Bill Tindall, E.Tn.
I was asked this question in an email. The answer:
There are two flammability issues. Will varnish burn? Yes, and a burning can of varnish in the shop will be a problem a small fire extinguisher can't fix. Can one generate enough combustible stuff in the shop air to explode? see below
One starts with data easily found on the Internet, the lower explosive limit, LEL, for mineral spirits. Below this concentration the flame from an ignition source will not propagate and the fuel air mix will not explode. LEL for mineral spirits is 0.7 volume percent. Translated, it means that 0.7% of the air needs to be mineral spirits before the air/mineral spirits mix will explode. What one wants to know is how much mineral spirits do I need to evaporate in a room of so many cubic feet to reach the LEL. To get to this value requires some recollection of chemistry gas laws to get a good approximation.
A mole of a volatile substance occupies 22.4 liters. 0.7% or 22.4 L is 0.16L. So 0.16L of mineral spirits vapor in 22.4 L of air will reach the LEL. 0.16L of mineral spirits vapor is 0.16/22.4 moles of mineral spirits. If we estimate the molecular weight of mineral spirits as about 200 we get 1.4 grams of mineral spirits vapor in 22.4 liters of room air will reach the LEL. A cubic meter is 1000 liters. 1000/22.4 X 1.4 g = 63 grams. It takes 63 grams of mineral spirits per cubic meter of air to reach LEL. A quart of varnish, at 30% solids would contain about 630 grams of mineral spirits, or enough to take 10 cubic meters of air to the LEL. A cubic meter is 35 cubic feet. That would equal a shop 2' x2' x 8' tall.
From here out you are on your own to make assumptions. What safety factor to build in, probably about 1/10 LEL. How far away is ignition source from mineral spirits evaporation? What is evaporation rate? What is rate of turnover of air in the room? Making reasonable assumptions yields t " use mineral spirits in a well ventilated space". Now where have you seen that warning before? Doing so will present no explosion hazard in the room." However, spilling the can of varnish on an ignition source will in itself present problems. I have no doubt that painters in 1950 smoked while painting rooms with mineral spirits formulated paint and varnish.