Viridian pigments

As some of you may know, I'm currently on a quest to produce a full set of artist's paints myself using chemistry. Just recently I decided to try a procedure for producing the pigment viridian, a hydrated chromium(III) oxide, using the procedure found here.
4 grams of sodium dichromate dihydrate(ceramic grade) and 13 grams of boric acid(roach killer) were mixed and added to a small iron dish lined with aluminum foil. The mixture is shown below.

The dish was then put into a rudimentary wood-burning furnace/kiln which was heated to roughly 500 °C for two hours, which was as much time as I could spare at the time. The high temperature was maintained by me manually driving air into the burning chamber of the furnace by fanning it with a board; I've got hand blisters to prove it



The dish removed from the furnace contained a large greenish plume of glassy material. It was filled with air pockets and had gained considerable volume due to the water vapor and oxygen released during the reaction. Inevitably some of the aluminum foil lining stuck to and partially fused with the mass, but the lining itself ensured that the contents of the dish were easily removed.



After allowing it to cool, the unsightly thing was broken into smaller pieces and tossed into a beaker containing ~400mL of water. The water in which the fused mass had been cast became bright yellow, indicating dissolved sodium chromate from the decomposition of the dichromate used as a starting material. Many of the pieces were quick to crumble in the solution and sink to the bottom, but those with the most intense green color and which contained the greatest amount of glass floated and did not break up further; these were collected separately from the green mud which precipitated at the bottom.



The floaters were immediately dried, ground into powder, and then ground again under water and re-dried. The solids that settled to the bottom of the beaker were first treated with sodium hydroxide solution to dissolve any residual aluminum from the lining of the heating dish, then filtered out, dried, and ground.

The picture below shows the pigments thus obtained from the process and a test with a brush for each. The pigment produced from the solids that floated upon being cast into water is at the top, and was more blue in color. The second pigment, produced from the solids that settled to the bottom, is at the bottom of the picture and has a more typical chromium(III) oxide color. The pigment in the middle is a mix of the two.



This was a really fun experiment to try out, especially given its rustic nature, and I'm quite happy with the results. On to yellows for my next paint pigments!


[Edited on 3-28-2015 by Amos]

Quote: Originally posted by blogfast25

Interesting. Any idea in what way this pigment differs substantially from plain Cr<sub>2</sub>O<sub>3</sub>, which is after all what gives 'Viridian' its vibrant green colour?

Since most of the resources aren't quite as scientific in nature, they're kind of vague, however, the consensus seems to be that the oxide readily hydrates after substantial heating and cooling. This paper also mentions that a byproduct of the calcination process includes a hydrated chromium oxide borate that is also green in color.

Oh, and if the rain and cold ever stops here, I'll be firing up the furnace again and hopefully decomposing chromium hydroxide to make chromium(III) oxide via another route, and I'll compare it to the viridian I made. I also want to make the viridian again and fire it longer, to see if I get more of the first pigment I described, with more of a bluish color.

[Edited on 3-28-2015 by Amos]

Quote: Originally posted by blogfast25

Interesting. Any idea in what way this pigment differs substantially from plain Cr<sub>2</sub>O<sub>3</sub>, which is after all what gives 'Viridian' its vibrant green colour?

To me it seems that the blue hue is causes by hydration or maybe hydrogen bonding between borate and hydroxyl groups. Is it possible to create a section on sciencemadness about artistic applications of chemcals? A board concerning, pigments, patinas, ceramics and glazes, painting mediums and filler materials. I believe a large percentage of us could contribute.

BTW that is good work Amos. I grateful that you shared this with us.

[Edited on 11/8/2015 by chloric1]

Quote: Originally posted by Bert

Quote: The high temperature was maintained by me manually driving air into the burning chamber of the furnace by fanning it with a board; I've got hand blisters to prove it I have used a cheap blow dryer from a yard sale on the cold air setting. Doesn't take much power to run, without the heating elements. I just use a small inverter and my truck batteries for a typical smelt, without even killing the batteries... Fueled with charcoal this will make an extremely hot fire, hot enough to ruin my makeshift refractory furnace lining!