Gas kiln and cement backer boards

[Luitreth]

Hello all, planning to make a gas kiln using the plans in Andrew Holdens book The Self Reliant Potter. The design calls for floor and walls of insulated fire brick, backed up with ceramic fibre blanket and then calcium silicate board on the outside, my question can I use hardie cement backer board in place of the calcium silicate board, main reason is hardie backer is cheap and readily available and calcium silicate board is not, the book was written some time ago so there may be more suitable modern substitutes I'm not aware of (in the UK). Grateful for any pointers, many thanks.

[neilestrick]

Hardie board is non-combustible, so it should work. It's just being used as a protective layer over the fiber blanket. I would double check with the manufacturer to confirm, though, and make sure it won't degrade with prolonged exposure to heat. You should calculate the temperature of the cold face of the fiber so you know how warm it will get. The brick and fiber manufacturers can help you with that. First figure out the temp of the cold face of the brick, then the fiber.

Wear proper protection when working with fiber- respirator, gloves, goggles.

[Luitreth]

Hi Neil, thank you for your quick response, yes it is just being used a s a protective outer skin if you like, the whole plot sitting in a frame of welded angle, regards, 

Daryl

 

[Luitreth]

As an aside, I note your comments about fibre, a lot of these products are now being marketed as "body soluble", should we take that with a pinch of salt? (and the proper ppe)

 

[Min]

That's what I was told a few years ago too; that it dissolves in your lungs and nasal passages. A quick google search shows that some are and some are not. I would err on the side of caution. 

https://www.unifrax.com/wp-content/uploads/2018/08/M0001.pdf

https://www.sciencedirect.com/topics/materials-science/ceramic-fiber

 

[Mark C.]

Some cement boards use a fiberglass like matt inside for strength . This material can be affected by heat. Check with the manufacture on how much heat it can take as Neil says.

 

as to eco frendly fibe I'm not a believer -wear a mask installing it.

[neilestrick]

Definitely get the fiber that is supposed to be body soluble, but still wear the safety equipment.

[liambesaw]

Yeah even the body soluble stuff is irritating as hell

[fergusonjeff]

The stuff will degrade at high temperatures.  I was told it would work as a damper in my wood kiln.  When the kiln was around cone 4 it collapsed making for a challenging first firing.  It should tolerate temps on the outside of a kiln, but the question is for how long?  Will it be exposed to the weather? 

I did use the cement board between the cinder blocks and the soft brick and hard brick layers under the floor of the kiln.  No sign of problems there but it is pretty protected. 

[Mark C.]

(I did use the cement board between the cinder blocks and the soft brick and hard brick layers under the floor of the kiln)

I like expanded metal in this spot as its strong and stay cool as air passed thru use 1/4 to 3/8 expanded steel.

[Luitreth]

I think it's probably best if I just stick my hand in my pocket for some calcium silicate boards, more expensive but I think I have found some locally, it won't be exposed to the weather as will be indoors, and hopefully not exposed to high temperatures as it will be behind hti  brick and fibre blanket. Thank you for all your helpful responses.

[Bill Kielb]

Just an add, lots of high temperature rigid insulation available these days. If you are spending the money, maybe worth careful examination.

 

[Luitreth]

I'm also wondering whether I would be better to use ceramic fibre board in place of the blanket/calcium silicate, kill two birds with one stone, I guess it wouldn't take too much knocking about but I'm not planning to be kicking the kiln (not often anyway), this what I'm going to build.

[Bill Kielb]

In my opinion I would go for best performance and lower operational costs as long as it was in my budget so yes there are many high performance insulation boards that will reduce your energy consumption considerably.  To me it would be a matter of first cost verses operational cost.

At top temperatures heat loss is exponential.  Losing 10% heating capacity in an electric kiln due to element wear usually results in a kiln that struggles mightily just to get to cone 6.  Anything you do now to improve insulation  will have a significant effect on firing time and energy use.

That being said I would look at being creative as well knowing that air gaps are a free form of  thermal insulation. Blankets and boards basically  trap air.

Best of luck with your project!

[Luitreth]

22 minutes ago, Bill Kielb said:

That being said I would look at being creative as well knowing that air gaps are a free form of  thermal insulation. Blankets and boards basically  trap air.

Interesting thought! thank you. No idea how long this project will take, given the amount of agonising over how to build the box, I think once I start thinking about the burners my brain may explode!