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I'm a beginner potter and I've tried wood firing pottery outdoors because an electric kiln is not very accessible for me. 

My small kiln is about a bit bigger than 2'x2'x2' and made from regular red bricks with a regular grill grate halfway to place pottery. As you can probably tell, it has terrible insulation and I can never get it to a high enough temp to fire glazed pottery. I can bisque fire and it works but It never gets hot enough to melt glaze, even the low fire glazes I use. 

I really don't want to ditch this project because I've already put so much time and material into it but I don't have the time/material to build a much bigger kiln or spend days firing. I also don't have a huge budget. 

What fairly inexpensive materials I can use to insulate my small kiln so that it can comfortably reach cone 06 and complete a glaze firing?

I've looked into getting ceramic fiber board to line the inside of the kiln with, fire bricks are very expensive and probably out of my price range. 

Any advise from someone with more experience in this field would be very appreciated, Thank you!

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Red bricks are not for kilns

You can use chimney bricks sold at lumber stores which will work for low fire and they are cheaper than fire bricks. They are made for fireboxes in house chimmneys .They are not stamped with any markings . Red bricks come apart when heated to much.They are the worst choice for making a kiln.

As to insulation behind the bricks mix up fireclay (dry powder) and vermiculite and keep it as dry as you can (less shrinkage ) 

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Red bricks are not fired very hot, and will not survive regular wood firing temps of cone 10+. They will likely not even survive cone 6, and won't last very long when firing to low fire temps. You need firebrick for a kiln.

There are lots of recipes available online for home made insulating castable materials that can be used on a wood kiln. Typically they contain fireclay, sand, a small amount of Portland cement, and organic material. Kind of like a high temp adobe mix. The kiln must be built in such a way that it can hold the castable, like a catenary arch. You can't apply it to a vertical wall.

The design of your kiln is also very important. Wood kilns are just as susceptible to the problems that gas kilns can have. It's all about fuel, air, and flow. I would start over with your project and use the right kind of bricks. You could probably use the red bricks as an outer layer of the kiln. Also post a drawing of your kiln plans and we can take a look at it.

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