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My forge keeps melting. Anyone know a good liner?


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So, I currently run a charcoal side blast forge. I basically filled it with clay and then the actual firebox has a cob lining made of refined earth clay, grass clippings, grob and a small amount of lime to act as a plasticizer (less water required to make the mixture wet). I get very good heat from it, some would say a little too good, So much so that the clay actually melts. like not little particles and clinkers but actual viscous, honey like liquid slag at the bottom of my forge. 

This doesn't overly affect the forging too much, as the tuyere is above the slag, but I don't like having to rebuild it every 2-3 forging sessions. Does anyone know a good liner or recipe, whether it be store bought or made (like my cob mixture) that could work better and melt less? Bonus points if it is refractory and doesn't draw heat away from the work piece. 

 

Thanks in advance

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Ash makes a good insulation to protect the bottom of the forge side blast and bottom blast.  Raise the air pipe (twyere) up to be above the ash. 

You may want to adjust the air in order to get the amount of heat that the project at hand requires.

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I've never had that problem using just clay or sand and clay in about a 3 to 1 ratio. It vitrifies like glass but has never become runny. What are you using for an air supply? It could be that you're giving the fire too much air. 

Pnut

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I also use a mattress pump. I just put a ball valve between the pump and pipe and it works pretty well. I wonder if it's something peculiar to the clay in your area. I've melted red clay bricks before but never the clay I've dug locally. 

I'm puzzled. Keep us updated. I'm sure some other folks such as Charles might have some thoughts on the problem. 

Pnut

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Separate the air supply from the air line and leave an air gap between the two.  Air more directly for more air, less directly for less air.  Spilling the right amount of air takes a little practice but you can catch on quickly.

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Clays, as natural materials, come in a range of properties; I've boiled terra cotta clay in a forge but had good luck with clays that have a higher temp range like kaolin clays.  Can you try another clay source?  (Check to see if cheap clay kitty litter is a good clay where you are at.)

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Yes, I can try other clays. The clay I dig up is an orange colour, but there is a clay source near me that is a white/grey colour. Would that be any different? Also I have used bentonite (cat litter clay) before, however it shrinks a lot when drying, though that could probably be fixed with a plasticizer like lime or ash. 

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Only make it damp enough to pack hard with a mallet or board. Do NOT make mud, you're not throwing pottery. 2-3 pts sand to 1pt clay works well. The sand provides voids and paths for moisture and steam to escape the first aids drying and prevents shrink checking, the second prevents spalling. (steam explosions flinging chips) Sand also allows the liner to move internally with thermal cycling and helps prevent heat checking.

The color of the clay doesn't tell us much, you have to try it and see. Make test coupons, dry and fire it to see what it'll take. Bentonite is a very high fire clay and routinely used as a plasticizer try it at 4 or more pts. sand to 1pt. bentonite. Remember just enough moisture it will pack hard. Bentonite needs to temper. After you mix it leave it sealed in a bucket or something air tight for a day or more so the moisture can distribute through the whole container.

Don't forget to test it, the sand may turn into clinker. If you have different kinds of sand look for some derived from slate or crush fire brick into grog. firebrick grog works a treat.

Frosty The Lucky.

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  • 3 weeks later...

Frosty,

I've seen your instructions before on the right consistency for forge clay. I've had the clay only just barely damp, not enough to leave moisture in my palm after a good hard squeeze, but as soon as I try to pack it into the container, it doesn't stick, whereas it sticks readily to my mallet, making it impossible to pack properly. Tried wooden and rubber mallets, and my forge hammer. doesn't stick to my fists but that doesn't stop it coming off the side of the bowl. For reference I'm lining a large ceramic plantpot. Is there anything I can do to make it adhere better without using more water? My current "mix" is simply a very sandy clay that I dug out of a cliffside locally, dried and ground to a fine floury powder.

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How thick are the layers you're ramming? Some clay is stickier than others, if it sticks to the mallet try adding a LITTLE dry sand/clay mix, stir it and try again later. Or you could just forget about it sticking and keep hammering. It doesn't lift the whole thickness when it sticks does it? Maybe start by pressing it in by hand until you can't compress it further then tamping it in with a mallet.

If it won't stay in the bowl it might be the bowl, I've always rammed clayey sand into a forge pan wooden box, etc. 

Frosty The Lucky.

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A half inch thick, I intend to add another layer if this works. As it is, it lifts the entire thickness of the clay on the hammer. Just smashing it in with my fists works well, but the dryer the clay the harder it is to compact, and I've already got plenty of microfractures in both hands so I'd rather use a hammer. I think I'll have to try just adding more sand to the mix. Do you happen to know if the salt from beach sand might affect the quality of the mix?

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1/2" thick is mighty thin, I typically start with around 2" and compact it to maybe 1" +/-. Perhaps the bowl is flexing and breaking the clay up as you ram it in? 

You really don't want the clay dry enough to be hard to ram in but suggesting ways to run field density tests to check compaction would be a bit much.

You certainly don't want to be damaging your :o hands beating on it! How about dipping the mallet face in dry sandy clay mix when the mix you're ramming starts to stick? Scrape the mix stuck to the rammer off in the forge, rub, maybe twist the hammer face in the dry mix for a release agent. 

I don't think salt will be a detriment, it's a common binding agent and  pottery glaze, it should melt and stick the liner together. Unless there's an excessive amount of course. I suppose too much could cause the liner to "melt". Look like it anyway, get soft, gooey and stick to everything.

Silica and has a relatively low melting temp so it could certainly be a problem making the liner melt. One of our club members is a caster specializing in bronze. The sand he uses comes from a local river and is composed of slate particles and has a much higher melting temp than silica sand. I can't recall the mineral type, Pegmatite maybe, I don't remember. I know you can buy molding sand for casting high temp metals. 

The easiest thing might be to crush up a fire brick into grog and forget the local sand.

Frosty The Lucky.

 

Frosty The Lucky.

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That little tip about the river sand is something I'd never thought of. We've got a big local pottery scene, relative to our small town, so I'm sure I can find some old boys who know the composition of the beach sand and other local sand sources. As it is I've just yesterday derived a new method of grinding up my dried clay, by means of a cheap food blender instead of a paint mixer in a reinforced plantpot, so I doubt I shall be quite so limited by my clay supply now. Sounds like I've got more experiments to run. Thanks your advice.

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You can crush up broken pottery for grog too. Ask around for some broken cone 10 pottery, it'll take a LOT more heat than silica sand and it crushes pretty easily.

There comes a point where I just do a work around, in your case I'd probably buy a few heavy fire bricks and put something together to heat steel in. 

I'm not saying you should but it's an option and it'll let you work at the anvil while you experiment with different sand mixtures. 

Frosty The Lucky.

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What I did to keep the clay from sticking to the hammer or wood ram was to lay a plastic grocery bag on top of the mix. It worked pretty good for me. Just have a couple ready, they don’t last very long...

David

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8 hours ago, Frosty said:

comes a point where I just do a work around, in your case I'd probably buy a few heavy fire bricks and put something together to heat steel in. 

Like Frosty said there's always this option. I've used it and like it.

 

Pnut

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How much Lime are you adding? I was under the impression that Calcium Hydroxide melted at about 600 degC (Wikipedia tells me 580 degC, 1076 degF).

Having experimented unsuccessfully with Waterglass (Sodium Silicate) as a rigidizer for blanket and as a binder for Zircopax, I have experienced the instant conversion from rigidizer to lubricant at around the melting point of the Waterglass ("about" 1100 degC, 2000 degF). The blanket just moves away from where the flame hits it and the Zircopax/Waterglass becomes a dribbly mess.

It may well be that you are seeing the same effect from the lime?

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I have melted hard fire brick and burnt the bottom out of the MK III with an electric bed pump. My working forge uses a “T” and a valve the air enters at 90 deg, and vents out the open valve. 
 

try substituting wood ash for the lime, and jar test your clay soil. My original experiments were with Oklahoma red clay strate from the bottom of post holes I was digging. A good Adobe mix of 1 pat clay to two parts sand works well, the addition of the ash helps with the tendency for slag to stick to the clay. 
 

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