Why did my furnace lining blister the first time I fired it?

The bubbling you saw is almost always due to trapped moisture, even if you think you dried it out for a long time. The initial burn is not just about getting hot, it is fundamentally about venting all the water out of the clay and cement before anything else. Most people underestimate how much moisture gets trapped in that initial mix, and a week isn't always enough depending on humidity and the thickness of the layer. What happened in your situation is that when you started letting in more air and the temperture climbed rapidly, that moisture turned to steam. Since the clay has very low permeability, the steam had nowhere to go but up and out, causing the blisters and the soft, doughy areas you noticed. Because the whole thing got a little too hot, you likely caused some localized thermal shock or cracking on top of the moisture issue.

If you're going to fix this, don't just patch the spots. You need to heat the entire assembly slowly, maybe for a day or two, just to raise the ambient temperature and encourage any remaining water to move and evaporate. A gentle heat source, like an old light bulb placed inside, works better than trying to re-fire it with high heat, which will just push more steam through the cracks.

As for materials, if you are only melting scrap aluminum, a cheaper liner like furnace cement might get you a little while, but it has limits. If you plan on pushing the heat or want a longer lifespan, you need something with a higher, stable melting point that won't degrade as quickly when exposed to the slags and aluminum fumes. Trying to save the current lining by just scraping the soft spots is a temporary fix at best, and the underlying material might still fail under stress.

A week of sitting in a garage does basially nothing if the mix was still wet going in, that's the real issue. When I cast my first liner I kept a bare 60 watt bulb burning inside it for a full week before the first fire, and even then the cure schedule matters more than the wait. Yours flattened back down, so the liner is probly fine. Next fire, stay low til a hand held near the exhaust can sit there comfortably for a few seconds, then step it up over a couple hours. If it holds, you're good to go for aluminum.

My first liner did the same thing and the problem was my mix. I was stirring it way too wet, like pancake batter, becasue it seemed easier to pack in. Second time around I used barely enough water so it would hold togather when squeezed, packed it tight, and the whole thing was dry enough to fire in a few days with a small charcoal bed going. Also, if a blister flattened back down soft, just press it back in place while the liner is still damp and let it cure, no need to patch anything yet. Cans and scrap aluminum won't stress that lining much anyway.

Total beginner here so take this with a grain of salt, but I had a similar panic with a small crucible furnace I made last spring. What saved me was just moving it outside for a few sunny days and rotating it every so often, then bringing it in at night. Way faster than waiting in a damp garage. One thing I'm still not sure about though, does anyone know if the spots that blistered but got pressed back down will end up flaking off later once they've gone thru a few heat cycles? Mine looked okay after but I only got maybe a dozen melts in before I moved house.

Been through this more times than I care to admit. Back when I ran a coal forge furnace, the old boys I learned from always poked a few pin holes in the fresh liner with a nail before it set, gives the steam a way out instead of lifting the face. Worked then, works now with castable. Half inch thick will dry way quicker than folks expect once you give it an actual path to vent. Also, don't judge dryness by the calendar, hold your palm flat against the outside of the drum. If it feels the least bit cool or clammy, there's water still in there. Warm and dry to the touch, you're good to go slow.