I’ve been sanding down a wood piece from the 1800’s. I performed two lead tests that came up negative but I’m still worried. I did the test patch by sanding it down to the bare wood, then rubbing the tester tube over the edge of the stain and the bare wood. When did they start to use lead in paints, varnishes, and stains? Does it go back as far as the 1800’s? Do I need to worry?
Lead was used as a pigment.
Lead (II, IV) oxide yields a bold red pigment.
Lead chromate (boy, there's a nasty customer for you) yields a yellow pigment.
Mineral pigments, including all these mentioned here, are naturally resistant to UV light, so very durable. Carbon black remains an ideal black pigment. Color pigments tend to be toxic minerals. (Or modern organic compounds that don't hold up under UV. Look at the nearest parking lot. See all the black/gray/white cars? Now you know why.) Old black is safe; old white/gray might be safe; old colors, lookout.
All of these things are pigments. They add color for the purpose of masking the underlying surface. You'd never find them in a varnish, whose job is to allow light to make a 2-way trip through the varnish and allow the underlying surface to shine through.
Use PPE, regardless.
That said, use PPE - i.e. a competent dust mask and safety glasses. Any dust WILL end up in your lungs, and there's a lot of other stuff in old varnish (and wood fiber itself) that you don't want in your lungs.
Also don't panic about lead.
It's lead, not plutonium. (Even plutonium isn't plutonium in the panic sense). There are a lot of materials we can live alongside safely, merely by not being idiots.
Lead was only ever a problem when paint was allowed to chip, peel and deteriorate. The reason lead gets all the press is that it's also a social problem - the upper classes living in newer suburban housing stock had titanium-pigment walls that were well-kept, so children were not eating paint that was peeling off the walls. Meanwhile, in the slums...
If the test shows negative you don’t need to worry. I don’t remember stain having lead (state required lead removal class).
Just to calm your fears, even if there was lead when you recoat it would seal the lead under the new layer (this is why on painted surfaces you score the surface because if the last coat is lead free the test will show negative). So I would not worry.