A Colour A Day: Week 41
By Ruth Siddall, on 3 January 2021
A Colour A Day: Week 41. 28th December- 3rd January
Jo Volley writes...
This weeks colours are seven beautiful Japanese pigments, gifted to me some years ago, but unfortunately their pigment identity is unknown to me. As with all the colours I make from pigments they are bound in my preferred choice of medium, gum Arabic, which is the hardened sap of certain varieties of the acacia tree which grow exclusively in the Sahel. It has a long, complex and sometimes a very brutal history and apart from its value to the artist it has been used in foods, medicine and cosmetics for centuries.
My favourite new fact about gum Arabic can be found in Dorrit Van Dalen’s wonderful book, Gum Arabic. The Golden Tears of the Acacia Trees, Chapter IX: Intangible Tears, where she explains its chemistry.
‘Gum Arabic is a complex polysaccharide composed of four sugars – galactose, arbinose, rhamnose and glucuronic acid, plus calcium, magnesium and potassium salts. These constituents are linked to each other in myriad ways, with a ramification of elements within the molecule. This is why gum Arabic dissolves so well and produces solutions of very low viscosity: the many ramifications allow it to hold much water. Solutions of gum Arabic in water become viscous only at concentrations of 30 per cent gum or higher.
Around the turn of the century it was discovered that the molecules of gum from Acacia senegal and Acacia seyal has another permanent element that had not been noticed before or that had been seen as an impurity. Chemists found that a protein that makes up just 2 or 3 per cent of the molecule explains the property which makes especially hashab, gum from Acacia senegal, so valuable: its emulsifying capacity.
In 2019, Peter Williams, professor of Polymer and Colloid Chemistry at Glyndwr University, explained to me the significance of the find. the protein that distinguishes gum Arabic from other gums is hydrophobic: it tries to get away from water, but close to oil if there is any around. The carbohydrate part of the protein, however, is hydrophilic and uses its ramifications to stay in water as much as it can. Now imagine a glass of water with some gum Arabic dissolved in it. Add some oil (Coca-Cola-flavoured, for instance) and see what happens: each molecule of gum wants to wrap one arm around a molecule of oil, and another round a molecule of water. Like couples on a dance floor, the new combinations will fill the glass evenly. So the protein component give the gum molecule amphiphilic characteristics: it likes to be near water and oil.’
Kordofan gum Arabic
All colours are bound in gum Arabic on W&N watercolour paper and read from left to right as a rainbow and dedicated to our Key Workers.