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:


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). Here are some safety recommendations for melting and casting:


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:


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"

Another option I tried where gypsum blocks for building inner walls. These are sold in Europe under the name YTONG and have good refractory properties (until 1450 deg C) and are remarkably cheap ( about $1 for a block of 60x25x8 cm). They are available at every builder supplier. They consists of virtually pure anhydrous calcium sulfate (CaSO4) 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:

But after 10 times (gif picture) of use (heating up from cold, melting about 50 grams of silver in a crucible and turning the torch off after 5 minutes) the condition is still remarkably good. There was only one crack in it (was already there after the first use) but can be kept together by wrapping a piece of annealed copper wire around it. The insulation works that good, that after 5 minutes heating while the inside is over 1200 deg C, the outside can still be touched by the naked fingers .
In a somewhat larger furnace with an inner diameter of 7 cm (2 3/4") into which a 50 cc crucible fits, I melted 180 grams of pure copper and it melted within 12 minutes.

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.


Obtaining very high temperatures

When one needs very high temperatures (> 1400 C), e.g. for melting platinum, iron, etc. the following possibilities are available. However, the higher temp is needed, the harder to keep it. This is because of the law of physics which tells that the heat loss by radiation equals the fourth power of the temperature difference with the surroundings. An example is the fact that a candle has a flame temperature of 1500 C, but no object (except the very tiny soot particles which are responsible for the candlelight) of a reasonable size can be heated over 1000 C.
Note: Except the oxypropane torch I've never tested or tried out these things, but I'd like to someday.
Use always dark (welding) goggles when looking into such hot objects, otherwise damage to your eyes will result.

Mail comments to
Klaas van Ditzhuyzen
Velp, The Netherlands

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Copyright 1994, 1995 ArtMetal

Author: Klaas van Ditzhuyzen, Velp, The Netherlands
ArtMetal Editor: Enrique Vega

Last Updated: Mon, Mar 11, 1996