Rev. ... 2002-11-14, 2003-03-01, 05-05, 2004-02-18, -07-01,
2005-09-18, -10-01, 2006-05-30
2009-05-07 (layout), -12-03, 2012-10-02
FURNACES Started 8/25/96 with moves from
Back to Equipment Sequence
|Electric vs Gas|
|Safety and Fuel|
|Building an invested pot furnace (one instance)|
|Building a Glass Furnace-Summary|
|How I built my first
glass furnace/glory hole
(Glory Hole of Insulating Fire Brick)
|How I built my second glass furnace (Building a cast dome furnace)|
|The Burner - Making a Burner (Moved)|
|Dual purpose barrel furnace/gloryhole|
In an pot furnace there is a pre-cast, pre-fired ceramic holder - the pot - for the glass that can (usually) be added and removed. In a tank furnace, the container for the glass is built of bricks fitted together. Tank furnaces are a choice if a large enough amount of glass is to be melted, but it has consequences as noted in the messages below. In the good old factory days a pot might be 3-4 feet wide and high and were made in the glass factory, although most used today are rarely larger than 2 feet. The former will hold a ton or two while the latter would be closer to 625 pounds and studios melting 130-225 pounds are more common. Glass held in the furnace too long starts to decompose, forming cords, etc., so overbuilding is not useful, besides taking more fuel to run. Most studios melt enough glass to use most of it in a week or less, recharging with batch or cullet weekly, twice a week or every night.
A small tank furnace can be built of brick-like slabs that are cut to size, so that there is a floor piece and four walls, but once beyond a certain size, it is built of side-by-side bricks. A tank furnace can not be well insulated because it keeps the glass inside the cracks by freezing it as it leaks out. One special form of tank furnace is the continuous feed. Here a barrier is installed part way across the tank with spaces underneath. Batch is added beyond the barrier and melted at a high temperature. The weight of the added batch forces the melted glass down under the barrier to the fore chamber which is kept at a lower working temp and the glass is fined partly by the action of being forced under the barrier. Glass is continuously used from the front and added to the back. 2004-07-01
Pots purchased for furnaces typically are rather fragile. This
is because of the tradeoffs made so the pot sheds relatively
little of its material into the glass during the months it is
running. The material in commercial pots will crack if heated (or
cooled) too fast. A typical pot has a recommended heating rate of
70°F per hour (one brand claims 300°F per hour) which means
taking about 30 hours to get up to heat (2100/70=30) which
absolutely requires a control system (more expensive) and means
the furnace must be built for continuous operation (not daily or
weekend manual runs.) Note that there is a bit of a Catch 22 here: The pot
being fragile requires a long time to heat up and cool down, but the pot is
fragile because of materials that will withstand being heated with glass in it
for a long time - the exit being that cooking a large amount of glass requires a
long time (overnight at least) and is corrosive, so the time to get started is
relatively long no matter what.
Within the modern pot furnace environment, there are two
choices: free-standing and invested. The choice is built on the
fact that eventually the pot is going to crack and most
glassblowers don't yank the pot when the first cracks appear at
the rim, but wait until it is seriously gone because of the
purchase and shipping costs of a pot. [Good procedure is to have a spare pot on
Tank furnaces are built of rather expensive large flat bricks. At the higher price and quality levels, these are be priced at $25-30 and up each and a dozen or more may be required for the liner, with additional blocks not costing quite so much, because they are not in contact with the glass, for the upper walls and roof and to back the other bricks.
There is an outstanding sequence of pictures here
2004-Intro of the Kokomo Glass furnace rebuild. Kokomo makes stained glass
and this 12 pot furnace is expected to run for decades. When a pot breaks
or has to be replaced, the wall in front of it is taken out, including the T
block shown, the old pot is removed and the new (preheated) one installed
without taking the furnace down in temperature. This is a recuperative
furnace as are most commercial furnaces, but it is not especially clear how it
works from the pictures. Rather obviously, the hot gases from the furnace
chamber flow up through the "eye", but then what? I think I am correct in
saying that the exhaust flows down through the "vent blocks" shown being laid on
days 12 and following.
The hot exhaust gases are passed through the recuperative chambers being rebuilt
as shown on many pages and the intake air is also passed through these. My
question is which way of: one way of recuperation is to heat one chamber while
the other is being cooled by heating intake air, then change the air flow to
pick up heat from the hot chamber and the other is to build two interlocked
passageways so that intake air is on one side of a wall and hot exhaust gas on
the the other. From the shape of the blocks on
Day 9 I would say the latter,
one passage being the square inside and the other the outside between the rims.
My problem is that they block off the ends of the squares,
Day 30-31 and I don't
see how the gases flow. Maybe I will find out. Neat pictures of a
rare operation in modern America. 2005-09-18
Electric vs. Glass (e-mail query reply) 2005-10-01
There is a lot of discussion of on the CraftWeb Glass Forum about electric furnaces, including from people who build them for sale. The overwhelming point against a glass furnace is the upfront cost - it is possible to get into glassblowing with a gas furnace at a fraction of the cost of an electric and back at the root of my site is "How do I it do it cheap?"
For as long as people have been building electric furnaces the formula has always been to build as small as possible with as much insulation as possible. And for as long as people have been building with Kanthol A-1 coiled elements, the risk has been that the elements are running near the upper limit of the material and they will be damaged if hit by glass or hot batch. Also, the cost of operations must include the time and cost of replacing the elements which seem to last 6-9 months at most. Some designers and builders put in 1 or 2 extra unconnected elements so they can switch to them on failure of one of the others.
Also, electricity in Texas will always cost more than gas for the simple reason that electricity in TX is generated mostly by gas (and coal and nuclear) and there are losses along the way. Electricity is only cheaper than gas where hydro-electric power is common, mostly in the NW US.
The consensus has become that Molydinum Disuliphide (MD) and Silicon Dioxide (SD) elements are efficient in glass furnaces. Both endure the extra heat of melting much better than coiled elements and are available in several commercial forms that a furnace can be built around. Both are fairly low resistance, so are fed off of transformers that reduce the voltage to about 60 volts and increase the amps correspondingly. Both are extremely brittle, especially after being heated, and may shatter if hit by a pipe or if the furnace is moved. Both are expensive (several hundred dollars per element) with SD being somewhat more resistant to glass spatter. The typical installation involves straight elements sticking in the space, standing clear of the walls and pot (to avoid local heating) with the connections on the top of the furnace being heavy bus bars leading to the side where the transformers are not too far away, but still protected from the furnace heat. Lots of insulation.
Because a glass furnace normally requires 1500 watts per cubic foot (in the estimates I have seen), the wattage rapidly gets into the area where 3 phase power is cheaper. Therefore, starting from an empty room with 3 phase available at the back wall (otherwise add the cost of connection to the power grid), there must be reasonably heavy wiring to a control panel, three transformers for MD or SD, three sets of elements, and the proper controller. If operation is attempted with single phase power and coiled elements, then really heavy wiring must be installed, with appropriate power panels and controls. I could do it, but I think most glass artists would hire an electrician. Costly either way.
In reading these notes, not a lot is said about safety because (in part) most articles are tightly focused on a narrow topic and because most of my equipment is used only when I am around it, so many safety features needed if I were to leave the equipment running while away or overnight have been omitted.
"My last posting recommended that a furnace should
be no closer than 18 inches (45 cm) from any combustible surface.
Eddie Bernard of "Wet Dog Glass" sent me the following
from the fire code:
http://www.regoproducts.com/LPmanual.htm has a lot of information on LP gas
and pipe capacity at various pressures. And tank location restrictions
Actually BUILDING an invested pot furnace (one instance) 9/20/93 8/25/96
This article tells of building a pot furnace with vertical sides of a cylindrical shape with a burner port on one side at the rear and a door. The description is based on a furnace built several times by an experienced artist in Texas. It typically lasts for 2 years, sometimes three, being fired for three months or so, three times during the year.
Start by ordering a commercial crucible (pot) from a reliable source (Ipsen, etc.) This will probably cost $125-225 plus $50-75 shipping depending on size. Choose the size to hold enough melted glass for the number of people blowing the number of days you wish blow, 120 pounds per 5 day blowing week for one person being typical, which requires a crucible about 13" OD by 14" tall.
Cut two sheet metal sleeves, one to fit the outer diameter of the crucible and a second larger by Pi times the thickness of the walls, 3" or 4". Both need to be taller than the crucible by at least the thickness of the base (3-5"). Using the 120# crucible and 4" walls, the inner sheet metal is 18"+ by 41" and the outer 18"+ by 67". This height, with the OD of the crucible, makes open volume above the glass greater than the volume of glass, a good ratio being 1.5.
Use sheet metal screws to make a cylinder of the outer piece, (or use a narrower sheet metal to make shallow ring mold) and use insulating castable (see below) to make a base disk (if 4"x 21"dia., 0.80 cu.ft.) It may be best to build the unit on the base where it will be used, so the lip of the crucible is just near waist level, although building the disk on the ground is usually fine.
When the base is fully set, place the crucible centered on the disk inside the metal shell. Pack insulating castable around the crucible and up the inside of the shell and level the top. The lip should be an inch or so above the castable. (0.5 cu.ft with 4" wall, allow .15 cu.ft for sloping sides of crucible.) When set, dust the top of the insulation with plaster dust or other separator to allow removal of the top if the crucible or base cracks. [One comment:: "This won't work, asking for trouble."]
By this point decide whether burner and access ports will be carved out of the set castable or whether an insert will be added. The latter will save castable and work. If inserts are chosen, they can be stiff metal or (more easily) carved Styrofoam. The entry port (if using a separate glory hole) should be about 8" diameter, the burner port about 3-4.5" depending on the burner size. While the burner port can be in the top, in this example it is in the side wall about 3" above the crucible lip. Both openings should be a blunt cone shape, smallest at the outside.
Remove the outer sheet and replace it above the crucible, somewhat over lapping the lower wall. Add legs if needed for support and alignment. Place the inner band of sheet metal (with screws on the inside) around the rim of the crucible and pack insulating castable between the two. (0.5 cu.ft with 4" wall.) Make the top level and even. When set, remove the inner and outer sheets.
To make the top refasten the outer sheet (or use the narrower sheet from the base.) Make a smooth mound of sand, clay or plaster about the diameter of the crucible and about 1.5-2" tall. Cover with soft plastic. Place the outer ring and pack insulating castable carefully. The dome will arch the inside for more even heating and stress. Making the lid thicker than the walls is cost effective because of the higher heat at the top. (0.8 cu.ft. if just over 4" at edges, 1.0 cu.ft. if 5".) Make sure the top is completely set before lifting it.
Use Mizoo castable.
When assembled, carefully heat with a small flame to bring the temperature up past 250 (water removal) then past 500 (setting the inner surface and vapor drive out) then up to 800-900°F to drive off chemically bonded water and then finally up to melting and cooking temperature (1800 to 2450F.) The whole heating process should take 18 or 30 hours the first time.
Uses a Giberson Ceramic burner nozzle on a Ransome [CA] venturi (about $? total) without a blower, fed high pressure propane from a 100 gallon (500#) tank filled by a delivery truck. The vent for the burner is the door, which originally fit too well and had to be carved back to allow a gap.
$20 Metal or block base for height, $165 500 pounds Mizoo castable Insulating Castable, $20 10 feet x 18 inch sheet metal, $200-300 Crucible, Ipsen, $175 Burner Plumbing & Regulator, Tank leased from propane supplier
Building a Glass Furnace-SUMMARY Description
Decide how much glass to melt, 30-50 pounds is a good middling number.
Buy or build a crucible (pot) to hold that much glass. (Build
is much cheaper, see Independent Glassblower #11 for several
recipes and construction choices; my recipe for a single choice
from them.) Decide whether enough natural gas is available to
provide 250,000-400,000 Btu/hour. If not, setup for propane.
Decide whether the furnace shall be "indoor" (but
certainly not under the house roof), semi-outdoor (roof, but
poles not walls) or outdoors. If to be built where there is
zoning decide whether to tell anyone about it (glass at 2400F
requires a special use permit in Dallas even when in Industrial
Glory Hole of Insulating Fire Brick (Include Sketch) Begun 1/24/93 Rev.2/14/93, 8/22/94, 9/19/96
As I build my equipment on a limited budget and grope my way toward being able to blow more than once a year, I have been following a sequence that seems logical: So far it has been annealer, flat grinder, and small glory hole. This is some notes on the last. This is a recipe, like in a cookbook. It tells how one person did it and why. NOTE: I now (8/22/94) feel that the most important step in furnace/glory hole design is the door. Each door design requires a different structure and thinking through the door will require that the frame work for that door be included in the overall design. My design (below) has very little structure above the base. This made adding doors and other features more difficult. My rolling door design works very nicely. It is best if there is a frame structure up to the top of the furnace, with the opening even with the front of the frame. This will allow adding any of the following: a variety of doors, bracing rails to hold in part of the furnace, mounting rails for the burner, sheet metal holders for cheaper poured insulation, and sheet metal holders for weather protection.
Originally, I planned on building a glory hole about the size of my final needs using ceramic fiber blanket inside a barrel (12-16"ID, 3" insulation.) I changed my mind and built a smaller one from insulating fire brick for several reasons:
Insulating fire brick (IFB) is sold by A.P.Green only in boxes of 25 in various temperature ranges. The 2300 degree bricks are $1.82 each, while 2600 are $2.41 (45.50/60.25 a box. Other places sell singles at over $3 each.) I bought the higher temp for future flexibility. I also got 8 ordinary hard fire bricks ($1.38 each). Bricks are 4.5"x9"x2.5" which means they are modular (line up) in two directions but not the third. [I should have bought 5 splits (4.5x9x1.25) also, did later, for the roof, see below.] I had an unblown burner, 78,000 Btu, from Seattle Pottery for $33.50, which delivers Btu about half the cost of other units (0.43 $/kBtu vs $1.16 for a 99,000 Btu, $0.71 for 200,000), because, they tell me, they buy them in bulk for the kilns they make.
After trying this burner for a while, I ended up buying a blower for $40 and building a burner head of pipe (covered in BURNERS) To avoid extra structure, I decided to make a unit that would be bridged (topped) by a brick. That meant 8" wide (1/2" support at each end) and 9" tall using IFB on end for the sides. I planned to use some bricks to reduce heat leakage at corners, simply setting them in place. [I later actually used mostly scrap insulating blanket from the annealer.] I could make various depths depending on how I used the bricks. If I went with 2.5" wall thickness I could get a lot more depth than if I tried for 4.5" all the way around. I had originally planned on 4.5", which would have required all the bricks in a box, but found that making an access hole for the burner flame was stupid unless it went through a brick in the 2.5" direction. For stability and fire proofing, I decided to build on a concrete square I just happened to have around (okay, it was a footer left over from leveling my house, 16x16x4" about $3 at the friendly local concrete lot.)
Problems during early use: Unclamped bricks separated with
heat, leaking flame. Originally the burner was installed in the
hole, resulting in blowback and unburned gases in the chamber
even though half the front was open. The burner was pulled back
out of the hole, firing its flame into the hole and carrying air
with it and the rear bricks were pulled apart to provide a vent.
Later a pipe was added as a chimney and the size of the vent was
controlled with ceramic fiber blanket. Even later, a blown burner
This picture shows the firebrick stacked unit with frax and fiberglass insulation added outside with the door made of clamped up IFB rolling on wheels at the bottom of the frame. The 20# propane tanks are manifolded using a commercial Y connector with a valve to share the flow to reduce freezing. The shield/yoke in the picture is still being used in 2005. Hose is not a great choice because if in a fire, it can burn through releasing high pressure propane. Hose must be rated for propane - not air compressor hose.
The firebrick propped up was removed not long after. I
replaced the lower IFB at the front entrance with regular fire
brick and clamped it, which cuts the opening permanently in half
until I rework it. I probably need to consider a door frame, a
holder, maybe for each part to keep support of the soft brick.
I melt glass cullet in small clay crucibles (pots) that I made
from a recipe in The Independent Glassblower, notes available. I
waited far too long to do this and tried several things along the
way, including clay pots, Corning Ware, and Corelware.
How I built my second glass furnace (Building a cast dome
I mixed vermiculite and water glass (sodium silicate) about 1 quart to a cubic foot to make a sticky mass and pressed it into the bottom of the barrel about 3" thick. After setting for a day, it formed a firm layer.
I mixed enough hard castable for a layer 1" thick and poured it on the vermiculite to set. I next should have cast a drain port from a small arch of Styrofoam and a paper form to make the emergency drain. I actually did this later and it was a lot more hassle. It should be as long as the total wall thickness. Cut a hole in the barrel just above the poured floor and line up the port with the hole.
I made sheet metal insert three inches smaller all round than the barrel (which is 24" so the insert was 18" in diameter, needing to be 58" long with the overlap. [58=18 times Pi (3.14159) ]) I used sheet metal screws from the inside to hold the circle. I again mixed the vermiculite and water glass and packed it into the space, working hard to keep the metal shell circular. Make sure you get heavy enough metal, mine was too light (I used leftover flashing, 26 gauge is better choice.) The top of the vermiculite should be 1-2 inches below the top of the barrel. I let that set for a couple of days.
After the vermiculite had set, I undid the shell and reduced its length by 6.3 inches for a diameter change of 2". I drew a 16" circle on a piece of plywood and cut the circle with a saber saw to form the base to hold the form shape and aid in keeping it down. I mixed hard castable and carefully placed it between the shell and the cast vermiculite, working around the form to keep the weight even and holding the base down with several bricks. I added castable up and over the vermiculite to form a top ledge.
Investing the Pot - If you are making an invested pot furnace, place the pot inside the walls and add insulating castable around it. The top inch or more of the castable should be Missou or other hard castable as molten glass will dissolve softer insulating castable.
Casting the Dome - I made some mistakes, which I may pay for in reduced life. If not doing an invested pot (see above), place the pot in the lower portion, on fire brick to raise it to the height of the lower lip of the gathering port. I tried to cut a disk with a hole in it for a lower support for the upper part. I should have cut the end off another barrel (bottom and sides) and cut the hole in the bottom because my efforts warped the flat steel. The hole in the disk should be large enough to allow castable to cover the edge of the steel.
Cut a piece of Styrofoam to the shape of the inside of the dome. I cut a rough pyramid from a block I had bought then rounded the shape to match the diameter of my opening and height above the pot I wanted. I then used long thin dowel to "nail" two cylinders to the dome molding, one for the burner port and one for the gathering port. Each of mine were six inches.
Saber Saw to cut plywood and barrel if desired. Welding Torch or Electric Saw.
History I fired up the furnace in the early fall of 1996 and had a whole series of problems with the burner plumbing which can be summarized by saying, one burner = one regulator+one blower. This may not be true with an industrial strength blower, but life is much easier if it followed.
It became obvious fairly early that not casting the dome on a flat plate was going to require stuffing some holes with castable or frax or both. That does not seem to have caused serious problems.
In the process of solving the problems, I twice let the glass
freeze in the pot, once when I couldn't get it hot enough and
once when I ran out of fuel. This placed a severe strain on the
pot and a crack appeared while reheating after the first freeze.
Then, in a maneuver I can't recall reasoning out, but remember
specifically doing, I turned the pot so the crack was in line
with the burner flame and when I got the problems with the
plumbing fixed, the crack enlarged and drained the glass into the
base of the furnace. Which is where I stand at this writing (11/26/96)
I am going to have to lift the dome (and maybe cast a new one),
remove the pot. Maybe chip out the base of glass. Install a new
pot and try again. Make another pot somewhere in here. [When redone, still being
used with second pot in 2003]
One of the problems of maintaining a web site is losing track of things, so
below I have the statement "I didn't write this down before" while above I have
detailed steps in making it which were much further up the page and many years
earlier, except for the last note. 2009-12-03
How I built my domed furnace
At right is a picture of my furnace as it has been sitting around for a couple of years (years???). The door shown here was made in a pie tin with a couple of bolts cast in place and broke during moving. A much better design is shown further down.
This unit consists of about half of a 55 gallon drum forming the bottom, a steel frame to hold it along with the rails for mounting the door and the burner (not shown), and a piece of the other end of the barrel to hold the dome. The barrel was cut with a reciprocating saw with a metal blade. The end holding the dome was trimmed to have a low edge under the opening and a higher back. Most of the middle of the flat end of the barrel was cut out, leaving about a 2" rim to support the dome, the rim being protected from the heat by the castable and a layer of frax between the two. [more views below]
of the Domed Furnace
Added long after most of this page was created, these pictures show views of the domed furnace being assembled and the relationship of the parts. Also, since they were added after the site was moved, these are larger pictures that can be clicked to be seen bigger.
This first image shows the use of the barrel lid as a base for the dome and the structure of the barrel body as well as the method of hoisting the dome. The view is almost directly from the front of the frame and base but the dome is turned right from its final position. The lifting method is a come-along to the roof structure of the rain shelter with added bars above to take the weight - over 100 pounds.
The nylon straps are added insurance due to lack of trust both in the steel wiring and the welding of the attachments of the wire to the base ring. The latter were trusted near the end of the process when the dome was much closer to the base and the straps were removed to keep them from being trapped.
This view shows the castable insulating refractory connection of the base of
the furnace to the 4" conduit flue riser along with the "valve"
As seen above the support for the connection runs up the side of the furnace to
the upper frame bar and a flat diagonal strap provides bracing. The box for the
connection is relatively thick sheet metal so welding to it is strong.
The is the door setup that I have just prepared to use. The bolts go to a steel band, not to something in the castable. The band is welded to a steel loop that was raised 1/4" off the flat so castable would flow under it. The ends of the the steel loop (at the bottom) are linked by a bolt so the loop is in compression on the castable. The door was cast by laying sheet plastic on a flat area, setting the steel frame on the plastic, then folding the plastic up over the frame and moving bricks in to the sides to hold the plastic up. Castable was then mixed and poured into place, rodded to debubble it and settle it and then the plastic was folded over to keep the whole moist while it set. Lines from the folded plastic show on the surface facing us.
Dual purpose barrel furnace/gloryhole
Steps in construction: (lots of details left out.)
Support frame and hoisting mechanism for furnace. Angle iron if use once, pipe slipped in pipe if repeat, as for portable. Weld a frame that extends from back about 6" and from front and middle about 2 feet. Along the line of the barrel, attach two (8-10 feet) long pipes or angles for pull handles. Tip the barrel on the back frame then down onto the mid frame and finally forward over the front frame. Under the back frame, add another frame to catch the back at this new height. Lift the barrel onto the back frame and add front frame to support new height. Add back frame, etc. Rocker frame or hoist frame? How many inches gained on each move with rocker? Hoist with lever across center of frame instead of dual come-a-longs or jacks. Raise with single hydraulic jack, bottle jack placed in middle (hollow frame)?
There are two basic designs for recuperative and they
both work, but the added expense is worth it only if you are
pulling a lot of glass.
From Craft Web Glass Forum with permission
Back to Equipment Sequence
Contact Mike Firth