BUILDING AN INEXPENSIVE SMALL SCALE FURNACE FOR MELTING "NONFERROUS" METALS

By Hank Kaminsky of "The Art Experience"
"Processes, methods, and apparatus presented herein have not been tested or verified by ArtMetal in any way. Anyone using any of this information is doing so at their own risk."

[Front view of furnace]
[Another view of furnace]


This furnace is easy to build and effective to rapidly melt up to 12 pounds of brass or 3 pounds of aluminum. By rapid I mean that the furnace can melt 12 pounds of brass in about an hour with the amount of air pumped in from the hair dryer pictured in the photo. Aluminum goes a little quicker. One time when the hair dryer was on call elsewhere I used a vacuum cleaner to blow in air. That furnace melted metal very fast but it would probably burn out fast too. Now I use a little squirrel cage blower I picked up at a supply house (Graingers) and it is reliable, not needed in the bathroom and is faster than the hair dryer.

The body of the furnace is made of soft insulating firebrick, bailing wire and ceramic fiber bat. The insulating firebricks and the ceramic fiber can be purchased at a ceramic supply house. I purchased mine at a refractory supply company in Tulsa, Oklahoma that also makes heat treating furnaces for industrial use. They seem to pile up odd pieces of the ceramic bat cut off in the furnace building process which they just gave me. Some places like that may sell their odd pieces at a cut rate. The stuff is immensely useful around the shop for patching a forge or an emergency heat seal, etc.

The only disadvantage I have come up with is that it tends to burn out over time with sloppy use. Metal dropped in the chamber will eat away at the brick and in time wear a hole in it. I stuff some more ceramic fiber bat in the holes until the laughter of my friends gets to be too much and I build a new furnace.

The burner is an assembly of a blower and some black iron plumbing fittings with no special orifices or complex welded parts. Its simplicity makes it easy to build but it does take some getting used to to operate. More about getting it working later. The gas used in the burner can be either natural or bottled gas. I have used both and the rig seems to works the same, no need to change a thing.

The crucible which is a number 4 refers to pounds of aluminum) was purchased at a foundry supply house which wouldn't sell less than a box of 6 (I think) so I bought the box and have sold most of my surplus to local metal melting enthusiasts and students in my sand-matrix design and sand casting classes.

The usual crucible handling equipment is some sort-of tongs I welded up out of 1/8" x1 1/2" strap steel. The tongs serve to pick the crucible out of the furnace and pour the molds. I have also used blacksmiths tongs such as the ones shown in the photograph which had a reach long enough to grab the crucible a few inches down inside. When the crucible is full (which means 3/4 full, never to the top) of metal it's hot and a little soft, (it is after all made of carbon). Stressing it outward will break it like a hot chocolate chip cookie just out of the oven. But if it is handled so that the stresses don't try to pull it open it will give long service.


Materials:

Furnace body

Burner

Tools needed:


Procedure:

Set up 5 bricks on their narrow ends in the form of a pentagon on a piece of paper on the table. (see photograph for orientation). Draw the outline of the inside of the bricks with a pencil on the paper. Remove the bricks and cut out the drawing.

Lay out 2 bricks side by side on their broad sides on the table. Lay the paper cutout from the inside of the five bricks down on the flat bricks centered and with a point of the pentagon on the line between the bricks. Transfer the outline to the bricks with pencil or scribe.

Using the old hand saw cut down into the flat bricks along the marks to a depth of 1 1/2" keeping the bottom of the saw cut parallel to the table. Cut straight and avoid twisting the blade which might snap off pieces of brick. When all 5 of these vertical cuts are made lay the saw horizontally and cut out the sections of the brick outside the cut lines and above the 1" level. (The bricks are 2 1/2" thick)

You will now have a two piece platform of brick with a 1" high ledge on the outside and a pentagonal raised portion in the center. The 5 bricks you laid out on the paper will sit nicely on the platform and snug up to the raised area. Using the iron binding wire make a loop big enough to encircle the 5 bricks and cinch them up to the raised area. Twist the wire enough to hold them in place.

The little triangular sections you cut out when you were cutting the platform now come in handy. They fit in the triangular areas at the top of the furnace between the 5 bricks. Use the saw to cut them to fit into each space and fashion another loop of iron wire to wrap around the top of the furnace. I wrap the wire loosely around the furnace and then fit each triangular piece in place before tightening the wire. A few of these flat sections of brick can also serve as a base block to sit the crucible on to raise it up about 1" above the furnace floor. This makes for a better combustion space.

The vertical spaces between bricks on the outside of the furnace is the place to slip in cut bats of the ceramic fiber. They usually stay tucked in place but a wrap of iron wire can hold them in place.

Two bricks laid flat side by side can be bound together around their edges with wire to form a lid. Once the firebrick box and lid is made a hole needs to be cut to accept the burner pipe. The hole is made with a 1 1/2" paddle bit at a level high enough to put the flame at the base of the crucible sitting on a base block. The pipe is pointed tangentially to the circumference of the crucible (and the furnace). Note the way the burner enters the furnace box in the photograph. The tip of the burner should be just inside the furnace when it is ready to operate.


Starting the furnace and keeping it lit.

This is going to be a matter of practice, but some general principles may help. The firebrick in this furnace being an insulator really helps to make the flame easy to start and control. Once a spot on the wall is glowing the flame will not go out. The gas is introduced into the mixing chamber through a valve at the tee. If the blower is in place even if it's not on the gas has to come out the open end inside the furnace and that's where you want it to burn after all. Once you turn the gas on avoid waving matches around the blower area if it's not on. The air comes from the blower which should be adjustable either with a vane on the blower input or a valve on its output. This adjustment is really only necessary if you have a bigger blower than you need or need to use the adjustment until you get familiar with the burner operation.

I have used two approaches to starting the burner. Turn on the air and allow a little flow to come out the end of the pipe. Turn on the gas and put a piece of lit newspaper at the top of the furnace. The burner will catch. Adjusting both the gas and the air flow will cause the flame to change. With the little blowers I use I adjust for the maximum amount of gas which can be consumed by the maximum amount of air available. My other method is to turn on both the air to maximum and add some gas and throw in a lit piece of paper. The furnace will go "wuump!" and be going full tilt from the start. Then fine tune the flame for the loudest sound of combustion. You can start the furnace with the crucible in place or out and put it in once the flame is going. It doesn't seem to matter.


Notes

Furnace:

There is a distinction between an oven, a furnace, and a kiln and forge but it is strictly a matter of common usage. My dictionary uses these words interchangeably with concessions to the high temperature associated with a furnace and the identification of the forge with metals. My own usage is to assign the oven to low temperatures and not much in the way of insulation for baking bread or sand cores, etc., the furnace develops high, rapidly rising temperatures and usually has sturdy, high quality insulation (although this one uses a high quality insulator it is fairly flimsy and not meant for long term or industrial use), kilns tend to steady or slow rising temperatures with good insulation for use in applications like ceramics firing or drying wood. Forges feature sturdy insulation and are specifically designed for the insertion of metal parts for raising their temperatures to forging heat. Return to link

Nonferrous Metals

Nonferrous metals are metals other than iron (which are called Ferrous because the Romans used the word fer to describe iron and we have a real attraction to the Romans). Nonferrous metals which can be melted in this furnace include the copper alloys, brass and bronze and all the other non iron metals like aluminum, tin, lead, etc.

Insulating Firebricks

Insulating firebrick are fireclay brick in standard sizes which are puffed full of air holes with the addition of a foaming agent when they are cast at the firebrick factory. They are sometimes known as soft brick. You can cut them easily with a saw or knife. They have insulating properties and come in grades according to their temperature range. I recommend the brick used in this furnace have at least a 2300 degree rating

The other type of firebrick you will encounter out there is the hard brick. It is refractory but has no insulating properties. Also you can't cut it except with a diamond bladed saw or the clever use of a chisel or pitching tool. I have used this type of brick to line furnaces. They last a lot longer but you must line them with some sort of insulating layer or the heat is transferred too rapidly to the air outside the furnace. They also take a lot longer to heat up and can be a bear to light a burner in them. But they are sturdy, last forever and can add to your stature as a real man (or a real woman). Return to link

Ceramic Fiber Bat

Ceramic fiber is a lot like glass wool or Fiberglas insulation but able to take high temperatures. It is a development of space shuttle technology. They use something like this stuff only in a much more dense form as the "tiles" on the downward surfaces of the shuttle to resist the frictional heat of re-entry. It has been a boon to the ceramics industry and is widely available. It comes in several grades and thicknesses. In this application it doesn't take much in the way of thickness to chink the cracks at the joints of the bricks. Return to link

Crucibles

Crucibles are pots made of metal resistant refractory material. That means they resist heat and can stand up to the outward pressure of molten metal inside. Crucibles also transfer the heat from the flame to the metal inside. The crucibles I'm familiar with are either made of clay or carbon. Carbon crucibles are the toughest and most common metal casting crucibles around. The carbon actually burns up but slowly and in doing so makes an excellent heat transfer material. Two types are available through foundry supply houses and some of the larger jewelry supply houses. Clay/graphite crucibles are the cheapest and serve for shorter amounts of time. They work just fine. Silicon carbide crucibles are made of silicon carbide bonded with pitch under steam pressure and seem to be quite tough and long serving. I prefer them because I'm hard on them and they just melt and melt for me. Some folks suggest "tempering" a crucible by heating it slowly to yellow heat with no metal in it and letting it cool in the furnace with the lid on. Sometimes I do that and sometime I forget. I can't tell if one way is any better than the other. Crucibles are numbered according to the number of pounds of aluminum they will hold at capacity (roughly 3/4 full). Brass is around 3 times the weight of aluminum so a number 4 crucible will hold 4 pounds of aluminum or 12 pounds of brass or bronze. Return to link

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Copyright 1994, 1995 ArtMetal

Author: Hank Kaminsky
HTML Editor: Roger Schmitt
ArtMetal Editor/Curator: Enrique Vega
Last Updated: Sun, Jan 21, 1996