Poured zinc into a glass jar tonight, lasted about four seconds

Same thing happened to me a couple summers back with a thick pasta sauce jar and a pour of aluminum scrap. I preheated it on the wood stove, sat it in wet sand, and it still let go with a loud snap before I was even half done pouring. The crack always seems to start the second the metal touches the glass and there's no amount of oven time that gets around it. Thick glass is acutally worse than thin for this because the inside and outside of the wall end up at very diffrent temperatures at the same moment, so the stress has nowhere to go.

The one that surprised me was a thin-walled spaghetti sauce jar I tried purely out of curiosity. It survived one pour. The aluminum picked up the ridge pattern from the mold seams on the glass and everything. I would not trust it to survive a second one though. These days I just do what you're planning. A coffee can packed into damp sandbox sand with a wooden dowel poked down the middle makes a perfectly decent crude ingot if you want round ones, or I use the steel muffin tin from the thrift store for flat ones. Cheap, ugly, and it never explodes.

I actually got one to survive once, but not the way you did it. I took a thin mason jar and hit it all over with a propane torch til it was nearly too hot to hold with bare gloves, basially the same temp as the zinc going in. That one held. The pour went clean and I got a decent ingot with jar rings molded into the sides. Problem is I never trusted it after that, because the second attempt the same setup gave me a hairline crack near the base that I didn't spot until zinc was weeping out into the sand and I lost most of a melt. That's a worse failure than a loud pop in some ways, since you don't see it happening.

These days I don't bother with glass at all. A soup can crimped a little at the top makes a fine loaf ingot, and the tin peels off with a couple of taps once it's cooled. For my fishing sinker pours I just carve a shape into dry play sand with a bolt or whatever's handy and pour straight into that. The sand eats the shock and you never have to wonder if the container is going to hold. One thing I'll add about zinc, it stays workable way longer than aluminum after the pour, so even a messed up pour like yours is easy to re-melt and try again. That freeform blob you dug out will go right back into the ladle no problem.

Back in the day the old boys at the foundry I apprenticed at wouldn't touch glass for anything hotter than lead. They used cast iron ingot molds greased with lampblack, and those things lasted decades. I did try a Pyrex measuring cup once thinking the borosilicate whould shrug it off. Same story, one clean pour then a crack on the second. Funny thing is the first ingot came out with a nice smooth finish, better than my sand casts ever do. These days I keep a couple of angle iron offcuts and clamp them into a V for long bar ingots. Ugly but bulletproof.