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Blacksmithing and metalworking questions answered.

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Claying Forges :

Coal Forge Linings

Many old forges had "clay before using" stamped or cast into the bottom. The reason for this varies and there are no manufacturer's instructions they explain exacxtly what they wanted. Claying doe not apply to heavy cast firepots, just the surrounding forge pan.

Reasons to line a metal forge with refractory material or clay.
  1. The manufacturer said so to make the forge more durable and avoid warranty problems.
  2. To create a better shape "pot" in a flat bottomed forge.
  3. To protect thin sheet metal pans from burn out.
  4. To protect thin cast iron pans from thermal shock.
Reasons NOT to line a metal forge.
  1. May serve no purpose.
  2. Makes the forge heavy (portable becomes NON-portable).
  3. Can accelerate rust due to trapped moisture in damp environments.
  4. May make the forge more difficult to use.
The first reason to line a forge may be valid if the forge is used very heavily but it had been demonstrated over and over that unless you build a HUGE fire that forges generaly do not need refractory linings. The manufacturer avoiding any possible warranty problem is a greater possibility.

The second reason, to make a better "pot" may be valid and is the ONLY situation where I have seen a manufacturer's diagram. This was in a flat bottomed sheet metal forge with a flat or shallow fire pot or tuyeer. The clay was shown as a ring around the joint between the sheet metal and the tuyeer. It formed a raised enclosure of maybe two inches with sloping sides to create a "firepot". This would help increase the controlability of the forge by keeping the fire concentrated in the middle of the forge. This makes a hotter fire and saves fuel.

The reasons not to line a forge are self explanitory. Many forges are used without lining and work well. Forges stored outside will rust rapidly due to the acidic coal ash unless cleaned very clean after every use.

The Clay

The clay used can be fireclay or common clay. It can be a commercial product or dug from a clay bed or even your back yard depending on where you live. The bottoms of small metal forges do not get so hot that they needs refractory clays. Common red clay like they make bricks from will work fine.

IF using plain clay it needs to be worked up like modeling clay. Mix as stiff as possible and then work on a plaster slab "bat" or "vat" to absorb moisture as it is made smooth and pliable. Its the extra moisture that causes the cracking. Some cracks are inevitable.

Cement is not necessary for claying a forge. When it is used add about 10% portland cement. If you are going to use a sand cement mix you should consider a castable refractory cement. It is overkill but it is very durable in this aplication.

Recipes and Methods

The mix I used (based on my grandpa's recipe) was
  • 3 parts mortar
  • 3 parts clean sand
  • 1 part fireclay
  • 1 part dry lime
I mixed it, poured in the hearth to a depth that was flush to the top of the firepot and raked it smooth, then sprayed only enough water on it until the surface was wet. I let sit two days before making a fire and it had hardened nicely with no visible cracks anywhere. I think the minimal amount of water helps eliminate the cracking.

- HWooldridge

REFRACTORY Here's the do-it-your-self castable refractory/mix from Jim Lindsay:
  • 4 parts premixed concrete (includes the cement and is very lean)
  • 4 parts fire clay (available at masonary suppliers)
  • 1 part vermiculite (available garden suppliers)
Mix dry first then mix with as little water as possible making a "stiff" almost dry mix. Compact into place. Be sure to let dry (as long as possible, several days - week) and bring up the heat slowly on first use.

The vermiculite is a good insulator, withstands high temperatures, adds porosity, and doesn't absorb a lot of water so the mix shrinks less than it would otherwise. Do not add to refractory cement as it weakens the already lean weak cement.

-- guru Friday, 10/15/99 05:10:00 GMT
Coal Forge linings, revisited

By clay, they mean clay of any type but preferably a clay with a placticity like good potters or artists clay. These work up stiff and harden without cracking severly. A refractory clay or mixture is slightly better but you do not need refractory temperature material for claying a forge. Clay suitable for making bricks is also suitable. It does it need to be a refractory cement or cement bonded clay.

The only manufacturer's diagram I have seen of claying a forge showed a ring at the joint between the flat bottomed forge and the flush fire grate. The ring created a "duck's nest" or shallow fire pot. This would help control the fire and partialy subtitute for a heavy cast iron fire pot such as the better forges came with.

Other shallow cast pans that had "Clay Before Using" cast into them probably wanted the clay as an insulating layer to reduce thermal shock to the cast iron. A uniform layer of clay about 1/2" - 3/4" (13 to 19 mm) thick would suffice.

Many folks use a mixture of clay and refractory cement or castable refractory cement, OR clay and portland cement to line their forges. This must be done carefully and with thought. Adding a layer of refractory to a forge pan makes it VERY heavy and what was semi portable is no longer portable. If the refractory is too deep the reserve volume for fuel is lost. In forges without firepots a deep layer of refractory can help form a firpot but only if the shape is correct. As noted above it could be a simple ring around the fire grate and a thin layer beyond. Heavily cementing a forge makes maintenance very difficult. Tuyeer and grate bolts rust rapidly enough and ocassionaly need attention. If burried under a thick layer of cement this may be difficult or require removal of the cement. Less permanent clay may have advantages in this case.

Castable refractory is a lean mixture of refractory agregates (usualy synthetic mullite), refractory clay and a high temperature cement. The cement only holds things together long enough for the dried mix to be fired and become calcined and partialy vitrified. Castable refractories are never as strong as fired refractories (brick). Unless castable is cured or fired through at high temperature the result is a very weak refractory.

Mixes of clay with a small portion of portland cement are probably better for claying forges than castable refractory due to the strength issue (as well as cost). Coal forge pans rarely get to the temperature needed to cure refractory cement and then only on spots near the tuyeer.

When claying OR cementing a forge the refractory/clay mix should be made as dry (stiff) as possible to prevent excessive shrinkage. Refractory clays are those with alumina and are generaly tan or white. The higher the percentage of alumina the higher the temperature rating but also the higher the firing temperature. When cements are used high alumina clays are overkill because the cement will not withstand as high a temperature as the refractory.

Pressed or welded steel pans do not need to be clayed and are best used as-is. Heavy cast iron pans with heavier fire pots also do not need to be clayed. The bed of fuel provides sufficient insulation in most cases. Even thin walled cast iron forges hold up well without claying if they are used for light work and a sufficient fuel bed insulates the forge.

Note that many old HD forges had the firepot set very low in a forge pan 6 to 8" (150 to 200mm) deep for very deep fires intended for heavy work. It is often beneficial to raise the fire pot a brick's depth (about 2-1/2" or 64mm) by using bricks to line the forge pan. This leaves a sufficient coal reserve and raises the hot spot to very near the bottom of the side cut out in the forge. This allows heating long slender bars to welding heat in a fairly efficient fire. Some modern forge makers just mount the fire pot in a flat surface and use the fuel bed to adjust the work height. The down side of this is reduced coal reserve.

Note that forges stored outdoors will accumulate corrosive water that has picked up acid by passing through the coal ash OR ?alkali from wood or charcoal ash. This moisture trapped under the clay will cause severe corrosion. Forges kept outdoors should not be clayed or bricked and shouold have all the fuel and ashes cleaned out between uses.

- guru - Friday, 09/16/05 19:22:08 EDT


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