Onions in the Varnish

The chemist-turned-cool-Holocaust-memoir-writer Primo Levi has a
story about a time he was working at a chemical plant that made
varnish. He was surprised to find that in addition to the chemicals
he expected, the varnish formula also called for a raw onion. At
first, he could find no reason as to why a raw onion had to be
added; there wasn’t anything in onions that was needed in varnish,
and even if there were, a single onion would be too little for a
large industrial vat.

After doing a little research, Levi found out that his
predecessors used to toss an onion into the varnish as a simple and
inexpensive way of testing its temperature. If the mixture was hot
enough, the onion would fry. With modern equipment, the need for
the onion had vanished, but for reasons they no longer knew, it had
become part of the recipe.