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Question about forge lining.


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I have built a three burner forge from a 30 gallon drum. Three inches of ceramic insulation with rigidizer. I coated the insulation with a layer of cement and let dry two days with some heat added from a forced air kerosene heater. I did the same with a second coat of cement. I kept heat blowing in for several hours on and off. The walls felt hard and dry. I fired up the burners for a couple minutes and now the walls look like they grew warts. I’m sure from trapped moisture. I am planning to coat last with Plistix. My question is do I try to smooth off some of the “warts” and recoat  with cement, simply add another coat of cement, or just coat with the Plistix?

Thanks. 

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Well that’s a lot smoother than my forge interior or any others I have seen for that matter. Interesting how linear some of the bubbles came up. Assuming it is now dry, I would just go with it.

 

PS FWIW I would have stayed with a single coat of refractory as less of a heat sink.

Edited by Charles dP
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You can always give it a topcoat of a real refractory once that falls off.  That is furnace cement, and it'll last three or four firings before it starts flaking off.  Save the Plistix for after you reline it with one of the better forge lining refractories.  This can be Satanite, which is pretty good and the easiest to use, Mizzou, which is better, or Cast-o-lite 30, which is among the best.  

 

Looks like you have a brick floor as well, which, while durable, is a big heat sink.  The Cast-o-lite is much better, as it's also insulating.  

 

Don't get me wrong, brick forges are tough and long-lasting, but they take a lot longer to heat up and cool down.  Not that big a deal for ordinary forging, but if you want to do a lot of welding it forces you to wait on the brick to come up to heat.  In an industrial setting that's fine, since it'll be running for hours at a time and the extra durability is a bonus.  In the home shop where a forging session may only be three or four hours, the extra warm-up time and cooldown time may become an issue.  

 

Apologies for the pessimism, you may be perfectly happy with it.  I just have strong opinions on forge and burner design. B)  Welcome aboard, by the way!  

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I'd run the forge until the Simond's mostly falls off, then recoat with Cast-o-lite 30.  The Simond's will stick in a few places, and what you can't just pull right off will stay well enough.  I'd be delighted if I were wrong about how long it'll take, but I suspect three to six normal forging sessions will do it.  Especially if you let it sit in a humid location in between.  The furnace cement is designed to be in thin layers between bricks or pipe joints, and as such is not engineered to withstand expansion/contraction on its own.  In a humid environment it'll turn white and flake away if left alone after firing.  

 

I do use it to patch the gaps around my coal forge firepot, and it lasts about a year in that role.  I've also used it to patch cracked bricks on the stove, and it lasts a month or so there.  If you ever decide to try for hamon, it's a decent material to use as a clay mask.  Bubbles up a bit much, but it works.   

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I'm going to second what Alan said. I've made three gas forges (one solid brick and two with Kaowool type material) and I used to coat them with Satanite. But by the 4th or 5th firing at welding temps, the satanite was peeling away and cracking. On the second forge I had made, I used some borax flux and it ate right through the satanite into the wool - hence the reason for the third forge. When the satanite on my current forge was looking pretty bad I tried Cast-o-lite 30 over top of what was left. I've used it for a about 30-40 hours of HARD forge welding and it still looks almost new. I'm never going back to satanite...

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I fell into the trap of using that stuff myself, although not to that extent.  I've used it more for quick patches, and it never seems to hold up after a few goes with the forge running. It might even turn into a black glassy like goo.

 

I'm looking at the floor of your forge and thinking, you might run into a mess there. Every hard fire brick I've used ended up cracked, crumbled, and glassy. Doesn't look like your case there. If you have enough castable, what I've done was take out the hard brick floor, and just put in the wool and refractory. Then I cast a slim "soap dish" like removable base (the dish is about 1/4in thick).  I don't weld, but this little dish catches a lot of the gunk that seems to pile up in my forge.

 

If your thinking on welding a lot, then you do have to consider that parts of your forge are going to be replaced often. 

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