Question and Answer Page (4) Questions and answers about glassmaking furnaces.
* Updated June 11, 2007 *
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Following are questions I have been asked about glass, along with my answers to them. If you have glass questions, feel free to post them to my Glass Discussion Board. I regret that I no longer have time to answer questions individually via e-mail..
Question and Answer Page (4)
Questions and answers about glassmaking furnaces.
Q. I have heard of tank furnaces and pot furnaces. Can you explain the differences and also explain why you use the type you do.
A. This looks to be a full page question...
For studio use there are two primary types of furnaces which then divide into several subcategories per type. The types are tank furnaces and pot furnaces. The primary difference between a tank and a pot (also referred to as a crucible) furnace is the method of construction of the part which contains the molten glass.
A glass tank is constructed of separate refractory bricks or blocks which fit together without mortar and form a (usually) rectangular container to hold the molten glass. The blocks are held together by mechanical pressure exerted upon them by the outer blocks of the furnace structure and the steel furnace frame. Because there are seams between the tank blocks, molten glass eventually finds its way through and starts eating into the backup insulation. It stops eating when it gets to a point where things are cool enough to solidify the molten glass. Because of this, the bottom of a tank furnace cannot be insulated very well (the better one is insulated, the more insulation the glass eats). Needless to say, this is not exactly ideal, but these furnaces can take a lot of abuse and are widely used. They are also hot to work near and consume a lot of fuel.
Day tank - The type of tank furnace most often found in a small glass studio - so called because it is filled with batch or cullet when everyone stops working at the end of the day. This is then melted overnight and used the next day.
Continuous melt tank - Typically used only where large quantities of glass are consumed, these are most often found in very large studios or industry. The back end is continually being filled with batch or cullet while glass is being taken out of the front. It melts new glass constantly in the very hot back end, and as the glass is used up this new glass flows to the cooler front end and is used. In a properly sized furnace, this new glass has sufficient time to melt and fine out (expel gas bubbles) before it gets to the front of the furnace to be used. .
In a pot furnace the molten glass is contained in a pot or crucible which looks a bit like a giant bowl. This is made out of a refractory ceramic material or, to put it in plain english, a very high temperature clay. There are many different formulas for pot clay, each with its own characteristics. For example, fireclay pots have been used for centuries and two of their typical characteristics are: a resistance to cracking from a sudden change in temperature; and the fact that they are attacked and dissolved gradually by molten glass. There are newer ceramic materials such as AZS (Alumina Zirconia Silica) which are much more resistant to attack by molten glass but have the tradeoff of being more susceptible to temperature shock. When using one of the glass resistant materials one must be careful about sudden temperature changes which can crack the pot. Alas, everything is a tradeoff. In general, a material that is more resistant to attack from the molten glass is also more susceptible to cracking from temperature shock. One must determine which is more important for his/her particular application and choose the crucible material accordingly.
Because a crucible has no seams, there are no places for the glass to concentrate its attack on the structure and leak through. The bottom of a crucible furnace can be insulated far better than a typical tank furnace and thus, all else being relatively equal, is more fuel efficient. Typically, they also melt higher quality glass. Most, if not all, optical glass is made in crucible furnaces.
Freestanding pot - A freestanding pot furnace is the type I currently use, due to the high glass quality, replaceable crucible, and energy efficiency. The crucible (pot) sits in the middle of a heating chamber without any support other than whatever the base of the crucible is resting on. The pot can be changed relatively easily when it is time to replace the crucible (typically after about a year of use). If the pot cracks, molten glass spills into the heating chamber and makes a mess. There is a drain port built into these furnaces to remove this glass in the event of a pot failure, but it is still a messy situation. Freestanding pots are not as durable as either a tank or invested pot furnace. They must be treated with some restraint and common sense to keep the pot from cracking. This is not a particularly good furnace for beginners for that reason. Beginners are often somewhat lacking in common sense and restraint.
Invested pot - Somewhat of a hybrid of a tank and pot furnace, refractory material fills the space between the pot and the backup insulation. This keeps the glass from leaking out if the pot cracks and provides some extra mass to keep the pot from cooling too fast and cracking in the first place. The pot will be useable longer because of this backup material, but when it does finally fail it is usually taken apart in the same manner as a tank furnace - with a sledgehammer. This furnace can take nearly as much abuse as (maybe more than) a tank furnace and melts better glass. This is what I built when I first opened my studio. Durable, easy to build, good glass quality, and energy efficient. This is what I would recommend for someone starting out, unless for some reason they needed a really large furnace. Then there isn't much choice other than a tank furnace.
All of these furnaces can be heated many different ways, but the most common in a studio situation is natural gas or propane, with electricity a distant third. As with the construction type, each has advantages and disadvantages. My first furnace was natural gas fired (the common choice, when available), and my current one (no pun intended) is electric, which I like very much. In an area where electric rates are not sky-high, I would personally ALWAYS build an electric furnace. (The fact that I have about 50 years worth of heating elements in stock doesn't hurt either.)
Some reasons I like electric furnaces:
Extended life of furnace materials and crucible due to even heating, no gas pressure fluctuations to worry about, simple and comparatively inexpensive safety system, no trips to the shop in the middle of the night to relight the furnace after a power failure (it restarts automatically), no flue necessary to vent combustion byproducts (and to waste heat), nearly idiotproof control system (depending on the creativity of the idiot), constant furnace atmosphere, and they won't blow up one's shop in the event of a disaster.
Last, and to me the most important - excellent glass quality.
There is more information about my electric furnace, including a crude wiring diagram of the control system, on this page. Questions about electric glass furnaces are also welcome on the discussion board of the website. There are many other people there who use, and/or have built, electric glass furnaces.
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