Lead was primarily added to paint in the form of white lead pigment, traditionally produced through a method called the stack process.
Historically, paint manufacturers used pigments to give paint its color and properties. One common and highly valued pigment was white lead (lead carbonate), known for its opacity, durability, and ability to mix well with oils. The traditional way to create this pigment involved a specific chemical process.
The Traditional Stack Process
The referenced method for making white lead pigment was known as the stack process. This technique involved carefully arranging lead and other materials in layers to facilitate a chemical reaction that produced the desired white pigment.
Here's a breakdown of the key steps in the stack process as it relates to adding lead to paint:
- Preparation: Earthenware pots were prepared. These pots contained a small amount of acid, typically vinegar (acetic acid).
- Adding Lead: Sheets or pieces of metallic lead were placed within these pots, often suspended above the vinegar without touching it directly.
- Stacking: The pots, containing both lead and vinegar, were then arranged in layers.
- Embedding: Crucially, these layers of pots were embedded in a layer of either tan bark or cow dung.
- Chemical Reaction: The organic material (tan bark or cow dung) would ferment, generating heat and carbon dioxide. The heat vaporized the vinegar, and the acidic vapor, along with the carbon dioxide, reacted with the lead over several months.
- Pigment Formation: This slow reaction converted the metallic lead into white lead (lead carbonate), which accumulated on the surface of the lead pieces.
- Processing: Once the reaction was complete, the white lead was scraped off, ground into a fine powder, and processed.
This resulting white lead powder was the pigment that was then mixed with a binder (like linseed oil for oil-based paints) to create paint. Therefore, lead was added to paint indirectly by incorporating white lead pigment produced through processes like the stack method.
