Firehole
a modest foundry furnace, forge & gloryhole

This design is not terribly original, although it has variations that are mine. A number of ideas were gotten from Forge and Burner Designs

Simply said, what I am discussing is a cylinder of insulating castable refractory, supported by a sheet metal skin, heated by a blower driven propane burner, held by a rectangular steel tubing cage that supports the lid, etc. The burner is a simple design and in fact, I use the same burner with a different head to melt aluminum cans in a cast iron pan. Melting Metal

HOW IT DIFFERS FROM A GLORY HOLE - Besides it being smaller, this hole has no back up insulation - frax or board which increases the insulation value. It will heat up and cool down faster and use more fuel at any given temperature.

BUCKET FOUNDRY
This is a shot of the first unit built to be a small glory hole/foundry for melting brass and other metals, and a gas fired forge for blacksmithing. By the time this picture was taken it was clear that the metal "popcorn pail" type container was not a good choice for leaving out in the weather - or it should have been painted with high temp paint. Since this picture was taken, the rusted metal has been stripped off (see below and above) and the body wrapped in a painted piece of sheet metal (with a hole for the burner as shown here) held in place with large stainless steel hose clamps
The lid in place is the one used for foundry work  It is not clear from this picture or the one below that the lid has a steel strap tightened around it- by the bolt through the bent ends on the left side of the picture above the lift handle.  The castable was built up so the strap is protected from the worst heat.  Insulating castable without compression cracks rather easily, so the strap holds it together.
  The hole in the side is the entry for the burner.  Unlike most of my burners, this one does not have a flared end, but the castable was shaped to provide that. The entry is low for foundry work, below the edge of the crucible, which sits on a fire brick.
There is no back to the unit. When used for foundry, it sits on a block or tray of castable, as shown. When used for forging a block is held against the back and the unit is laid on its side, with the option of removing the back or using this door, so long work can be heated in the middle. When used as a glory hole, the back would be blocked. Inside diameter is about 6".

This unit was built by making an inside form of  a fairly stiff piece of sheet metal, about 22 gauge (0.030") and bending it to an overlapping cylinder about 3" smaller in diameter than the can. It was held in shape with fiberglass strapping tape and inserted in the can.. A fairly thick mixture of insulating castable was made up and troweled into the space between the can and the shell; the thickness preventing flow of castable below the shell. When set, the form edge was pulled inside to reduce its diameter and free the tape and then pulled out. The burner hole was cut, first in the sheet metal with a hole saw and then in the castable with an old paddle bit -THIS IS NOT A GOOD IDEA unless you have a use afterward for a really dull paddle bit. A better idea would have been to cut the shape of the burner port from Styrofoam or rolled paper, cut the hole in the can before pouring the castable, inserting the core and sealing it with tape, then removing the core, by burning or tugging, after the castable is set. 2001-09-29

Below, left to right, the framed hole standing on a larger frame holding other hot stuff.  In the first image the PVC to the right is where the blower for the burner attaches.  The lid is set to one side in one parking position on fire brick.  The center image shows the rain shield raised up and the pole that supports two hooks.  The lower hook catches the chain from one side of the lid like a hinge or pivot - the chain runs from the lid in the picture.  The right image shows the lid hung by the other chain to the upper hook, which I use so I don't need a layer of firebrick to lay it on. 2008-01-06

These 3 pictures show the hole with the lid in place and the large link chain resting on top.  The two chains are attached to the compression band around the lid, one welded and the other to the tightening bolt. The center image shows one of the crucibles in the open hole resting on firebrick to raise it near the top.  The heating flame comes in from the right rear behind the pot which is centered when in use. The corner and centering pieces can be seen at the bottom of the frame.   Tools on right described below image. 2008-01-06

Tools for handling the foundry pot. The pot must be lifted out straight up with a grip below the lip, the gripper having to fit between the pot and the wall. The loop is set on a fire resistant surface (sand in the casting pit) and the pot set inside the loop which is then lifted to pick up the pot for pouring.  The image shows one end in the main picture and the other in the insert. The loops at each end of 1/2" square tubing for different sized pots. The clamps for lifting the pot had to be quickly adjusted to squeeze between the walls of the hole and the pot. Note that they overlap when closed for a smaller opening. I rebuilt [2003-10-30] the lifting tongs, welding 1/2" square steel tubing to the straight sides for strength, still keeping the bottom end thin.   The original clamps were pretty weak when lifting several pounds of molten brass that might splash on my legs.

BUILDING A FIREHOLE STEP BY STEP

• Read through directions first.
• NEEDED: Insulating Castable, sheet metal, plastic film, hose clamps, scrap Styrofoam, steel flat stock, rod, trowel, mixing tools and containers, support board, sheet metal screws, ruler.
• Buy 24 gauge sheet metal. The cheapest way if you have other uses is a full sheet from a sheet metal (air conditioning) shop.
• Cut two sheets. The outer sleeve is 1/2" taller than the inner. My outer is 14" tall.  The ID is 8" and the OD 12"
• To determine the length, multiply the diameter by 3.5 or 4 (instead of 3.14159) so there is an overlap. [28-32" for the ID,   42-48" for the OD to match mine.]
• Buy 4 oversized hose clamps (or enough to go around the outside in two locations, they will interconnect.)
• Use the clamps to form the inside sleeve, then hold the inside sleeve with sheet metal screws installed from the inside.  Remove the clamps.
• Use the clamps to form the outside sleeve.  Do not use screws here to allow tightening later.
• Cut a piece of Styrofoam into a cylinder the size of the burner head/tube and trim one end diagonally so the port will be tangential (or nearly so) to the inside wall.  Cut a matching hole in the outside sleeve about halfway down, and insert the core.  Secure the core with glue or tape, etc. during assembly. Core does not penetrate inner sheet metal, just outer.
• On a flat board (plywood) mark an X for centering and draw circles (4 or more) from just inside the ID to just outside the OD. Lay clear plastic film over the board.
• Place the sleeves on the plastic, centering as needed.  You may wish to take a small amount of insulating castable and make spacers - about 2" cubes - to keep the sleeves evenly separated.
• Make a rather stiff mix of enough insulating castable to put about 2" in the bottom of the space between the two shells.  Drop it into the space, gently, working around the slot.  Check the spacing.  Give the mix a chance to set up (start to stiffen), keeping it damp, leave the top surface rough for bonding with next pour.
• Make more mix, rather thinner (normal) so it is pourable.   See castable working suggestions under meltmetl.htm  
• Carefully pour the mix down the sides, moving around so the weight does not get uneven and distort the shells. For extra insurance, you might lay a 10" board across the inner shell and put bricks or other weights on the board. Use a rod to settle the mix without stirring too much which will separate the ingredients.  Fill to inside rim..
• When the castable is set hard, unscrew the inside metal sheet and work it free.  Let the shell air dry for a couple of days.  Burn or carve out the foam core for the burner port.
• Meanwhile or at the same time, to make the lid bend a steel band into a loop with the ends bent back [or 2 half loops to match mine], drilled for a tightening bolt.  If you are making one like mine, opposite the bolt, weld a loop or insert an eye bolt for lifting.  The length of the piece(s) will depend on the diameter. See right image above of lid hanging from chain.
• Place the band on a surface covered with plastic, prop it up about 1/4" (6 mm) in three places and form a thin plastic sheet as a bag to be lining and edge shape - cutting it 2.5 times the diameter of the top. [30" for my 12" lid.]  In the center place a Styrofoam disk or greased tin can to make the vent/access hole. 
• Pour in a fairly stiff insulating castable mix, pulling on the thin plastic to shape it, and pull up the plastic into a bag shape and tie or twist tie it off so the castable is actually above the band (as well as below because you propped up the band below.  When it has set, cut away the extra thin plastic, remove the inner core and air dry (the other plastic will burn off although you can tear it free.) Tighten the band before using.
• Either repeat the process for a back door/plate or make a metal tray about 1" deep and pour IRC into it to make a base plate.  Cover with plastic to aid setting, then when set, remove the plastic to air dry.
• Assemble the unit, adding the frame, etc. if you wish, and put the burner in place and gently heat the thing with a candle flame (yellow, low volume, no blown air) until well heated (over 300F.)  Then add air and bring the burner up to volume for the first full heating.

It has occurred to me today, (2002-01-13) after reading Dudley's book again, that the fire hole could be a glass melter for a vertical access furnace! Duh. It could then later be a small glory hole when a furnace was built.
 

This is a mini version of the Firehole, cast from insulating castable around a shaped Styrofoam core with a piece of clay to form the burner port. While casting was done the sheet metal outer covering was held with a large hose clamp which was replaced with a piece of wire twisted to tighten it. It has a bottom/back wall.  A Hot Head torch is used to heat it.  So far it has been used to heat the ends of rods for forging and heat for annealing copper and brass being hammered for bowls, etc.  The rough clay crucible shown is about the size of a muffin. 2003-07-20.