I have a question for all of you experienced blacksmiths, my brother has been experimenting with forging using a firepit and bricks as a makeshift forge. He asked me if i would like to come out and help make a knife with him out of a railroad spike. Long story short i got hooked on blacksmithing. Anyway's we have recently required a nice metal tank and were thinking about making it into a better forge than the fire pit that we have been working out of. We are thinking of cutting the top half of the tank off and using the bottom half as our new forge. My question is to prevent the coal from heating through the metal at its high temps, would lining the sides using wood ash as a refractory be sufficient enough to run a safe forge to make knives in? Or do i need to coat it in something else or nothing at all? Hope that makes sense and thanks!

Welcome to the brotherhood. You would be better off by searching this site or checking u tube for making a brake drum forge.  lots of good information on this site avaliable by searching.  U tube also has lots of vidios.  Check blacksmithing and forging.

New here, but I have the same worry for my own forge Im building, so I went to my local pottery store to get some fireclay for liner. Pretty much every pottery supplier carries it; the owner said that its raw material needed for mixing pottery clay, and he sold the stuff to farriers and people building pizza ovens. The bag he sold me was labelled Hawthorne Flint Clay shipped from Missouri, paid 22 dollars after taxes for 50 pounds of the stuff, so pretty cheap. Owner told me to mix it with water just like concrete, mold into whatever shape or place required and let it dry COMPLETELY. And once you think its dried out, take a propane torch to it and dry off any moisture left in the clay to prevent cracking. Hope this helps.

Forges are a heat balance: heat in vs heat out. The nice cool air on the other side of the steel helps keep the firepot lower in temp as does a typical non burning layer of coke as air doesn't make it all the way to the edge of a typical firepot.

I've been using a firepot made from a "Banjo rear end axle cover"---from the '30's?---for about 25 years now with no lining and lots of forge welding.

It is getting a bit thin; luckily I bought 2 of them that had been used as jackstands for $3 at a fleamarket and so switching to the other one should take me till my 80's...