A Metal Melting Furnace

MELTING

A Metal Melting Furnace

by Klaas van Ditzhuyzen


"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."


Introduction

NOTE: Units used are metric and amounts in US dollars, unless specified otherwise.

For hobbyists it is possible to build a gas fired metal melting furnace for small amounts (50..300 grams) with the following features:

  • Capacity till 200 grams of cuprous or silver / gold alloys.
  • Max. temperature (inside the crucible): 1300 deg C (== 2500 F) within 15 mins (melting cast iron); 1100 C (== 2000 F) (melting copper) within 5 minutes. The space between the crucible and the inner walls is about 100 C hotter.
  • NO pressurized air, hydrogen peroxide or oxygen required: runs on ordinary natural drag of air
  • Cheap refractory, especially when the air-spaced gypsum blocks are used.
  • Runs on propane gas, usually sold in ordinary propane canisters available at many hardware shops.
  • Clean burn and high (compared to other models) efficiency, so low impact on environment.

History

My first coal fired furnace

I started long ago in 1973. I used the bottom third of a 200 liter oil drum, filled it with sand and left a hole in the sand of about 20 cm wide and deep, put some ordinary (nonrefractory) bricks around it and that was the furnace. I used anthracite as fuel with a reversed vacuum cleaner as fan. To light the coal I burned some wood first. It reached temperatures until 1600 C (2900 F) and I used stainless steel 'crucibles' for melting copper ... which leaked after two times of use, because the alloy components of the stainless steel dissolved in the liquid copper. And I started with home made foundry sand which I later replaced by 'real' foundry sand for better results. Later on I also got a 'real' 500 cc graphite crucible. The melting capacity was about three kilos (6 1/2 lbs). The furnace wall bricks were completely cracked and 'slaggified' on the inside.

I suspended this hobby for 20 years and I resumed it in 1992 with a large furnace from refractory concrete which is able to heat 500 cc crucibles for melting 3 kilos of copper in 45 mins with a roofmaker's propane burner. The furnace was just an oil drum of about 30 cm (12") diameter and height and I lined this with a layer of 7 cm refractory concrete. From the top of the drum I cut about 7 cm, filled it with that same concrete (leaving an exhaust hole just off the center) and that was the lid. Now I call this furnace the 'roar furnace' because it roars so loudly that once on a summer day when I was melting bronze a neighbor thought that a jet airplane was flying low. I use it very seldom, only when I make large castings. A year later I also made smaller castings and started silver- and goldsmithing. I bought high resolution oil-based sand (purchased at a silversmithing supply store) for small (< 100 grams) bronze and sterling silver castings and home made sand for coarse (cast iron) and larger castings (bronze). The small furnace can be used in the garage too because I have a hood with a vent. The description below is about the small furnace. In October 1995 I discovered that one of the cheapest buiding materials (air - spaced gypsum) turned out to be one of the best and most insulating refractories. I could make the furnace so hot that it is able to melt cast iron and shorten the melting times of copper, silver and gold alloys further.

Necessary Considerations

These items are required:

1. Safety
A list of required items for your safety (description below).
  • Protection goggles or face shield.
  • Dark welding goggles when temps above 1300 C are used.
  • Oven gloves.
  • Large flat iron pan to put the mold on.
  • Running cold water or if not available, 10 liter bucket (preferably not plastic) full of cold water.
Here are some safety recommendations for melting and casting:
  • Check all the gas couplings by 'painting' them with water with a little detergent in it after installing the furnace or replacing the gas canister. No bubbles of leaking gas should appear. Propane (and natural gas) is explosive when mixed with air! After use always close the main cock on the gas canister for safety. When using propane and working in the basement, please note that propane is heavier than air!
  • Do not put any flammable materials (paper, wood, plastic, etc.) within a meter (3') above the furnace. They might catch fire because of the hot gases. Also keep flammable liquids (gasoline, kerosene, etc.) away from the furnace. A hood with a chimney above the furnace is the best. Work only in a well-ventilated room, as propane consumes twenty five (25) fold its volume of air to burn. For methane this figure is 10.
  • Avoid use of asbestos. The fibers are very carcinogenic. Rather use firebrick, chamotte or other refractory to put hot objects on it. For heat-resistant gaskets (if you need them) other materials are available.
  • Put the mold on an iron pan (dry !) to prevent spilled metal running over the workbench (or even dripping off it onto your feet !). Filling it with sand is safer, but spilled metal (especially when it's silver or gold) can harder be reclaimed.
  • Always wear an eye, or better a face, protection mask during pouring the metal or adding metal into the crucible. This is to prevent unexpected splashing metal drops into the eyes.
  • When using metal molds, preheat them to above 100 deg C to remove any moisture. The smallest amount of moisture in contact with liquid metals will cause splattering away the metal through the workshop! Moreover, when working with gold, it will be expensive too!
  • When using sand molds, the sand should not be too wet.
  • When adding metal to the crucible in which is already liquid metal, the new metal should be completely free of any moisture!
  • When using zinc-containing alloys (e.g. brass), be careful of the zinc-oxide dust which comes from the crucible. Zinc burns very easily with a bright green flame. Do not add zinc to liquid copper or brass, as the boiling point of zinc is lower (906 deg C) than most copper, silver and gold alloys in their liquid state. The effect will be the same as putting ice to liquids above 100 deg. C.
  • Use dark goggles when looking to objects which are above 1300 deg C to prevent damage to your eyes by excessive infrared rays.
  • Put hot objects to be cooled (e.g. metal molds just cast or used crucibles just removed from the furnace) separate from tools to prevent touching them by bare hands. Otherwise painful burns will be the result.
  • When burns happen ,immediately immerse the hurt body part in cold water and keep it immersed for minutes. When a cold water tap is available, use the tap instead.
  • When working with larger amounts ( > 500 g) wear shoes which are easy to kick off (no laces) in case droplets of metal fall in your shoe(s).

2. Environment Consciousness
As we live in an era of greenhouse gases, waste dumps, exhausting of resources and endangered species, some environmental care in casting and silversmithing is a must and in books, FAQs, etc. little attention is still given to this hobby or business. For the average hobbyist, some recommendations to work as energy-efficient and environmentally conscious as possible:
  • Use as much from the exhaust heat as possible (e.g. preheating metal molds).
  • Combine as much batches of metal melting as possible. Reheating a cold furnace cost lots more energy than putting a still hot crucible in a hot furnace with only new (cold) metal in it.
  • Bring all fluxes, slag and dross skimmed off the metal bath to a local hazardous waste collection point and do not dump them into the trash can or the toilet. Metal oxides are hazardous waste and should be treated to recycle the metals from it.
  • Avoid use of cadmium, lead or mercury in your alloys. These metals are heavily toxic and can also be a threat for your own health. For making hard solder for silver or gold with a somewhat lower melting point, the cadmium can easily be replaced by zinc. Notice the precautions for zinc too.
  • Pickle (which is usually a solution of about 10 % H2SO4 sulfuric acid in water is used for cleaning silver and gold items, especially after soldering) can be used a long time before dumping (not in the sink or toilet but at a chemical waste collection point !), until it is blue. When only light blue it can still be used for cleaning. When it is getting dirty with solid particles, just filter it with an old coffee filter.

Estimation of Temperatures
Temperature scale



A little practice lets one estimate the temperature. For guessing temperatures, this table applies:
480 C / 900 F
Barely red in the dark. About the threshold temperature that a body emits visible light.
600 C / 1100 F
Dark red
800 C / 1470 F
Cherry red
950 C / 1760 F
Orange. Barely visible in bright sunlight.
1100 C / 1760 F
Orange yellow (light yellow in the dark). Even visible in bright sunlight.
1300 C / 2370 F
Light yellow, nearly blinding. Above this temperature dark (welding) goggles are required.
1500 C / 2730 F
Nearly white, blinding.
The values are the same for all materials. When one has (access to) a thermocouple, the best one is a platinum/platinum-rhodium (Pt + 10% Rh) couple, which allows measuring temperatures to 1600 deg C. But, don't let it contact with liquid metals as the expensive platinum wire will dissolve ! Optical pyrometers (with which the user compares the glow of a wire with the background glow) are very good, but hard to obtain.
Refractory
A heat-resistant brick, usually based on high-melting point ceramics such as silica (SiO2, mp 1710 C) or alumina (Al2O3, mp 2050 C). Some high-grade refractories are magnesia based (MgO, mp 2800 C). The most common refractories are:
Chamotte
Most commonly used and can be used until 1300 C. It can be obtained at a building supply store which sells stoves or open hearths. But chamotte is a rather heat-soaking product. It does not insulate very well. This can easily be checked out by melting a piece of (red) copper on a chamotte brick with a blowtorch. When it melts in the flame and then contacts with the chamotte, it freezes again. Another disadvantage is that after a few times of usage it probably cracks.
Refractory cement
Usually used for masonry in chamotte based furnaces. Available at a building supply store which sells stoves or open hearths. Small furnaces can be built entirely from cement 'armored' by crunched chamotte bricks. I made such a furnace and it resists well temps until 1400 C and lasts long.
Refractory concrete
A type of concrete based on alumina which resists well to 1650 C. It looks virtually white rather than gray. Better than chamotte, but harder to obtain (professional builder shops). It should be handled like ordinary concrete.
Gypsum blocks
Not an 'official' refractory, but just discovered by luck. These are gypsum buiding bricks described below.
Ordinary cement or concrete should not be used in furnaces at all, as these are subject to crack and crumble when heated. When using refractory cement or concrete, the furnace should be set aside for a few days (read the manual provided with the cement and follow the first heating instructions ) before 'first fire' is set.
Torch or Burner
The best fuel is propane gas (C3H8), as a natural gas household connection usually has a low pressure (less than 50 mbar) and the propane provides higher flame temperatures. Butane (C4H10) is discouraged too, because its boiling point is 0 deg C so its pressure is not so high at room temperature. The torch I use is suitable to 1.5 bar pressure and consumes 140 g / hour which is equivalent to 2 kW heat capacity. The usual propane canisters sold in the hardware store are suitable for such torches, It only requires a controllable reducing valve (usually 0.5 ... 4 bar). Such a torch costs not more than $40 and the valve about the same. It may be better to replace the nozzle by a shorter one (I did it from steel pipe) because the temperature gets higher. In this case, the torch only keeps burning while in a furnace, because the combustion speed is higher. The torch I describe here is (in a furnace) able to melt 300 grams of copper in 15 minutes (t = 1083 deg C), starting from a cold furnace. So it is sufficient for most hobby needs. When larger amounts are required, a bitumen melting torch (used by roofmakers) and a larger furnace are an option, which allow to melt 3 kilos of copper in 40 minutes in a 500 cc crucible.
Crucibles
This depends on the type of metal or alloy.
Copper, silver and gold alloys
The best crucibles are the graphite ones, usually made of a composition of graphite with refractory. They can be heated quickly and last long and are available from a silversmithing supply store from about $15 onwards and can withstand temperatures to 1500 deg C. They are available in several sizes, from 5cc (about 50 g silver when full) to 1 liter onwards. When one handles more alloys, there should be a separate crucible for each alloy. So I have a copper crucible (for bronze), a silver and a gold crucible. These crucibles are excellent for copper and precious metal alloys.
Cast iron, nickel, platinum class metals and white gold
For cast iron, graphite ones should not be used, as the carbon dissolves in the metal which results in a shorter life of the crucible and a too high carbon content of the cast iron which makes it a thick slurry unable to pour out. A better choice for may be the Hessian crucibles which are made from Hessian clay (Hessen is a state in Germany where Frankfurt is). They are cheaper, but less durable. Other alternatives are alundum (Al2O3) crucibles but they are not suitable for heating in an open flame as alundum is very susceptible to quick temperature changes. For platinum metal (alloy)s including white gold, the same applies, as palladium, platinum and nickel also extract carbon from a graphite crucible.
Low-melting point alloys
Low-melting alloys (aluminum or even tin and lead) can be easily molten in an iron pan or a stainless steel soup dipper. A furnace is not needed at all: the pan can just be hold in a flame. The furnace described here is for metals in the 700 - 1300 C melting point range.

Other tools
Crucible tongs
For taking the crucible out of the furnace and holding it. Can be bent easily from iron bars. Try it out in a cold furnace with a cold crucible before using it 'hot' to be sure that it fits well and pours out well.
Gloves and face shield
As described above, wearing a face shield is strongly recommended during the pour and oven gloves can also be useful.

Installing
The furnace should have a cilindrical shape, the best size is between 1.2 and 2 times the diameter of the crucible (1.5 is optimal) and 1.2 to 2 times the height of the crucible. In the bottom of the wall a hole for the torch should be made with tangential entry, which forces a rotating movement of the flame and hot gases around the crucible. When the flame enters straight to the center, the cooler part of the flame hits the crucible, thus cooling it. The torch hole should fit exactly, otherwise hot gases exit here with overheating (and melting if it is made of brass ) the torch nozzle and more heat loss. It should be open on top. The lid should have the same diameter as the furnace itself with a hole in the center with about the same diameter as the torch entry point. Here the exhaust leaves the furnace and the user can watch the melting process (again: use face shield when getting close).
Schematic view



YTONG Gypsum Blocks, an Alternative "Refractory"
4) and are air-spaced and thus provide very good heat insulation. They are also easy to work with. A worn-out ordinary saw can be used for cutting the material and an ordinary screwdriver can be used for other workings, such as carving the hole for the torch and crucible.
I tried one of 8 cm and made a cylindrical hole in it of ca. 5 cm diameter and 8 cm length in it and in the bottom a hole in which just the torch fits in. I put it on another one wit a flat top and put the crucible with ca. 40 grams of copper in the hole. It melted within five minutes. But there are some (minor) disadvantages:
  • Calcium sulfate melts at 1450 deg C, so close to the torch mouth the stone will melt.
  • When using a new piece, it decomposes a little and so it releases SO2 which is rather unpleasant and attacks aluminum exhaust pipes.
  • It is attacked easily by fluxes (borax Na2B4O7) or other molten salts. So don't spill flux when adding it to the metal in the crucible.
  • Spilled metal droplets which are frozen after cooling are hard to remove without damaging the furnace walls. But traditional refractories (chamotte) do have the same problem, only is a lesser extent.
  • Be sure it is calcium sulfate, not air-spaced concrete, because the latter is not heat-resistant at all!
gif picture

For use as soldering blocks this material is excellent. It is soft (nails to fix the work can easily be pushed in), heat-resistant and cheap. Now I don't need the far more expensive soldering blocks sold by silversmith suppliers anymore.

The amounts of this blocks needed are small, so it suffices just to look at trash heaps at building sites.


Melting & Casting

MELTING POURING

Here are some guidelines for working with molten metals. Of course, the mold should be ready before melting the metal.

  • Weigh the metal and fill the crucible and put it into the furnace.
  • To light the fire, put a lit piece of (news)paper into the furnace between the wall and the crucible and turn on the gas slowly. When it catches, the torch can be set to full power and the louder it roars, the better (Earplugs are not necessary). Experiment by moving the torch in and out.
  • When it smells badly, then the combustion is not complete because of the cold walls and the lid should be kept off for a few minutes to allow combustion of still unburnt gas.
  • When everything goes well, the bottom of the crucible is yellow-hot within 5..10 minutes.
  • Do not overheat the metals, but also, the metal should be molten entirely before casting. Before pouring out, add a teaspoon of borax which dissolves all oxides and other dross.
  • When melting pure copper, add some powdered charcoal to prevent oxidizing as cuprous oxide (Cu2O) dissolves into it and makes it (and its alloys) brittle.

Obtaining very high temperatures fourthcandle
Note: Except the oxypropane torch I've never tested or tried out these things
always
  • Oxyacetylene or oxypropane torch. A small one available in hardware shops 2H2) is hotter, but harder to obtain. Special requirements apply with acetylene tanks.
  • Small blast furnace. Especially useful for larger amounts of cast iron. One can make a shaft of about 50 cm long and 10 cm inner diameter from high temperature grade refractory (e.g. refractory concrete) and fill it with charcoal briquets mixed with pieces of iron. On the bottom the charcoal can be lit by a torch and then be blown by a reversed vacuum cleaner. This will get so hot (>1500 C) that the iron will melt and precipitate to the bottom. Charcoal chips are too light: they will be blown out of the furnace by the air vent. Anthracite (hardly available) works well too, but it releases lots of SO2.
  • Thermite. This is a mixture of 75 mass % iron oxide Fe2O3 (rust) and 25 mass % aluminum file or saw dust. It should be ignited by magnesium ribbon or by a little potassium chlorate KClO3 with sugar and glycerol. This yields a temperature of 3000 C (yes you read it well: C, not F). However, the reaction is hard to control and is hard to lite off. It is used for welding rail tracks. Note: virtually no material can withstand the temperature, so forget the item on which you let the reaction take place.
  • Electrical arc heating. Using a welding transformer (a 150 amps one costs less than $150) and attaching carbon electrodes (can be gotten from old zinc-carbon flat 4.5 Volts batteries in which there are three) to the wires also yield temps up to 3000 C.
Copyright 1994, 2006 ArtMetal

Author: Klaas van Ditzhuyzen, Velp, The Netherlands