We had a wonderful science session - Alison’s plan gave us chemistry, art and history in one session. We saw some practical and historical applications of chemistry: we made some paints which were used in medieval times, and then painted with them.
First we looked at sources of paints, and some paintings which they were used in, from a book published for the recent National Gallery colour exhibition, which several of our group had visited. We passed round samples of lapis lazuli and malachite, and heard how they were ground up to make pigments, but that lapis lazuli was expensive. Verdigris was used as an alternative source of blue, although its colour was more turquoise. Alison and Angie had prepared beforehand some verdigris by standing a strip of copper in a small pot within a sealed jar containing a small amount of 3m acetic acid. It had only been in place for two days but plenty of verdigris had developed.
The children prepared their own artificial verdigris, by mixing copper II carbonate with dilute acetic acid. Acetic acid is the main active ingredient of vinegar and, for example , cider vinegar contains about 5% acetic acid and this is what makes it useful for medical purposes. We noted bubbling as a sign that a reaction was occurring, then saw the colour change from green to turquoise as a solution of copper acetate say formed. We let the unreacted copper carbonate settle, then poured off the top liquid and filtered into a beaker. Then we heated this solution in evaporating dishes over burners. We used retort stands with rings and pipe clay triangles. The turquoise residue was scraped out into a well of a spotting tile.
In other wells of the spotting tile, we placed charcoal that we ground in a pestle and mortar, red iron (iii) oxide (red ochre), yellow iron hydroxide, tin oxide, and chromium oxide. Thus we had as our pigments the following:-
Charcoal - one of the oldest artists’ materials, known traditionally as ’
carbon black’
Iron (III) Oxide (Red Ochre) is mined as iron ore, and is also used as a pigment. Its chemical formula is Fe2O3
Yellow Iron Hydroxide (Yellow or Gold Ochre) has the chemical formula FeO(OH) and it is ‘hydrated’, which means it contains water of crystallisation.
Chromium Oxide is also known as chromia or the pigment viridian, and it was originally prepared from a rare mineral called Eskolaite. It was probably not in widespread use until the Victorian era when a process was developed for deriving it by chemical means from a more common mineral.
Tin Oxide SnO2 - this was our white pigment and is commonly used by potters. It is the main ore of tin, and its mineral form is known as cassiterite.
Making Paints
Next, egg tempera was prepared. We separated egg yolk from the whites, washed the intact yolk, then punctured the yolk membrane and squeezed out the yolk. Some yolk was added to each sample of pigment, and they were mixed thoroughly. A little water was added where necessary to get a smooth consistency.
Alison had printed traditional icon outlines, and the children painted these using our home - made paints. All of our paints were similar to those in use in the medieval period. Finally, when the paintings were dry, they were gilded using imitation gold leaf.
We had never seen the children so quiet in a science session!
