Anyone have any pros or cons on using kiln shelves for a floor and top on a gas forge ?

Pros:

1. If correctly selected can be fairly flux resistant
2. If forge is designed well can be replaced after cracking or damage
3. Depending on thickness will have some thermal mass and act as a little bit of a "heat battery"
4. If good selection, can be used for direct flame contact (i.e. set directly opposite your burner)

Cons:

1. Have virtually no insulating value so must be backed by insulation to ensure efficiency
2. Are brittle and don't always respond well to rapid thermal changes, so can be expected to crack over time and should not be used as a "structural member" (i.e. to hold up a forge "roof" or support burners)
3. Are difficult to cut openings in.

Bottom line is I recommend thinner high alumina shelves for floors, but not tops.

Thanks for the info 

They only thing I would add to what Latticino wrote is that I like to trap the shelve floor in recesses in to forge shell so that they can be easily pulled out and replaced.

The other con with kiln shelves is they absorb moisture and crack when exposed to thermal cycling if they aren't dried out slowly and what ever you do don't drop them.

whoops!

My local source i got IFB from didn't have shelf so I just cast a quick plate with the Mizzou I used for the ribbon burner using the top to a plastic storage container.  About 3/4" thick and should work fine for protecting the soft brick.  If it doesn't hold up well I think I can get 3/4" hard brick from them.  I was afraid of not mixing enough for the burner so I went over and my calculations for what I needed were actually spot on.

If you have soft brick or can get it, I have seen where someone used some iron flat stock or angle and threaded rod and clamped the brick lightly together so it held together over the span.  They had a large area that they could brick in an area depending on the size of the job they were working on.

What internal size are you looking to build?

I think Kast-O-lite 30 makes an excellent alternative for kiln shelves, especially for people who can't get genuine high alumina shelves. If you can get the real thing, then I think it boils down to a matter of preference. Genuine high alumina kiln shelves are tough, but a lot of mullite substitutes are being pushed as high alumina shelves; they are nowhere near as good.