Building the trench forge, and a dirt cheap clay recipe.

This may not be the fastest forge to build, but it was worth the time. These ingredients have been used on all my clayed forges. The secret is slowly curing and really rehydrating the kitty litter before hand:

A dirt cheap recipe for fire hardening clay in forges— Works great for projects big or small; I just winged it loosely off of some other recipes. This is an approximation. That was modified as I went along and needed more. I just went for something that “looked right”.. not that I knew what that was. I started off drier and got wetter to keep the large mass from cracking:

-1 20lb Bag kitty litter (the unscented bentonite clay kind) is barely covered with water in a wheel barrel.
-Add three full shovels of sifted red ground clay. (Sifting is really just a formality, but it should be done dry.. if it’s from a deep enough hole.. like a charcoal barrel pit then it’ll be well clumped and moist almost pure clay)

-Stir it all together and wait at least 24 hours. Occasionally stir to make yourself feel productive. =P This will soften the clay pellets as they absorb the water. Additional stirring will make sure all the mix stays wet. (The weather was mild, but cool at night when I did this so use less water in winter, and possibly more in summer)

– Once about half the standing water is soaked up or evaporated. Then add about equal parts wood ash and kitty litter to the mix until it’s to the desired consistency. For a single layer about an inch that’s something that clumps together in your hand without crumbling.
This mixture should to cure very slowly.

I would apply it in about one inch thick layers around firebricks letting them dry 24-48+ hours in between. I used a heater and halogen light at night because it was getting into the high low 60’s. I’d check on each layer to keep the outermost “skin” moist. The goal was to avoid lots of steam from the inside escaping and cracking a dry outside. After many layers were applied I waited maybe just over a week with 75-degree heat and air movement. Then I built a small wood fire in my new forge. I built it up to a nice sized fire taking up the whole forge over a few hours and then let it burn HOT for half an hour before letting it slowly cool.

This forge wasn’t a single afternoon project, but it’s served me for a long time now:

I used coal in the forge, but wasn’t really happy with it compared to charcoal. I may try it again with coked coal now that I have a nice supply.
The PVC goes to a steel pipe.
The end can be used or the lid can be opened.
I’ve actually loaded a lot of wood on top of charcoal. The heat with out a lot of O2 (with the lid closed) can allow for charcoal production while forging. But sap is something to consider with green wood.. And be CAREFUL when opening the lid us a fire rake or pair of tongs, the flames will flare up.
The entire leangth of the forge can be used or clay can also be used to block off air holes in the pipe.

A tip with all clayed forges used with coal is to leave a thin layer of ash in the forge, or even apply a layer mixed with very little water. This will help prevent clinker from bonding with the clay. Instead it bonds with the ash.

Hardwood charcoal or bamboo charcoal are what I prefer to use n the trench forge. But coal can be used in a pinch.
Some relative information on clay:
Clay when used in bladesmithing for heat treatment.
I went on a quest to find out the science of how clay hardens… Instead I found debate!
I’ve been introduced to some new vocabulary that is very interesting: Sintering.

How to light a coal fire.

I’ve tried a lot of methods to start a fire, my favoured is simply a sheet of news paper and charcoal. They produce a clean fire quickly. Everything should be self explanatory, except perhaps F: drier lint is useful, but smelly if you happen to have a hairy family =P: A wind and sun screen is useful when the fire is in it’s infancy.
Here you can see a close up of coal, coke, and charcoal respectively.
N: When using an electric blower controlling the air flow can be done in many ways. I use a shop vac directed to a T fitting. On the side of the fitting, opposite from the opening and going toward the forge, I’ve fitted a ball valve to allow excess air to be channelled to a side draft style hood/chimney.
O: Additional air can be vented from the ash dump as well by sliding it open a bit.
Before a new day’s hearth can be lit: sifting the ashes must be done, removing occasional bits of clinker. I toss the coke into one bucket and more questionable sifted shovels go into a bucket of water. The coke floats, and the ash clinker and coal sinks.
First a ring of coke is made at the bottom of the firepot. Some charcoal is present.
A little more charcoal is added. Charcoal or dried wood tender will ignite much easier than coke. If wood is used do so sparingly. I don’t know that you can’t weld in a fire with burning wood, but I do know it takes up hearth space and doesn’t burn as hot as the coked coal will.
A single sheet of newspaper is lit and placed in the center of the hearth.
Many people will use lots of paper or ball up coke/coal fines into the paper. I’ve tried many methods; this is what works for me. Although a separate wood fire is always nice to steal burning coals from.
Some larger chunks of charcoal are quickly added while the air supply is on very low.
I would use less kindling, just enough to catch fire before the paper burns out.
The kindling or charcoal only need to burn long enough for the coke to catch.
The key at this point is not suffocating the fire. The blower is still on but you may find too much air causes smouldering kindling that won’t catch. Once again charcoal or even charred wood is much more forgiving.
This bamboo charcoal burns hot but quickly. I use it to supplement my coal more than primary forging, so I added more than necessary.
Some coked coal is added.
When you are sure the coke has caught pile more on. The blower is blowing lightly. If you used kindling you can even carefully pull out the burning wood and seal it in an airtight container, or extinguish it some other way. This will make for an easier fire next time.
Remember that floating coke, now is a perfect time to layer it on. This will buy the coal a little time to cook. You may increase the air some, but you don’t need a lot of air yet. Just enough to keep things going.
Pile your wet coal around the center of the hearth/firepot. I snapped this picture right after the wet coke lit, just before cutting the blower off.
A handful of mostly uncoked coal (from the bottom of the water bucket) was then placed on top.
Then a small scoop of wet coal was added. (This picture didn’t turn out well and I failed to notice at the time.)
The coal has heated up around the hearth and begun to melt and stick together as it’s coking. You can make a small entrance with your rake. Within this cave you’ll be able to keep an eye on your steel and see the color of it while still surrounding it with heat from almost every direction. The fire pot is 3 inches deep and the mound is about 5-6 inches above the forge table.
Note – the fire is about at welding temperature.
This is the fire after welding. You can see it’s beginning to burn hollow– there isn’t enough burning coke inside the cave. While enough fuel is present inside this cave to work, use less air and water to allow coking to catch up.
A hollow fire is a result of not enough coal coking because you’re keeping it too wet, or too much air being introduced. Too much air and not enough burning coke will cool your fire and create scale.
I use a ladle that’s easily held with my tongs to avoid steam. I can’t tell you when exactly you need to sprinkle the fire and surrounding coal with water, but the idea is to keep the fire from migrating out of the firepot (generally whenever you see large sooty flames). If the perimeter of the firepot becomes well coked it may need to be watered until it’s time to be slid inwards. Occasionally, after the blower is stopped, the coking coal atop the fire will ignite and go out as soon as the blower is cut on again. That is often a good sign to add more coal. I believe it also means the burning coke just below the top of the mound is consuming excess atmosphere from around the fire.
This post is about building and maintaining a coal fire. Forging completely with coke is an entirely different animal.
Heat cokes coal. Coal will burn, and it’ll stay aflame simply from atmospheric air. Coke needs more air introduced or blown towards it to stay lit. I know of no reason to burn coal other than to create a supply of coke and help maintain the shape of certain types of fire.
It’s time to sprinkle some water on the fire.
Time to add more fuel.
If heat becomes a problem then clinker build up could be the source. Clinker can be prevented from blocking the air orifice by firepot design, installed clinker breakers or simply by pulling it all out in one large sticky clump with your fire rake.
To the left you can see clinker and flux from lots of welding. The middle and right are two different types of clinker from seperate coal sources.

I’ve found this sort of fire useful to heat several inches of steel. In this particular fire I forge welded a small hook out of 6 inches of half-inch square. Then finished working out the edge of a knife with a seven-inch blade.
I use many different types of fires and I am by no means an expert. Working odd bends and unusual shape or sizes without destroying a coal fire is difficult for me. It’s easier when extra coked coal is on hand, but coke still has to be raked back or added. Time must be spent waiting for the fire to get back up to temperature once burning coke is displaced. Proper fire management is one of the most basic skills anyone wanting to learn blacksmithing should know. I’ve had to teach myself most of these skills but I’ve by no means done it without the advice of others. I’ve not seen a good step-by-step pictorial on the subject so hopefully some may find this helpful. I plan to post a similar post over at It has been asked about an awful lot lately and I know many people like me are unable to attend classes. Around here, even at the events I’ve been able to attend, gas forges have been prominent or coal fires were started early and maintained irregularly and without discussion.

Side-blast solid fuel forge

This is my side-blast solid fuel forge.
shopvac, or any other air source.
(optional) 3/4″ PVC, ball valve and preferably on a T-fitting.
1 inch steel pipe
tin sheet
clay slurry (clay, water and woodash)
Fired clay bricks. (Not cement blocks)