Furnace Working

Glass Blowing, Sculpting, Paperweights, Casting

By Mike Firth
Furnace working of glass involves melting the glass to a honey like liquid in a furnace and then manipulating that liquid with heat resistant tools or pouring it into molds.
A. What it costs.

Furnace working is the most expensive form of glass working. Because of the effort involved in heating the glass it is necessary to have an annealler large enough to hold several hours work, which usually means serious electric power requirements. To be practical, five major pieces of equipment need to be built before a single piece of glass can be blown and several tools obtained which can easily add up to $500 or more.
It is possible to blow a piece of glass having spent as little as $400. I did it. It is much more logical to build equipment that will last longer. That will run the cost closer to $1200-2000. Always remember that these costs assume a suitable place to set up. The lowest cost I have heard for starting with a bare piece of land and building a structure and equipping it for production blowing was $5,000.
The five pieces of equipment needed for a furnace working studio are an annealer for properly cooling the glass, a furnace for melting the glass, a glory hole for reheating the glass, a work bench, and a marver, a flat heatproof surface for rolling out the glass. Most hot glass workers will also need at least one grinder to cold work the bottoms of pieces. It is possible to combine, in some cases, the furnace and the glory hole and the marver and the work bench, to avoid building extra structures, but not in the long term.
The tools needed for working the glass include pipes to inflate the glass, punties to handle glass off the pipe, jacks to shape the hot glass, and shears to cut and form, Roughly speaking each one of these costs about $100 and most furnace workers have at least two of each. Some gaffers (the title of the person responsible for working the piece) prefer wet wood blocks for shaping the glass, another $50- 100, but many use wet newspaper held in the hand, by far the cheapest item on the list.
The real problem in stating costs is the huge variability in each item in the list. Almost all furnace working equipment is built by the glassblowers, although much can be bought for high fees, while almost all tools are bought, though they can be made. The following paragraphs discuss the ranges of cost.
The site to blow can range from a bare concrete pad open to the air to an air conditioned building. In most parts of the pure country open air is not an option although I do it in Texas and summer camp schools often have open walled structures. Wood floors are not an option. Dirt floors get muddy and won't take the weight of wheeled furnaces. Most glassblowers use existing old warehouse style structures in areas without strict zoning. The second most important requirement is a ceiling 10 or more feet tall to allow swinging the glass and to allow hot air to dilute with vent air. A studio should not be under the same roof as a residence or shop containing valuable equipment.
Most studios are not in buildings with air conditioning, but all must have good ventilation, usually by large openings in walls - garage doors - and large exhaust fans. The latter are often scrounged as they can be expensive. Usually the hottest items - glory holes and furnaces - are in a line with sheet metal walls forming an area to catch the heat and be separately vented from the working area.
Furnace glass blowing requires a considerable amount of gas - natural or propane - and enough electricity to be a nuisance - 220 volts at 40-100 amps. At some sites, including many homes, there is not enough natural gas capacity - pipe size or pressure - to run a glory hole and a furnace. Changes can be expensive - $500-2,000 to hire someone to do it. Propane tanks are usually provided by the delivery company and the cost may be low once they see how much propane is being used. Electricity wiring is not difficult for the worker to do, once the code rules, are learned for safety's sake.
The first equipment usually an annealer, since no glass can be saved unless the annealer is working. The annealer consists of a box and a controller. The cheapest solution is to already own an electric pottery kiln with a usable controller, but these often too small in the long run. Most annealer boxes are built by the worker because they are big and bulky. An annealer is a metal box with insulation inside. Often the metal box was once a refrigerator or freezer although environmental rules on freon and its safe collection have made this choice more expensive. The insulation can be ceramic fiber and fiberglass, ceramic fiber board, and insulating fire brick. If the box is only used as an annealer - max temperature 1000F, brick need not be used and nichrome dryer heating elements can be used. If it is used for slumping (1300F), fusing (1600F max) or casting (1800F) also, different choices must be made, including Kanthal for elements and providing a lot more electricity. Lowest cost of an annealer container is about $150 for blanket, shell, and element. Highest cost is about $1,000.
Controller costs range from almost nothing (with a lot of time and attention while annealing) to $2,000. Most glass workers use Digitry or competing programmable controllers. Digitry's GB-1 controls one device and costs about $700 per device. Their GB-3 controls up to 5 devices and costs about $1500 plus about $100 per device for relays (solid state or contactor) and thermocouples. At the low end, a knowledgeable person can make a ramping controller for about $20 from digital chips, plus about $30 for a voltmeter, $20 for a solid state relay and $20 for thermocouple. (Old timers used Variac transformers, but cheap high amp Variacs are not easy to find any more.) Almost anyone can buy a digital, single ramp, learning controller with display for $200 plus $20 for thermocouple and $50 for solid state relay. There are also systems which work with old personal computers and cost about $800 (plus $100 per unit) to control up to 8 units. Most glassblowers have 4-5 units requiring temperature control - perhaps 2-3 annealers, a color kiln, and a garage. Some control their furnace.
After the annealer, the next device is a glory hole, which is either a barrel lined with very high temp insulating material or a box lined with insulating firebrick. Money and space may be saved by building a furnace with a big enough opening to reheat the working glass over the melted glass, but that is unsatisfactory because the melt gets too hot and the glory is not hot enough. A 12" opening glory hole made of a 30 gallon barrel, using vermiculite, insulation board, and castable insulation with an iron pipe burner will cost about $175. Using one of the ceramic head burners (Giberson or Wilton) will add about $130. The unit will weight 150-200 pounds and will require a frame that has to be welded. Cost can range from $30 with self-welded scrap steel and no wheels to $200 with hired welding and good casters.
A marver is a flat piece of material, usually steel about 1/2 to 1" thick, set on its own frame or on a table at about hip level. Getting a good piece of steel can be a matter of cheap luck at a surplus steel place or costly investment at a machine shop. It needs crisp straight edges. 1 x 2 feet to 2.5 x 5 feet, $10 to $200.
A work bench for furnace glass working is built of steel angle or tubing or of wood with steel edges on the long arms. Most glass workers follow the model of the one they learned on. Bench materials cost $20-40.
The furnace can be built with a large door for use as a glory hole or with a much smaller gathering port just big enough to get in and out with the last gather made, 6" max. In the long run, it is best to have a separate glory hole so the furnace can be as efficient as possible and the glass held at the best working temperature. A furnace may be a tank furnace or a pot furnace.
A tank furnace involves very high temperature glass resistant hard fire brick backed by insulating brick, bound together by a heavy steel frame. Some of the individual bricks can cost $25+ each. A tank usually holds 200 - 500 pounds of glass and is used where many gaffers are working (student teaching) or when large amounts of glass are needed for casting. Such a furnace can cost $1500 or more. Often regulations require industrial gas controllers that can easily add $2000+.
A pot furnace contains one or more ceramic pots which may be free standing or invested and 125 pounds is a common capacity. The most common is invested where the pot is bedded in insulating castable to reduce thermal shock and so that when (not if) it cracks, it can be used longer. Glass melting pots have to be formulated so they can stand the heat and won't dissolve in the glass or throw chunks into the glass. They can be fragile. Commercial pots require careful slow heating to avoid cracking. The pot costs $100-200 and shipping can double this. The furnace is usually cylindrical and can be built in a 55 gallon drum or with sheet metal forms. The top is cylindrical or domed and covered with added insulation. Cost beyond the pot is about $200.
A durable burner costs about $150. Since the furnace usually is run for weeks or months at a time, common sense as well as regulations will require some form of safety equipment to cut off the gas if the electricity fails and perhaps restart the furnace if conditions are correct when power returns. Solutions range from venturi burner with high pressure propane (no power to fail), add $75, to a simple cutoff of gas and electricity to a blown burner, add $100, to a trigger restart with sensor, add $200, to a controller that maintains precise temperature and safety, add $1-2000+. A Charles Correll regenerative tank furnace with full controls, installed per code, costs $10- 14,000.
I blew glass at a backyard site after having spent $400 on annealer, $75 on workbench with marver, a gloryhole melting glass in the bottom for about $200, and about $100 in propane tanks and regulators, doing my own welding. I had also spent about $1,000 on classes by that point.
When I started, I used newspaper for shaping, made my own jacks from carefully bent flat steel stock, a pair of stainless steel scissors for shears. After careful consideration, I bought, for $65, a pair of diamond shears, figuring no way to make them. My pipes and punties I made from solid rod and stainless water pipe (1/4" IPS about 1/2" OD) which I found after calling around to all the used steel places in the area. Proper steel is non-magnetic and when heated with a small torch does not transmit heat well. I have since bought one pipe. It is possible to make pipes, but many steel companies have high minimum purchases which they sometimes apply to each product. There are several sources for each of the tools needed for glass working and, as mentioned above, they typically cost about $80-120 each. Each gaffer would normally own 3-4 pipes (2 minimum), 4-6 punties (2 min.), two different jacks (1 min.), 2-4 various shears (1 min.), a puffer, and various wood paddles and blocks.

Mike Firth, Hot Bits furnace glassblowing newsletter
mikefirth@ticnet.com / http://users.ticnet.com/mikefirth/start.htm
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last updated 31 Mar'97
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