Making Lead White Pigment

With the warning out the way, let’s see if we can make Lead White, a heavy white pigment composed primarily of basic lead carbonate, 2PbCO3·Pb(OH)2. It was used in art from antiquity until it was first replaced by zinc white, then titanium white starting in the 19th century.

Early methods of production

Crude lead white can be produced by suspending lead above a pot of vinegar and allowing the acid vapours and carbon dioxide in the air to react. This is relatively slow at room temperature, and as the atmospheric concentration of carbon dioxide is low, the yield of lead white is also low. An improved method developed in the 17th century used horse manure around the vinegar-lead assembly, which provided heat from the decomposing manure and an enriched carbon dioxide atmosphere.

Using pure chemicals

I don’t fancy messing about with piles of horse poo; lead is bad enough as a reactant. I’ll use pure chemicals and make the lead white up atom by atom, the way things should be made. First, we need to get the lead, a solid metal, into solution. We can make the wonderfully toxic soluble lead compound Lead Acetate.

Making Lead Acetate

Reactants:

2.5g lead
10ml glacial acetic acid
20ml water
a couple of drops of 30% hydrogen peroxide

In a beaker you will not use for anything else, add the lead and the water. Dropwise, add the acetic acid. You may see a few bubbles form as the acid eats away the lead oxide film on the metal. Dropwise, add hydrogen peroxide; lots of bubbles should form, but nothing should foam. The solution will rapidly heat up, and the metal should quickly dissolve. If you’re too generous with the hydrogen peroxide, you may need to add an ice cube to the solution to cool it down. If, after 10 minutes, there is still some remaining lead, you can try another drop or two of peroxide.

Making Sodium Carbonate & Hydroxide solutions

Sodium hydroxide is corrosive, so keep it off your skin and eyes. I don’t know how much of these solutions we need, so I made them fairly concentrated. It also avoids having a lot of leftover lead-contaminated solutions. I used ~2g of hydroxide in 20 ml of water for a ~2M solution, I also made up a roughly 2M solution of sodium carbonate - 4.45g in 20 ml water.

Making lead carbonate

The lead carbonate synthesis is the easiest of the two. There’s not much that can go wrong. There’s only one possible end product—the one we want.

Reactants

50% of the lead acetate solution
sodium carbonate solution

Add half of the lead acetate solution to the sodium carbonate solution. Immediately, a fine white precipitate of lead carbonate will crash out. Lead carbonate is very insoluble. There may be some carbon dioxide bubbles as any remaining acid reacts with the carbonate. You can add more sodium carbonate to the solution to ensure all the acid is destroyed.

Making lead hydroxide

This step can go wrong; it did the first time I tried it. If there’s any leftover peroxide in the lead acetate solution, the desired product will oxidize and become a form of red lead.

Reactants

The remaining lead acetate solution
Sodium hydroxide solution.

Add the sodium hydroxide solution dropwise to the lead acetate solution. A white precipitate should form, settling to the bottom of the beaker. Stop adding the sodium hydroxide when no more precipitate forms.

If you see a red precipitate forming, then there was leftover peroxide in the lead acetate solution. You can continue the process, but you’ll get red lead instead. This is another pigment and can be used, but it won’t be very pure.

I saved the red solution for further experiments with neurotoxic medieval pigments and made a tiny bit more lead acetate with 0.7g of lead squished out into a long, thin strand. This has a large surface area and reacts quickly with the acid, with only 0.5 ml peroxide needed.

In the second attempt, to doubly ensure no peroxide was left over to spoil the reaction, I heated the solution to a gentle simmer. Hydrogen peroxide is unstable with heat, and it quickly breaks down into oxygen and water. I simmered for 10 minutes, then added an ice cube to cool the solution down (probably unneeded)

Finishing up

The last step is to combine the two solutions we just made. Carefully pour the lead carbonate into the lead hydroxide solution. You can wash out any lead carbonate stuck to the beaker with a little more water. All that remains now is to wash and filter off the lead white we made and dry it. The liquids should be lead-free but disposed of properly (check the regulations for where you are).

Annoyingly I didn’t get an image of the final dry product. Just imagine some dry white powder here.

Bonus yellow chemistry.

How can we be sure the white powder is really lead white? Let’s do some analytical chemistry!

Given the only elements in the chemistry we did to make lead white are lead, sodium, carbon, oxygen, and hydrogen, it’s unlikely we made any other white pigments. There’s no zinc or titanium contamination, for example. However, we did put white powders into the solutions and got a white powder back – can we be sure we didn’t just make some sodium compound?

Testing for lead.

As a bonus, there’s a nicely specific test for lead that will also prove to us that we have carbonate in the pigment, too. We react a sample of the pigment with dilute acid and add a crystal or two of potassium iodide - if there’s lead, you’ll get a brilliant yellow colour.

Reactants

A sample of the pigment (a tiny amount will do)
A drop or two of dilute acid (acetic or hydrochloric work best)
A crystal or two of potassium iodide

Put a small quantity of pigment in a test tube or on a white plastic weigh-boat. Add a few drops of acid; carbon dioxide will be released if carbonate is present in the pigment – the solution will bubble slightly. We’re trying to bring some of the lead atoms in the pigment into solution. We don’t need to dissolve all the pigment. After a minute or so, add a few crystals of potassium iodide - if there’s lead present in the solution, you’ll get a brilliant yellow precipitate of lead iodide.

I did it slightly differently when I videoed the test. I made the potassium iodide into a solution, but I’m unsure of the exact concentration. It’s unimportant; it’s just some solid in a few ml of water. For the lead to test for, I took a sample of the water layer above the lead white pigment as it formed and added a drop of hydrochloric acid. Then, I combined the two solutions and got bright yellow lead iodide.

Lead iodide isn’t particularly stable. Although it has been used as a pigment in the past, it quite rapidly decomposes and releases iodine, which stains and corrodes whatever the pigment has been painted on.