Monday, April 4, 2011

Firing Options for Silver Clay

Posted by Mary Ellin D'Agostino
Technical Advisor

People often ask about what they need to fire their silver clay. Do they need a kiln? Can they use their old ceramic or glass kiln? What kiln should they buy?

Let me say first that silver and gold clays do not care what your heat source is. Heat it long enough and hot enough and you will have a good result. Heat it too hot and it will melt. If it doesn’t get hot enough for a long enough time, it will be brittle. There are many firing options available and all have their pros and cons mostly having to do with expense, convenience, and consistency of use. In this article, I discuss several why or why not you might choose any particular firing method.

Base metal clays have additional issues to consider when firing. These clays are still in their infancy and the most reliable firings methods are kiln based. Many metal clay artists and manufacturers are working on coming up with reliable firing alternatives.

Hot Pot: The reason to purchase a Hot Pot is if you are a new user of silver clay, don’t know if you will continue using it, are concerned about using a torch (often this means you are concerned about the open flame or of the possibility of melting your piece), and only firing the low fire metal clays. Hot pots are a good choice for people who match one or more of these criteria. The Hot Pot, WHEN USED CORRECTLY, gives an adequate firing–I do not recommend it for firing rings or other items that need to be very strong unless the maker has no other firing options. These items should be fired in a kiln for a longer/hotter firing schedule than is recommended by the manufacturers. Even when firing such items with a torch, it is highly recommended that they be fired for longer than one might fire a pendant. The choice to fire twice is often made with a torch and kiln as well as the Hot Pot and reflects more on the particular piece and the person making it than on the type of firing system used. Finally, part of the concern over the Hot Pot may be due to people using the wrong fuel for it. When purchasing replacement fuel, it is important to buy the right kind. Not all gel fuels are the same! Unfortunately this means that the safest choice is to buy the Hot Pot branded fuel as the heating characteristics of gel fuels is not listed on the various brands found in hardware or camping stores. Hot pots are only for low fire silver clays (PMC3 or ArtClay650)!

That said, the torch is more versatile and I prefer it for that reason, but many people are concerned about using an open flame. A person using a torch to fire metal clay should be very clear on what is meant by “glowing orange” as many people have under-fired their pieces when their “glowing orange” is different than what experienced metalsmiths and torch fire-ers understand the term to mean. This is one of the biggest drawbacks to learning from books and instructional media. Nothing beats learning from a good experienced teacher in person.

Speed Fire Cone or Stove Top: A stove top can be a good firing option if you actually have a gas stove. These are not as common as many people think. While gas is cheaper to run, it is a lot less expensive to install an electric stove and that is what many builders have chosen in the USA. In my experience, you can’t really just set it and forget it because you may need to adjust the flame while firing. Also, you need to be sure to use a stainless steel wire grid and not standard hardware cloth, which is usually galvanized and will deteriorate during firing. Camp stoves are far more problematic than the household range as the flame does not remain consistent–it changes as the level of fuel in the propane or butane canister goes down and as the canister becomes colder while firing. Hence the Speed Fire Cone™, which concentrates the heat AND comes with a pyrometer so you know what your temperature is. The SF cone needs to be monitored as well. Also, the most consistent result will be gained if using a large BBQ type tank for the butane/propane–an adaptor is needed in this case for the SF Cone. Note that the Mini Speed Fire is only for low fire silver clays like PMC3.

Kiln: Kilns give the most consistent firing assuming one has a reliable pyrometer in the kiln. These are available in a wide range of options, but if one can afford it, a kiln with (or that can be retrofitted with) a digital controller and pyrometer is the way to go. The kilns designed for use with metal clay products are the best for using with silver and gold clays because they are designed to have a very consistent temperature throughout the firing chamber.
Lots of people like the Ultralite™ kiln because of its small size and relatively low cost. It is comparable to the SF cone at its base price, but the special metal clay inserts may make it a little more costly. It does take longer to heat than the SF Cone, but does run on electric.

The easiest and most reliable method for firing any type of silver or gold clay is a small programmable kiln with a ceramic fiber or refractory brick muffle. A muffle is the insulating part of the kiln. Ceramic fiber is a good choice for silver and gold clays because it is an excellent insulator, lightweight, and the heating elements for the kiln can be embedded in the sides. Traditional refractory brick insulation is bulky and kilns made from it tend to weigh a lot, take up a lot of space, and have the kiln heating elements exposed on the interior of the kiln.

Ceramic fiber is a space age material that can be formed in a mold with the kiln elements in place. The embedded elements are not exposed so you can load, unload, and crash cool the kiln without worrying that the elements will be damaged. The exposed elements in a traditional kiln require periodic replacement because they can become damaged if the kiln is opened while hot or by glass, glazes, and other substances in the firing chamber. On the other hand, traditional brick kilns are a lot less expensive and the elements are inexpensive to replace when they wear out.

If you plan on firing bronze, copper, or other base metal clays using the “bury it in carbon” long firing schedules, brick kilns with exposed elements are preferred because the long firing times wear out the elements more quickly and, if you do a lot of firing, you will appreciate their easy and inexpensive replacement. When the elements go in a ceramic fiber kiln, you have to replace the whole muffle, which is expensive.

The size of the kiln is an issue because the larger the kiln, the more difficult it is to have even heating throughout the firing chamber. By “small” I mean a kiln that has an interior chamber that has a footprint less than or equal to 12″ at it’s widest point; a little bigger will be ok, but the typical 18″-20″ kiln may give less even heating. A large ceramic kiln will have hot and cool spots which can make firing metal clays chancy. This is not to say that they can’t be used, but you should know your kiln well and place any silver or gold clay objects near the thermocouple or cones so that they are heated to the desired temperature.

Top loading and front loading kilns also have different heating characteristics. When firing base metal or PMC Pro clays in a carbon filled box, a top loading kiln is preferred because the box is heated on all sides. Front loading kilns do not usually have heating elements in the door and the side of the firing box facing the kiln door remains cooler than the rest of it.

Glass and silver clay kilns tend to have maximum operating temperatures of 2000°F/1093°C and extended firings at the hotter bronze/copper temperatures will wear out the kiln elements relatively quickly. These kilns are designed to be fired at glass fusing and silver clay firing temperatures and will last for years when used for these purposes. If you are doing the long carbon filled box firings for copper and bronze, you may want to look into using a kiln designed for firing ceramics. These cost more because they are made using materials that are durable at the higher temperatures required for firing stoneware and porcelain.

Happy Firing!


Anonymous said...

This is great info for new and seasoned clay artist. Thanks MaryEllin!

Earl Roberts said...

Great post Mary Ellin. If I may I would like to add to your analysis.

When choosing a kiln there are many factors to take into account, including element life in fiber kilns versus brick kilns. In designing the Model 360, we accounted for the fact that the elements would be fully embedded in fiber, therefore the elements we used are made of an iron chrome alloy. They are of sufficient wire size such that the watt loading minimizes the reduction in element life when fired in the 1700F range for extended periods of time. Iron chrome elements can operate in air to temperatures exceeding 2300F. The firing profiles used for firing metal clays are such that it is unlikely that the elements would ever exceed 2000F for more than a short period. Once the kiln goes into the hold mode the kiln is operating at less than 100% power and the differential in element temperature vs air temperature would be on the order of a couple hundred degrees. At these temperatures element aging is almost non existent. In the 15 years+ that I have been involved in the design and marketing of fiber kilns I have seen very few element failures due to element aging. So, if I were choosing a kiln, I would put more emphasis on my intended use of the kiln, rather than this factor.

One of the biggest considerations is top loading versus front loading. Our front-loading Model 360 is designed to be very uniform in temperature front to back. While the front of a steel pan would be facing a cold door, once the kiln reaches firing temperature heat transfer is done primarily by radiation/re-radiation. My opinion is that the temperature variations found in a steel firing vessel are not solely the result of a “cold” door, but also the container itself. One solution for obtaining uniformity in a front loading kiln is our Fiber Firing Container, which has been tested by Mitsubishi and shown to have very uniform temperature distribution.

Another consideration in the top versus front loading area is whether you will need to be moving things in and out of the kiln while it is hot. You should always take precautions when opening any hot kiln. However, unloading any vessel from a hot, brick lined top loading kiln is a bit of a challenge as the operator is exposed to a significant amount of heat energy when the lid is raised.

With regard to glass kilns, most have nickel chrome elements which operate best below 2000F. However since they do not “age” like iron chrome elements, they operate for years at close to their maximum temperature without dramatically shortening their life. My observation has been that in kilns with exposed elements, most element failure is caused by mechanical abuse or contact with foreign materials, glass, glaze, etc. so caution in loading and unloading is a must. Also, while I have not observed this, I would suspect that the oxidized steel particles from firing in a stainless steel container could shorten element life in a brick kiln if allowed to collect in the element grooves.

fire pot gel said...

1 inch from the top. Use FIREPOT Fuel Gel. Do not overfill. Carefully light fuel gel with a fireplace or grill type lighter. Enjoy the ambiance of the dancing flames. When finished, use the metal snuffer to cover the flame and extinguish the fire.

Anonymous said...

What brand is the little white kiln you have pictured? I don't recognize it.

Jen said...

The kiln you see is an older model of our kiln line. The updated version of the kiln is our Eclipse, which comes pre-programmed with PMC and Art Clay firing profiles.