Parker Glass Furnace


Parker Glass Furnace














Electric Glass Furnace

(Continued from Home page.)


Propane costs for a round gas furnace with recuperator and free standing crucible can be above $2000. a month. One of the problems with a gas furnace is that they have to have a flue to exhaust combustion gasses and the BTU's go out with the gasses. Another problem is that if the electricity goes off, a gas furnace has to be reset, where as an electrical furnace will come back on and has no flue exhausting heat. The idea is to eliminate or at least minimize potential problems while striving for efficiency.$2000 a month Propane Glass furnace

A further adaption of the round furnace is the moly crucible furnace. This is a chamber with a crucible and and a firing chamber above it twice the size as the crucible. Here the moly elements are exposed which can lead to breaking if bumped. The elements are very expensive, can last for years and are cost efficient to operate yielding several hundreds of lbs. of glass. This furnace will accommodate many glass blowers. The electrical boxes are similar for a moly furnace and a resistance coil furnace except a moly furnace requires an additional transformer. The moly furnace is an adaptation of a gas fired furnace using the old 2/3 - 1/3 ration.

The Columbia glass furnace is more efficient because the elements are heating a firing chamber at the bottom and sides of the crucible, eliminating the top 2/3 of the furnace, except for the gathering point.

The furnace is a smaller crucible furnace, 3 feet by 3 feet, less than 3 feet high, using coil resistance elements which cost considerably less than moly elements. Under best of circumstances coil resistance elements can last six months (longest life of elements is 8 months and still going) and take 5 days to cool the furnace, replace the elements from inventory and bring up to melting temperature.

Parker Glass FurnaceI want all my equipment on wheels, 2 stationery and two locking swivels. So we start with a steel grate and 4 wheels. I will also mention that only one out of four of the 1" bars in these grates are welded to the end piece, and all of these should be firmly welded before painting. Cover the grate with heavy duty 18" aluminum foil available from your big box store, shiny side up. Overlap the edge.

Parker Glass FurnaceApply 1" of 1600 degree F fiber board and over that 4 1/2" of 2300 degree F soft brick and cut to fit hard refractory pan. The next layer is the 2650 degree minimum no/low iron hard refractory pan. The reason for this is because iron in the refractory fluxes the refractory. This pan has a collection recess for errant glass spills. I have used this type of hard refractory in multi crucible gas furnaces and with a layer of old ceramic fiber over a layer of iron less sand, glass contact easily comes off with little damage.

Parker Glass FurnaceOn the outside of the ring is a keyed recess for the 9 hard refractory element holders. They are of the same material as the pan. The pan key is slightly larger than the element holder and this element holder contains the exit holes for the elements, which we will at a corner of the furnace to protect the element cover.

Adding the next element holder which contains a hole for a type R TC.

Adding a third holder with another TC hole and this time it is on the other corner to protect the TC. There are 2 TC holes fit is necessary to have 2 TCs with a limiting controller. This furnace doesn't need a limiting controller because the elements burn out at 2400 degree F, preventing a runaway.

They are in the back to protect them.

Parker Glass FurnaceParker Glass FurnaceInstall the rest of the 9 element holders. Push all the element holders to the outside of the keyed pan recess. Then realign towards the center.

This is the location of the the TC. The type R TC has an alumina jacket and is located at the middle height of the chamber. It is small so that it is more vulnerable to temperature changes.

Parker Glass FurnaceParker Glass FurnaceAround the hard refractory is one inch of 2600 F insulating fiber. Lift to the top of the hard refractory and fill around the soft brick.

Two boxes of 2300 fiber are applied for 5 more inches of insulation.


Parker Glass FurnaceLine up pre punched element holes in the 24 gauge stainless steel exterior with the hard refractory and enclose fiber.

Lets back up here and talk about installing the nine elements. So before you put on the fiber you can wrap a rope around the outside of the element holders as soon as they are installed to secure them. Taking another rope place a piece of colored tape on the end an place it into the top element holder so that the tape is not visible. Then place the rope in the groove of the element holder and mark the other end so that the colored tape doesn't show in the groove.

Parker Glass FurnaceParker Glass FurnaceNow you have the length that the coiled element needs to be stretched. Cut the loop off the end of the element and clamp the lead of the element and the end of the rope with the tape on it to a solid object. I use my marvar. Double check the clamp before stretching for safety and pull the other end of the element from the lead with a pair of pliers. The idea is not to over stretch so pull in repeated motions until the element is the length of the rope inside the taped ends. If you over pull start at one end and push the element back together over the whole length of the element and re pull. Best to do it right the first time.

Parker Glass FurnaceClip off the round at the other end of the element and begin the cleaning and coating. Because the elements are wound on a lathe there is machine oil on them. Prepare a bath of denatured alcohol and run each element through the alcohol, removing the oil. Hang to dry. This is important because the next step is to heat the elements in a kiln to 300 degree F in a kiln, which might ignite any residual alcohol. Prepare a bath of milk of magnesia and dip the heated elements one at a time and hang them to dry. The manufacturer of the wire sells a product that costs a lot more than milk of magnesia and the manufacturer says will make the elements last longer. Your decision here. The idea is to setup a barrier on the element to lessen the degradation of the element. Before installing elements, vacuum the element grooves and apply a wash of alumina hydrate in water to the groove. This is another barrier between the element and the element holder.

Install 1 1/4" rolled stainless steel angle to the top of the stainless wrap. This ring provides safety and structure to the furnace structure.

Install elements and ceramic insulators. The element connectors are oversizes tin plated copper split bolts. Tin plating lessens corrosion of the copper and the over sized bolt is a heat sink to lessen shorts.

Parker Glass FurnaceParker Glass FurnaceInsert nickel hinge wire next to element in copper split bolt and tighten. Insert the other end of the nickel hinge wire into the distribution block and tighten. The 9 elements are connected 3 in series, 3 in parallel. Twenty minutes after installing retighten connections, as copper relaxes after installation.

Cover the connections with the stainless steel cover plate. All parts are stainless to lessen corrosive metal degradation and heat transfer.

Before installing crucible cover floor pan with no iron silica 30 mesh sand. Cover with old ceramic fiber to catch spills. Install crucible stand and center crucible. Looking at the crucible installed, it becomes apparent that charging with glass batch exposes the element to the glass fluxes and charging with cullet and the resulting "popcorn effect" of exploding cool glass in a hot chamber will flux the elements, destroying them in short order.

Parker Glass FurnaceBecause I make my own crucibles, the idea was developed to install a second ceramic piece on the top of the crucible, made of the same body as the crucible, as a throat through the removable insulating top of the furnace.

Parker Glass FurnaceThis is a side view (cross section) of the furnace showing the tan color as insulating material, the dark grey as the element holders and pan, the lighter grey as the crucible with orange as glass and the added throat, or tophat. The crucible and tophat are an actual photo of raw clay pieces photoshopped for the graphic. The red wax on the rims is to lessen the water evaporation on the edges while drying. It is not only the formula of the crucible and tophat, but how you form them also.

Parker Glass FurnaceThe crucible is 16" in diameter and 12" high OD. The throat of the tophat is 8" ID. All parts are in place and the top of the furnace with lid are in background.

Parker Glass FurnaceDuring initial heats of the furnace and always run a second TC, type K, as a second monitor to the type R TC. Type K reads more accurate at lower temperatures and type R at higher temperatures. At 1500 degree F the type K is no longer necessary. Connect type K to a second pyrometer for accuracy.

Parker Glass FurnaceStuff the gap between the insulating furnace top and the tophat with old fiber. This seals the heating elements in their own chamber and the hard refractory tophat and the fiber act as a collection area for any errant glass or ceramic parts. Because the lid has a concave top with a diameter outside the diameter of the glass throat, ceramic particles falling into the hot glass is lessened considerably for a pot furnace.

Parker Glass FurnaceAnother important part of this furnace is the infrared safety switch. The 9 element holders are roughly shaped like a circle and a Tesla effect is generated by the elements and this charges the molten glass which carries current, roughly 120 volts at 1/2 amp. Just enough to give you a little jolt as your hands, wet with sweat, relax off a metal blowpipe with several pounds of molten Parker Glass Furnaceelectrically charged glass on the end. So a second relay is installed with an infra red reader and when the lid is lifted for a gather, this IR reader sees the mirror on the other side of the furnace and shuts off the second relay and all electricity to the furnace until the lid is replaced over the gathering hole.

Most glass blowers are used to side gathering furnaces. This is a top gathering pot furnace and it is different. The lid in this image has been counter weighed to offset lid weight. The lid has no hinge and resets on the back stainless steel plate. A gather is taken and the gather string is broken by moving the pipe to a 90 angle and turning it. If the heat of the gather does not break the glass thread on the end, turn up the furnace a few degrees. A furnace with less glass will need to run hotter than a furnace full of glass.

Parker Glass FurnaceGather from the side of the crucible closest to you and when you pull the gather out move the pipe to the back of the furnace to drip off the glass in another area other than from where you gather. This leaves an even gather and eliminates dripping off in a water bucket. For example, in a side gathering furnace, the glass blower gathers at an angle and after removing the pipe from the furnace, drips off in a water bucket to achieve an even gather.

Parker Glass FurnaceNext the glass blower moves to a pipe cooler because the pipe is hot from the side gather. Following that the glass blower has to reheat the glass before beginning to work it. With this pot furnace those steps are eliminated. The gather is even from the draw and the residue goes back into the back of the crucible and not into a bucket of water. Because the pipe is not hot, a water cooler is eliminated and the glass blower goes to the bench or marvar to work the glass.

The elements are 11 gauge Kanthal wire which draw 45 amps at 240 volts.

The controller is a Yokogawa setpoint controller with a type R thermocouple. It is in an 120 Volt electrical box by itself to separate if from EMF (electro magnetic flux) from the SCR.

The main relay is a Watlow Dyn-a-Mite SCR (silicon controlled rectifier)

The SCR must be cool to be efficient. There is a fan blowing into the box, a fan blowing across the SCR heat sink and a fan exiting the box.

Parker Glass FurnaceThe furnace requires a safety relay because molten glass conducts electricity. Because the molten glass in this furnace is surrounded by a cylinder of electricity it contains an electrical charge despite no contact of the glass to the element.

When the furnace lid is opened it breaks a light beam which deactivates a mercury relay which shuts off all power the the furnace unit. Pilot lights and an ammeter are visual checks confirming power output. If your state outlaws mercury switches, then it is necessary to install 2 mechanical relays. Mercury switches fail off. Mechanical switches and SCR will fail on.

The relay box is 240 volt with a 60 amp shutoff and 60 amp breakers. Each of the electrical parts in the box has its own cartridge fuse. The power in is a 50 amp plug and the power out to the furnace is a 50 amp receptacle.

This furnace was developed for a production shop. The idea was to take the resistance technology as far as possible for a 100 lbs crucible and melting excellent glass successfully and efficiently.


Jason at the Sonoran School, Tucson, AZ.
Columbia Electric Glass Furnace, Portland, Oregon

Currently in stock:

60# furnace with crucible 30 amps 240 volts