gelatinous substance made from animal hides that made the sheet stronger and water resistant.[ix] After my sheets had been left to one side to dry for a few hours, I decided to experiment by writing on them. I had not applied size to any of my sheets, so found that when I wrote on them the ink spread out, giving a sort of blotting paper effect.

(Left) After pressing, the sheets are dipped into large tub containing size. This step is important if the paper is to have a slightly waterproof quality that enables it to be written on without the ink spreading. (Right) Writing with ink on untreated sheets results in the ink spreading out across the paper. (Image credits: Left “Papermaking. Plate XI" The Encyclopedia of Diderot & d'Alembert Collaborative Translation Project. CC BY-NC-ND 3.0. Right Hannah Wills)

(Left) After pressing, the sheets are dipped into large tub containing size. This step is important if the paper is to have a slightly waterproof quality that enables it to be written on without the ink spreading. (Right) Writing with ink on untreated sheets results in the ink spreading out across the paper. (Image credits: Left “Papermaking. Plate XI” The Encyclopedia of Diderot & d’Alembert Collaborative Translation Project. CC BY-NC-ND 3.0. Right Hannah Wills)

 

After having size applied, sheets in an eighteenth-century papermill would have undergone a number of finishing stages. These included polishing and surfacing, processes that gave the paper a more uniform appearance.[x] With my own sheets of paper, I found passing a warm iron over the surface achieved a similar effect, removing some of the creases and wrinkles that had appeared during drying.

My finished sheet of paper, trimmed down into a small square ready for use. (Image credit: Hannah Wills)

My finished sheet of paper, trimmed down into a small square ready for use. (Image credit: Hannah Wills)

 

It is after these final finishing and drying processes that sheets of paper are ready to be packaged up and sent to the stationer’s.

Replicating historic crafts and processes is not new within the discipline of history. One of my favourite examples is a paper that was published in 1995, in which the historian Heinz Otto Sibum recreated the experiments of the scientist James Prescott Joule (1818-1889) in determining the mechanical equivalent of heat. By trying to recreate the experiment from Joule’s notes, Sibum revealed that Joule made frequent use of the bodily skills he developed while working in the brewing industry, such as the ability to measure temperatures remarkably accurately by using only his elbow.[xi] Often, attempting to replicate an experiment or craft will reveal just how much it relies upon implicit bodily skills, or tacit knowledge, the kinds of ‘knacks’ that are not written down but are simply known to those who perform an activity regularly.

In attempting to replicate the craft of eighteenth-century papermaking, I really only approximated the process, making substitutions for equipment and improvising a number of techniques, particularly when it came to removing my delicate wet sheets of paper from the mould. I think the biggest lesson I learnt was to have a greater appreciation of the material, and just how many skills and processes went into crafting each sheet of paper in the eighteenth century. Characteristics of individual sheets such as colour, texture and markings had not caught my attention in the archives previously, but I now find them fascinating for what they can reveal about the nature of the fibres used, the construction of the paper mould, and the processes followed by each individual papermaker.

 

 

References:

[i] Dard Hunter, Papermaking: The History and Technique of an Ancient Craft (New York: Dover, 1978), 178.

[ii] Ibid., 309-33.

[iii] Ibid., 153-55.

[iv] Theresa Fairbanks and Scott Wilcox, Papermaking and the Art of Watercolour in Eighteenth-Century Britain: Paul Sandby and the Whatman Paper Mills (New Haven: Yale Center for British Art in association with Yale University Press, 2006), 68.

[v] Hunter, Papermaking: The History and Technique of an Ancient Craft, 177.

[vi] Ibid., 114-23.

[vii] Ibid., 125-27. See also Fairbanks and Wilcox, Papermaking and the Art of Watercolour in Eighteenth-Century Britain: Paul Sandby and the Whatman Paper Mills.

[viii] Papermaking and the Art of Watercolour in Eighteenth-Century Britain: Paul Sandby and the Whatman Paper Mills, 71.

[ix] Hunter, Papermaking: The History and Technique of an Ancient Craft, 194.

[x] Ibid., 196-99.

[xi] Heinz Otto Sibum, “Reworking the Mechanical Value of Heat,” Studies in History and Philosophy of Science Part A 26, no. 1 (1995): 73-106.