So to start off hello everyone, I've been trying to start, but i'm having some worries about how i can build my forge, at the moment i was going to use the box forge that's pinned but i haven't had time to build it, so my father decided to do it but it seems like he went a bit overboard this thing is absurdly large something like 3ft x 3ft and a foot deep and he used what i think is treated wood (I know treated wood had arsenic in it and couldn't be burned im not sure if its still like this today, but is it safe to heat it? aka being the box? ) now in the last few days someone heard i was trying to start blacksmithing and gave me a whole bunch of firebrick he had from a chimney or something,

so i'm wondering is the box safe to use, or should i just do something with the firebrick? or line the box with it? 

sadly i don't have access to many tools or cash or i would have probably just rebuilt it or made something else but im hoping someone can let me know what i should do or at least give me an idea of a direction to head

I would prefer not to use treated wood as hot metal is going to get laid on it sometime.  As it's already been done: why not just slip a section of angle iron over it to "protect" the wood where stock might touch it.  If you can't find any angle bed rail can be used and can generally be sourced for free...

If your forge is going to catch on fire, you need a better design.

The forge sounds big enough it won't catch on fire when filled with dirt/firebricks.  However hot workpieces might be laid on the edge causing localized toxic smoke generation. Hence the suggestion to armour them.

If you use a sprinkler can to wet down your coal, drizzle a bit over the wood rim at the same time. It will keep the wood from getting too close to the ignition point. 

Welcome Aboard D~W...You have just entered the realm of all things Smoky and Rusty!!!