Tuesday, July 20, 2010

About Lead White















Lead white is one of the oldest artificial pigments in the artists's palette. It is also arguably the greatest pigment in the history of oil painting, used by humanity for thousands of years.

Lead was once the only decent white available to oil painters. The last century witnessed the introduction of alternatives, first zinc white, then titanium white. The result is that its use has declined significantly. Now that there are alternatives, why would we want to still use lead?

We are only beginning to understand the complex chemistry of paints, and conservation scientists have noted that paintings with lots of lead in them are usually in much better shape than their counterparts. In some cases, parts of a painting with lead are the ONLY areas still in good shape. The consensus is that lead is strengthening the structure of paintings. By contrast, the Smithsonian has recently completed a 30 year study linking the use of zinc white to major cracking, disintegration and delamination of modern paintings.

The great downside of lead is its toxicity. Even small exposures will accumulate in the body over time. Lead poisoning was noted even in ancient times, but little was ever done. Most of the risk wasn't to artists but to the individuals exposed to lead during its production. Little protection was available to workers and they were exposed to large amounts of lead as a result. Thousands would have died in this pursuit. One 1884 source I found suggests most of the lead factory workers were women. These and other concerns lead to its decline. Over the past fifty years developed countries have highly regulated its use. The day may come when its use is banned completely.

It came as a surprise to me during my research to learn that modern lead white is not the same as it once was. From Roman times up through part of the 20th century, the best lead was made with the Dutch "Stack" process. Below is an illustration of how this was done:

Note: Lead is highly toxic. I would NOT recommended trying this process on your own, it is not worth the risk.

Lead white is basic Lead Carbonate. Alchemists discovered a simple way to make this successfully using lead, vinegar and manure.















First Vinegar is added to the base of clay pots. Special pots would have been made for this purpose.















Curled sheets of lead are placed inside the pots, with a spacer in between them and the lead, so that only the rising vapors will contact the metal.















Fresh horse manure was collected from Dutch breed horses, for added authenticity. This was mixed with leaves and filled in around the pots. The pots would have been placed in a special sealed building, in many rows of stacks. The "Stack" is where the process gets its name . I used a large plastic container.















The vinegar vapors contain acetic acid, that chemically corrodes the lead, forming lead acetate. The decaying manure releases carbon dioxide and moisture, which reacts with the acetate making lead carbonate and lead hydroxide. It also generates heat that speeds the process along. It will take several months for the conversion to take place.















The box after two weeks. A significant amount of white has already been produced.















Over a month has passed, and the process is mostly complete. The lead can now be removed, coming off in large flakes. This is where the traditional name "Flake White" comes from. I ground and washed the lead, to remove impurities.















It appears the raw lead I used had copper impurities in it. These were washed out as much as possible.

The pigment was left under the sun several days to dry.















At this point, the lead white was ready to be sold as is or mixed for paint.

Lead produced in this manner has unique properties that are highly desirable for artists. Under the microscope it's particles are larger than modern lead whites. This makes it more opaque and gives it special handling qualities in oil. Lead acetate and other impurities accelerate its drying.

Stack lead white is nearly extinct. One of the only companies I know that still sells lead produced this way is Natural Pigments.

1 comment:

Raj Garewal said...

Thank you so much for the info, I've started my own batch and was unsure about a few things, you cleared em up. Btw, Michael Harding now sells tubed stack white, but it's Michael Harding paint and stack lead so it's really expensive, but very good.