Using refractory to build a kiln?

[joshturner84]

Hi I'm new to glass and am starting to build a kiln seen Marks threads about the stove pipe and mailbox annealers and was wondering if I could use refractory instead of the frax and fire bricks. Will the refractory work because I have some on hand left over from a foundry I built a while back and there isn't a place to get frax anywhere near me.

[Gaffette]

I assume you are talking about a castable refractory. In my experience the castables tend to degrade faster than soft brick when directly exposed to the constant heating and cooling. It will work I just don't think it is ideal, and even with the refractory you are still going to need the frax for insulation. Of course everyone has different opinions on materials so someone may disagree with this. There is a reason that almost every kiln I have ever seen is the frax and soft brick combo. It insulates well, lasts long, and is relatively simple to build with. You can order frax online. Look at different sites to get the best deal on shipping and what not relative to your location.

[crec]

Ok I'll toss my two cents in.

Like Kris I am assuming by "refractory" that you mean "castable refractory". Now without knowing which type of castable (lightweight, dense, etc..) the advise will be somewhat generic. You can make a kiln out of castable, just understand the limitations of the material that you are working with. Castable refractory is not a very good insulator, so your energy usage will be higher than if you had some form of insulation (like frax) behind it. Also it will require more time to come up to temperature than a frax or frax/IFB kiln as the refractory will absorb some of the heat that you are putting into the kiln (frax reflects heat). That said the refractory will also take more time to come back down to room temp then a frax kiln as the absorbed heat will be released more slowly. If you make a monolithic kiln insert, like is usually the case for small foundries pay attention to the shape that you create. Most small home foundries are circular in shape allowing the forces in the wall to be more equally distributed when the foundry is heated. Most small kilns are rectangular in shape so the tendency to crack is much greater when it is heated. If I was going to use castable to create a kiln I would probably cast individual "bricks" to the size and shape required for the walls, roof, and floor and the assemble them in a frame. Also I have read in some of the foundry sites of individuals adding some sawdust or styrofoam beads to the castable to provide small air pockets in the matrix to add insulating qualities to the castable (the sawdust/foam beads burn out when fired the first time). Care has to be taken not to add to much or the castable will not have any strength. Personally I have not tried that with castable, but I have made my own foundry using a styrofoam bead/clay mixture that I made from clay from my back yard.

Here is a short list of pros and cons to some available materials.

Castable refractory pros
Can form any shape you want.
Durable
Does not fracture into airbourne fibers
Retains heat and gives it off slowly (good for when power fails)
Cons
Heats up slowly
Not as good an insulator as other materials
Can spall or crack if heated too quickly the first time
Can crack due to expansion/contraction if fixtured too tightly

Frax pros
Easy to shape
Heats quickly (actually it reflects heat)
Decent insulator
Allows for expansion/contraction as it is heated
Cons
Breaks down forming airborne fibers if disturbed
Soft and easily damaged
Does not retain heat ( when power fails, heat in zone crashes)

IFB (Insulating Fire Brick)
Soft easy to cut/shape.
Insulates better than Castable but not as good as frax.
Retains heat.
Does not break into airbourne fibers
Cons
Heats slowly (absorbs some heat)
More durable than frax, but not as durable as castable

This list is by no means comprehensive, and I am sure others can add to it.

Many small kilns are made out of only IFB which is not a great insulator, but it is still better than castable. You can source some of the materials (frax/IFB) online. Even if you do create a kiln using castable, you will probably find that you will want some extra insulation on the outside to make it more efficient.

YMMV

Carlos

[silberrucken]

If you want an overview of kilnbuilding with the pro's and con's you might try Dudley Giberson's book "A Glassblowers Companion". He writes in a style that is a monilogue that is part philosopher, part historian and part handyman. Some folks don't like his book, but in addition to telling you what does work, he tells you what doesn't work, and why. Has a rundown of different rafractory materiels. Talks about everything from ovens for making glass from rocks to making the ubiquitous mailbox annealing kiln New from Joppa Glass is $35, and sometimes available used on Amazon for half that.

Anither good book is "Glass Notes" by Henry Halem $40 new from Artco among others. Glass notes is a more "finished" book with more information, but IMHO not as entertaining to read.

Another possibilty is to buy a pre-made kiln from Glasshive as they have an open ended "lay-away" plan and they have kilns that will do annealing as well as slumping and enamelling.