For those thinking of making their first furnace, I actually would not recommend using a keg.
It’s bigger than you really need. But if you plan on doing bigger melts this is an option for you. This furnace will fit a #6 or #8 crucible.
That’s after I sized it down. I cut 3 1/2” out of the middle to size it down. Do that right away unlike what I did.
I prefer the furnace I made out of a propane tank. It’s smaller so it heats up faster, hotter and uses far less fuel. And is plenty big enough for most of my melts. I use a #4 crucible in the propane tank furnace. You can find that video here:
How to build a foundry out of a propane tank in under 5 minutes: back yard Foundry
   • How to build a foundry out of a propa...  

Cost of build and material used.
I bought most of the material from local fire brick stores in Minneapolis so prices will vary.

Keg-$25
Mizzou refractory cement, 55lb- $50
2”x 2’x6’ of ceramic insulation- $90
1/2”x2’ x2’ ceramic insulation- $20
Sairset mortar, 1 gallon- $20
Greenpatch 421 mortar, 1 gallon- $30

Note: Some of these things I had left over from other builds and it’s not necessary to get each one of these products. For example I used a very small amount of the refractory cement. I’ve seen people but metal inserts around the rim to strengthen them instead of using cement. That would be a cheaper option.

Chamber size. It’s 10”wide by 11” tall. Remember the bigger the chamber the more it takes to heat all that space up. And the more mortar and cement you have in there the longer it will take to heat up and the more fuel you will use.