Mixing It Up

The past weeks of research have been packed with activity. In order to best tailor our experiments to historic paint applications, I began the summer by making green paints according to traditional techniques. In agreement with research from Shelley Svoboda, the paintings conservator at the Colonial Williamsburg Foundation, we chose to combine of the blue dyes azurite, Prussian blue, and indigo with the yellow dyes gamboge, stil de grain, and reseda lake to make optical greens. The paint making process starts by making small mounds of dye on a roughed glass plate. Drops of linseed oil are added to make a paste. Once satisfied with the color, we added enough oil to create a spreadable consistency. Then a large glass pestle was used to grind together the grains of dye and the oil to create a homogeneous mixture. Grinding each of the nine mixtures for 15 minutes each was definitely an arm work out! It was worth it though: the paints came out gorgeously.

Paint making in action.

Most interesting was the different character of each dye. Stil de grain, a yellow dye of ground buckthorn berries, was chunky and fought the pestle all the way to the end. Though the powder itself  was very yellow, it refused to release its hue to the mixture, leaving them much more teal than green. Prussian blue, on the other hand, dominated the mixtures with its deep, saturated hue. The blue powder also seemed to make its way into every exposed nook and cranny. My hands were blue for hours despite best attempts to protect them with gloves. Due to the differing qualities of each component, each unique mixtures will need to be treated and analyzed differently.

Oil based paints take quite a while to dry, so analysis on the mixtures has just begun. The end goal is to correctly identify both components of a green optical mixtures. The difficulty lies in the different techniques used to extract blues and yellows. Indigo requires a long acid hydrolysis which can potentially destroy a yellow dye. Striking a balance will be key. Luckily, with our newly made paint mixtures, there is plenty of opportunity for experimentation before we attempt analysis on samples from actual paintings.

Check back for future results!

Mary Matecki

Comments

  1. i<3science says:

    Hey Mary this stuff is pretty interesting. I was wondering why different color dyes require different treatments? Specifically why the different treatments are categorized by the color of the dye. Is it because they have all have a similar chemical structure or is it something else. Keep up the good work!