What substances can I find to stain and protect wood that are produced without serious pollution and can be disposed of safely?
Linseed oil can be used for varnish. It is obtained by pressing seeds of the flax plant, which sounds more sustainable that anything petroleum-derived.
It's worth mentioning the safety aspect of using linseed oil. A pile of rags with linseed oil on can spontaneously combust, thanks to the linseed oil reacting with oxygen and producing heat that (when the pile of rags is big enough to work as effective insulation) can build up to the point of combustion.
[Thanks for comment by Highly Irregular]
- soak rusty nails in vinegar for two weeks, then strain
- same with pennies (for blue), or tea+metal (for black)
- soak walnut husks for several days, then strain
- chewing tobacco soaked overnight in 50/50 water/ammonia, then strain
- strong tea
- strong coffee
- not mentioned in the linked articles, but many plants can be used for dye and some of these dyes may be suitable for staining wood; search pfaf.org, tick the "Dye" box under "Other uses"
Be sure to test your stain and/or varnish on a piece of scrap wood that is of the same type you're finishing so that you can see how it will turn out.