By Daniel Ballard
Whether using a Torch or Electricity to melt your precious metal, you need to know the pros and cons of your melting method.
Gases used in melting precious metals run the gamut of flammable compounds. Many are derived from the hydrocarbon family of compounds. These gases need a boost of oxygen gas or compressed air, usually delivered via a torch, to get hot enough to melt gold. And remember: All flammable gases are dangerous if mishandled!
All torch melting depends on the experienced eye of the operator to regulate temperature and metal flow. You can use an immersion pyrometer to check the metal’s temperature, but very few jewelers do. Without a pyrometer, you need to be a very good caster to get consistent results. A good caster can “tend” the melt various ways, including proper fluxing and adjusting of gas pressures, gas balance, and heat.
This is the craft of casting.
No matter what gas you use, the heat energy is coming from the reaction of elements. The ultimate goal with torch melting is complete burning, without getting the torch too hot (usually visible as a short, hard blue oxidizing flame) or too cool. Whenever the torch gets too hot or too cool, you create unwanted compounds such as copper oxide, or run into problems from the carbon in these hydrocarbon compounds.
Incomplete combustion also creates troublesome, even lethal byproduct compounds. Soot is one of these byproducts–Look at an acetylene flame before the oxygen is added and you’ll see the shop filling with floating bits of carbon. Another important example, familiar to us from our homes: Natural gas appliances that do not have adequate air flow and venting can result in the tragedy of carbon monoxide poisoning.
In some areas, such as the cold Northeast, the content of the natural gas you receive can vary with the season or the availability to your city source. In addition, the supply pipes have their own problems, including rust, varied pressure, and condensed water in the line. You must also be certain to have excellent ventilation for safety.
Volumes of books exist about combustion physics. We’ll spare you the science in favor of practicality.
Sometimes called “city gas,” natural gas is a mixture of several hydrocarbon family compounds, primarily methane and ethane. This gas is a bit weak on heat, but there is a natural (forgive the pun) safety advantage-the gas comes from the city pipe as needed, so there is not a tank of explosive gas sitting in the room with you. You are, however, still stuck with that tank of oxygen. In some areas such as the very cold northeast, the content of the gas you receive can vary with the season or the availability to your city source. This is the most commonly used gas in small to medium capacity shops. Natural gas is also the only gas commonly used in blow furnaces, which are found in many large silver casting houses. For large melting jobs, a forced-air blow furnace offers a great reducing atmosphere limiting the formation of oxides.
This is by far the cleanest gas you can use. Hydrogen is a simple element, with no bound-up carbon at all. The byproducts of burning hydrogen are heat and water–hence the clean burn, a very practical advantage. Hydrogen is also the only gas recommended for melting platinum alloys. All the other flammable gases (compounds all not elements) provide hydrogen with another attached element, such as carbon.
Hydrogen is a high heat gas and very, very explosive. (High heat and explosive potential always come together.) Hydrogen is extremely light, so it rises away from you quickly. The gas molecule is so small it will flow through almost any opening in the ceiling or roof material, which usually allows it to disperse harmlessly. If hydrogen becomes trapped, however, it is extremely hazardous, so adequate ventilation is essential. You’ll use far less oxygen to boost the heat with hydrogen than with other gases. Less oxygen is universally a plus when melting precious metals, since fewer oxides will form.
Propane is a hydrocarbon family compound commonly used in areas where natural gas is not available. This gas burns cleanly, but it needs plenty of oxygen to boost the heat. It tends to be a bit hard on the metal, since with higher oxygen use, oxidation becomes a proportionally greater problem. This gas is stored in a small pressurized tank in a liquid state. Regulators are simple and inexpensive, and refills are as close as the nearest gas station that sells propane to the RV and barbecue crowd. A safety caution: Propane is extremely heavy and explosive, so be sure to ventilate properly.
Butane. For our purposes here, butane is the same as propane, although refills are harder to find than propane. I run across this gas very rarely. Butane. For our purposes here, butane is the same as propane, although refills are harder to find than propane. I run across this gas very rarely. Butane. For our purposes here, butane is the same as propane, although refills are harder to find than propane. I run across this gas very rarely. Butane. For our purposes here, butane is the same as propane, although refills are harder to find than propane. I run across this gas very rarely. Butane. For our purposes here, butane is the same as propane, although refills are harder to find than propane. I run across this gas very rarely. Butane. For our purposes here, butane is the same as propane, although refills are harder to find than propane. I run across this gas very rarely.
Acetylene. This gas is the dirtiest gas I see being used in the jewelry industry. Acetylene is what they use at the local muffler shop to weld steel. Acetylene offers lots of heat–and lots of carbon soot as well, making it very hard on your gold. In addition, acetylene can react with copper and silver alloys, making it less than ideal for use in jewelry manufacturing. Acetylene tends to stay in pockets rather than dissipate, and as with all the flammable gases, acetylene can explode if it builds up. It is also shock sensitive, so cylinders must be handled with extreme care. Acetylene is one of the least expensive gases to refill, and the regulators for it are like hydrogen regulators in cost.
Making Electrons Work for You
Electricity is the energy source for both induction and resistance melters. Both types of machines are relatively safe, clean, and quiet, and like everything else that is technology based, they have been much improved in the last couple of years. Jewelers generally find fewer restrictions on electric melting than on gas melting from both land lords and fire departments. This is at least partly because in electric melting, the heat is contained in a very small area.
In addition, the idea of having ranks of explosive, high pressure gas in the same room with people worries some landlords and firefighters. (An explosive controversy?!)
If your shop shares space with a retail showroom, you also run into stricter f-ire safety rules imposed on public spaces. Any non-retail shop in an industrial area will have an easier time using processes, such as torch melting, that can present a safety hazard. Public areas face tougher safety regulation, for very good reason.
All electric melters depend on a thermocouple to regulate the heat. The more accurate the thermocouple, the more consistent your results will be. Herein lies the source of many problems.
Most of the troubleshooting calls we get at PM. West have to do with temperature control.
One easy and fast way to check your thermocouple’s performance is to melt some fine silver. This melting temperature is constant at 1,762F so you can calibrate accordingly. Alternatively, you can call an electronics technician for a more “professional” (i.e., expensive) adjustment.
Electric melters also generally have a tough time isolating gold from oxygen in the air, which causes oxidation. A neutral “gas cap” like argon is the most common solution to this problem. A more aggressive method is using a reactive cover gas such as hydrogen, perhaps mixed with inert nitrogen. This mix is commonly known as “forming gas.”
Resistance melting is best known in the small, lower capacity machines, such as Kerr’s Electro-Melt or Memco Electro-Vac casting machines, although some continuous casting machines also use resistance melting. All resistance melters use some kind of high temperature wire (like the wire in light bulbs) wound into a spiral outside the crucible. When electricity moves through, the wire offers resistance to the electricity and becomes hot enough that heat radiates through the crucible to heat the metal. Resistance melters are safe to use and very simple in construction, which makes maintenance easy. They’re also relatively inexpensive, especially compared to induction systems, and safe to use. A complete casting machine is usually less than $10,000, and a small dedicated melter runs about $800. Crucibles can be delicate and a bit expensive, ranging from $35 to $60.
Induction melting is becoming much more common in our trade. This powerful technology used to be reserved for the large-scale caster or melter, but not any longer. As platinum casting, stone in-place casting, and mass merchandising of light goods have become more common, they have encouraged more firms to up-grade to induction melting. The induction method uses radio frequency energy at medium frequencies (6 to 12 kilo-cycles) or high frequencies (300 to 500 kilo-cycles) to “induce” heat directly into a special part around the crucible, the crucible itself, or the metal. Different frequencies have different qualities and produce different results. For example, medium frequencies offer the major advantage of actually stirring the molten metal. The current trend is away from high frequencies, but both types have their adherents. Despite many phone calls to machine makers and visits to shops, I’ve never found a consensus on which type works best and why. Many shops that have a new induction melter make the mistake of overheating the metal at first. Because the induction method is so powerful, and the timing is quirky, many folks “overcook” the melt, just as many of us have done to dinner with a new microwave oven. Keep in mind that timing is very important in induction casing. When the induction coil shuts down, the temperature of the gold falls quickly–much faster than most thermocouples can measure.
There is even a new ultra-high-tech type of pressure/vacuum caster that has variable frequencies. This variable setup solves the control problems associated with time delays inherent in thermocouples, as well as the myriad inconsistencies caused by the differences in one melt versus another.
Another variable to consider when using induction melting is the differing rate of heating between the crucible and the alloy. This is addressed by data access and artificial intelligence software, such as that found in the Neutec J-5 and the Romanoff / Yasui casting machines. With the advent of scene-in-place casting, many induction-melting casting machines are also showing up with pressure vacuum capabilities. Induction melting requires lots of juice. Often you need to install special high voltage circuits to feed this energy hungry animal. In addition, these melters need a supply of cool water to pump through the “induction coils” (really the antenna for the radio energy) so that they do not overheat.
The cost for induction melting systems can vary widely. The range is from about $5,000 for a dedicated melter without casting attachments to $70,000 for a pressure/vacuum casting system featuring induction melting and artificial intelligence software. In addition, with all this technology maintenance can be a big part of operating costs.
A less expensive level of induction melting costs roughly between $15,000 and $30,000. Memco, Inresa, Erschem, and other suppliers offer a wide variety of these machines. These induction melters are usually part of a complete casting machine. They can be bought as separate devices, bur problems with open-air oxidation reduce the advantages of separating melting from the casting machinery. The solution for oxidation problems is a neutral gas cap such as argon or nitrogen, or even a reactive gas like flaming hydrogen.
A disadvantage with the “all in one” casting machines is you cannot alter the speed of the pour during the cast, as you can with a torch and hand pour. ‘The precise casting equipment setups are too varied in detail to discuss here, but all attempt to melt rapidly to protect your precious metals from oxidation, and they all automate as much as possible to ensure consistency. These machines can isolate you to a greater degree from your processes, which can be a good or bad thing, depending on your management philosophy the advantages of isolation include consistent results during repeated castings of similar type, some additional safety features, and fewer problems when personnel turn over. A perceived advantage is a lower pay scale in the casting room. The disadvantage is that when anything goes wrong or you change the casting process, whether it’s changing to a different karat gold or moving to stone-in-place casting, there’s no one there to teach the machine and the operator what the differences are. The machine can hardly teach the crew!
A common misconception is that if you buy a highly automated machine, the person who runs it does not need to be highly skilled. Some will disagree with me here, but I believe that the more skilled the person, the better the results month in and month out–regardless of the machinery’s sophistication. There is no substitute for highly trained and experienced people-with all that implies, including a good salary.
Besides casting, the most common use of electricity is to melt metals for a bar or ingot. In this case, the metal is either extruded (continuous casting style) or poured into a steel or graphite mold. When choosing an induction unit, be very specific and tell the salespeople exactly what you will melt, and what kind of crucible you will use. The sales and technical folks can advise you about how much power you need and what frequency range is most suited to your needs. Flask sizes can be important for efficiency and crucible types are varied and have their own material and design criteria. The sales rep can help you sort through all these variables and find the machine that’s right for you.
If you’re considering adding a new process like casting to your shop, be thorough in your research. When you understand the tools available as completely as you understand your shop’s needs, you’ll make the best choices.
This article ran in the October Issue 1997 of AJM Magazine