On Jan 24, 5:06 pm, "Jimmy" wrote:
I mixed some blo and thinner in a glass fruit jar. Is it safe to put a tight
lid on this and keep it in a cool dark place

Yes, no problem. There isn't anything in there that would react to
form a gas and increase pressure significantly inside the jar.
The only time I can think of that finishes could "blow up" under normal
conditions (without extreme heat, sparks, etc.) is via spontaneous
combustion. This occurs when the finish polymerizes or catalyzes, and
the polymerization builds up heat, and when there's enough heat, some
fuel, and some air, spontaneous combustion can occur. The main way
I've heard of this happening is when a finish-soaked rag is wadded up
and left for a while, especially in a warm area - the rag provides lots
of surface area and possibly some insulation, so the finish can
polymerize quickly and build up heat. As long as the rag is spread
out, the heat can dissipate and it shouldn't be a problem, or I know
that some people submerge used rags in cans of water.
There isn't enough surface area for this to happen in the jar/can - if
anything, you might get a skim of polymerized finish on the surface of
your jar, but I've never heard of spontaneous combustion happening in a
partially-full container.
Good luck,
Andy