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Protective coat for Re-lined forge

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I recently pulled all the interior out of my gas forge and rellined the 5 gallon bucket I was using with a couple inches of kaowool. I used the same refractory cement i used the first time, but it started flaking and chipping off. It was not exposed to rain but it was to the high humidity. The refractory insulation I had bought looked like chipped up insulation mixed with a little cement.

I went to a ceramics store hoping to find some kaolin ( the woman running the shop had no idea what I was talking about) or at least 'porcelin' mix which i figured was laregly kaolin. She was out of it, so i bought some of the ;highest' fire ceramic clay. She had no idea of the componenets and when i went to the manufacturer's website, they did not disclose it either. They talked in terms of what cone you would put in the kiln to fire the stuff.

I added a fair bit of the ceramic clay to the refractory cement I had on hand to thickly coat the kawool. It has taken all week to to dry enough for me to risk firing up the forge. I used some pure clay to fill in any cracks/weak spot i could see as the interior dried.

Today i fired ithe forge for 3 minutes.I had some sputtering and flaking of 'plaster'. I remembered at that point I had not thoroughly cleaned the opening of my burner. After it had cooled, I turned the forge on end to let the flakes to fall out. Later I fired it for almost 8 minutes, and this time I did not see any flaking. I was impressed with how fast the interior turned to reddish orange.

I put a piece of steel inside to see how fast it would heat. It sdeemed to heat even faster than it did before. (Might be wishful thinking though). I will let the forge cool overnight an see if I need to paint on some more of the ceramic clay slip I made.

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I am going to guess that the high fire clay that you used was likely to be cone 10 which equates to 2381Deg F.

Ideally the coating should be rated for temperatures equal to of higher than your refractory insulation, and the maximum temperature to which you fire your forge.

Below is a formula for a cone 14, or 2500 Deg F, porcelain casting body which I believe would be a good candidate for forge coatings:

Ingredients parts by weight

Kaolin - 54.8
Silica - 26
Feldspar (potash) - 21
Sodium Silicate - 2

The sodium silicate acts as a liqufier to allow the use of less water to achieve flowability. Probably also acts as a flux.

If any of you here in IFI have experience with ceramic casting bodies or clay slip bodies please join in join in and share information on their possible application as forge coatings.

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I've been really surprised at how little ceramic shops know about clay. Kaolin clay is porcelain once fired, it has a large component of al si and is more resistant to fluxes than high silicate refractories. Adding sodium silicate not only increases the fluidity but sets up more quickly than simply drying. however the more silicates and sodium silicate (water glass) you add the higher the silicate % and the more vulnerable to flux it is.

Personally I like a high phosphate or phosphate bonded refractory for flux resistance though ITC-100 is darned resistant. ITC-100 is kaolin clay and zirconium flour. Zirconium is slightly less tough and more heat resistant than diamond, happily it's lots less expensive. <grin> Zirconium oxide ceramic has a working temperature in the 3,800-5,000C range, heck it doesn't fire till it hits mid 3,000C. I don't know why they don't make shuttle tiles from the stuff. Oh well, too late now. <sigh>

There are a lot of strategies for refractories and it's a popular topic here. I'm sure there's a thread here somewhere but I don't know where.

Frosty The Lucky.

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