Warm Glass
Sagging & Fusing

Rev. ... 2002-10-13, 2003-02-12, -05-15, -07-20, 2004-08-22
2005-03-09, 2007-09-18, -09-25, 2008-04-28, 2010-03-11, -12-11

Return to Sitemap

What Can I Do With Glass? Warm Glass

My Warm Glass
Play sand Box to make curved clay molds
First Bowl Mold
Bottle fused to bowl
Clay half cylinder mold for whirly
Spinner ends for whirly
Sagged bowl from recycled plate glass
Fused bowl of fragmented thin window glass
Fused bowl of thick window glass fragments
Thin clay bowl molds - oval and heart
Clay form water features
Clay molds and suncatchers
Wind Chimes NP


Intro: I had not intended to have a separate page for Warm Glass as I don't do much of it and there is a good site devoted to it. Brad Walker's http://warmglass.com/
But I did some when I first built my annealer and I have done some recently in connection with the wind related pieces and the Empty Bowls project, mostly using one technique, so I shall put some pictures and comments here.  Most recent thoughts at bottom


Warm glass is that portion of glass work done in kilns, falling between hot glass done at the furnace and at the torch and cold or flat glass involving stained glass, etching, painting, etc.  Warm glass has come into its own since 1980 with the development of glass tested to be compatible among all its colors, led by Bullseye glass, with kilns that have element placement favorable to sagging glass, and with lower cost easy to use controllers that will ramp down for proper annealing.
Warm glass has three major aspects: sagging, fusing and casting with the first two often combined. Sagging is the forming of glass by heating to a softened state so the glass drops into a mold or through a ring.   Fusing is the welding together at a higher temperature of pieces of glass to form a color or texture pattern.  Fusing may be used to form a colored pattern that is then sagged into a mold.  Casting takes place at the highest temperatures and involves melting the glass until it flows into a mold.  Like all glass, warm glass must be annealed with the additional factor that the thickness of the mold material must be allowed for in the annealing time.
Because glass changes viscosity over a wide range of temperatures, unlike metals which abruptly melt close to some specific temperature, it is possible to control the effects with time - stopping sagging in mid drop or fusing until just the  desired merging of pieces has occurred.  Because glass colors do not mix, one common operation in fusing is stacking layers of colored glass to fuse into a thick sandwich which is then sawn into slices which are arranged into a checkerboard or other design and fused again to produce a sheet that can be sagged to a plate, bowl or plaque.  Fusing can result in raised pieces having very slightly rounded corners, having a slighted domed over look or fused flat to look like a sheet with colored details.  Sagging may be controlled to stop glass in mid-air or just as a dropped center touched the base.  A common form of sagging is making curved windshields for cars and curved corners for furniture.
Among the more recent developments is the roll-up in which a fused sheet made using kiln techniques is reheated and picked up on a hot glass collar, sealed on the join and blown to form a vessel.  [Older techniques involving arranging murrini or cane on a plate and heating/fusing in the glory hole normally has no detailed design pattern and is picked up on a clear bubble of glass rather than just the collar.]  2008-04-28

My Warm Glass

Samples of glass work of Mike.During the time between building my annealer (after my first glassblowing class) and getting some small form of furnace going, I used my annealer to fuse and sag items.. Since removing some of the insulation to increase the inner size, the kiln with a single 12 amp element does not reach fusing temps. I am still using the old element, hooked where it is broken, and using both allows me to get up to fusing temp. My elements are Kanthal which has a higher temp range. If it were nichrome, which is okay for annealing, it would sag or oxidize badly at fusing temp (about 1450-1550F)
The two not-round pieces in the picture to right that are fused represent two directions in my work. Both were sagged on clay forms. The green piece is Coke bottle glass, cut apart and arranged on a clay sheet into which had been pushed an aluminum tea cookies sheet, so the clay carried the form of ridged squares. The glass was laid on in pieces, so the square textures would enhance the shapes of the pieces.
Windchime with fused glass elements and cherry wood hanger.The piece at the right is clear and blue tested compatible Bullseye laid out on a gently undulating clay form, using just the portion with a concave curve. A nichrome wire fused between the clear and blue makes the hanger. Most of the items made in the last style were tied up on shapes of cherry wood to make small wind chimes as shown at right.

 I have been working on various ways of controlling the form of the thin clay shells so that I can sag to make glass wind foils and forms. One problem that needed solving was making the compound curve for the end of the whirlyjig.
The image at right shows a preliminary idea, but the clay form has got to have lips/ledges to support the glass before it sags, so it doesn't sag over the outside.
Slump mold
This box was originally (over-)built to provide a strong step for a person to get into my van meanwhile holding tools.  The lid is just 1/2" plywood resting on 2x6's with routed in end handles and a rope loop shoulder strap.
When I started casting, it dawned on me that I could do rough casting of extra metal using play box sand, plus set the invested molds for pouring with support of the sand around them, plus have a fire proof place to set down the crucible of molten metal when transferring to the pouring handle.
Box of sand used for casting and laying out clay forms
It took me a bit longer to consider using it like this: scooping out a shape in the sand, which has been wetted down to hold its shape, rolling a clay layer and forming it in the scooped out area.  The water in the sand moderates the drying of the clay.  For better or worse, some sand imbeds in the clay.  This is the form for sagging a blade for a 3 blade upright glass whirly. 2003-07-20
BOWL MOLD - Here is a bowl mold and two bowls made for the Empty Bowls project from grey smoke 1/4" thick glass I found in the trash.
The mold was made by laying a Corning Corelle ware breakfast bowl down on waxed paper on the counter, rolling a thin layer of white potters clay and forming the layer over the top of the bowl. As the clay started to dry and shrink it began to crack, so the clay was lifted up and the bowl removed.
Bowls donated to Empty Bowls made from smoke glass cut irregularly
 The layer seemed too thin (about 3/32"), so additional clay was rolled and pieced around the rim on the bottom side to make it stronger. A wet finger and added water smoothed the wrinkles in the bowl and it was fired in the kiln to at least fusing temp. It was then painted with kiln wash, which being thick, filled some cracks and bubbles. I did a rough drawing on paper, starting with a circle the size of the depression in the mold and adding a wavy scallop edge. I then put the paper under the glass and informally followed the drawing as I cut the glass. I have not got a lot of experience with cutting thick glass and was using an ordinary cutter, not a blunter one commonly used on thicker glass. After I grozed off the more awkward points and used the sickle stone on the sharper edges I set the glass on the mold and warmed it over a couple of hours to 890F and then took it up to 1450F. The first time I did not have vent holes in the mold and did not take it up hot enough, so it only sagged slightly, so I drilled 3 holes and ran it up higher, looked to see that it had sagged. The second one was done in one run. I use the kiln wash from Paragon kiln made for glass work. 2002-02-07
FUSED BOTTLE - After taking apart a beer bottle, it was laid in the bowl mold above for both fusing and shaping. As discussed at the link, it was a failure because the small pieces from the neck used as filler slipped and/or sagged out of position. The small pieces were used for compatibility of glass, but with more time, they should have been sagged flat or to a proper curve before use. Suggestions about labels. Bowl shape fused from cut apart bottle
CYLINDER MOLD - This mold was made to form hemi-cylinders for a whirly gig The goal was to have a base for strength and a curve. This piece was made from half a dozen pieces of window glass laid in place. The mold was made by finding a curved piece (of conduit in this case) which was laid on a flat counter on waxed paper. The clay was rolled out on a thin plastic sheet which was laid over the curve and shaped on the end. Fusing a semi-cylinder on a thin mold.
The curves were carefully worked in to make a smooth transition. As with the bowl above, when it began to dry, cracks showed at the corners and the core was removed carefully to let the clay dry fully. It was then fired, cooled and kilnwashed (the pinkish color).  While I was photographing this, it occurred to me to wonder why I hadn't just planned on cutting a bottle in half lengthwise for the shape of the glass. Two or three answers are 1) a choice of glass color - I was thinking of stained glass. 2) The high failure rate cutting across the bottom of bottles and the limited choice of size of bottles. 2002-02-07
Here is the piece above and another mounted on a flat whirly jig. They were both made of fragments of window glass fused and sagged in one step. Kiln fused glass glued and mounted for testing on flat whirly gig.
These pieces were made for a three armed flat whirly, like the metal one. This form was made by drooping a cloth across a raised wood edge and down across a shelf and then putting the thin rolled clay sheet on the cloth. The pieces were cut following a rough drawn pattern and quickly cut (note the broken corner on the green and rough edges.) They were rested on the curved top and slowly slumped by programming the kiln to go up to 1250F over 6 hours and shutting off. They were then glued to formed arms and assembled on an axle (no pictures before I took it apart but see below.) Flat whirly end arms
The pieces above made the arms turn, but not very quickly. Since the hemi-cylinder fused unit turns constantly, I decided to make a new mold and re-fuse.
The first attempt used a half of a conduit section, but the result was too narrow inside, so I placed a cloth on the clay, wetted it, covered with plastic to soften the clay.
To get a rim on both sides, a board and a piece of 2x4 were shimmed up to be even with a half section of 4" PVC thin wall (Sch. 20) drain pipe 18" long. The clay sheet was pushed down into the valley and formed repeatedly until smooth. After firing to something over 1300F the (almost) cooled form was painted with kiln wash and leveled with a small piece of fire brick and the glass from above placed in it. Again the glass was slumped by running it up to 1250F on a 6 hour cycle while I was at work. 2002-06-01 And here are the three mounted to arms on a bearing on a post in the front garden where they turn anytime there is a breeze and no tree branches in the way. 2003-05-15
Because of this change, I am thinking of making a wind tunnel to test anemometers and whirly shapes. 2003-12-16
Revised flat arm whirly
Three prong horizontal sagged end whirly
Four more pieces made for the Empty Bowls below were fused using 1/4" plate from a broken table top I found at curb. The awkward cutting and only fusing to round edges slightly were intentional.  All were run to 1400F. Two at bottom are just rough cutting a blank to mold, one mold with rim, other without, although triangle is a whim.  The two immediately below are more complicated. The one just below (B) was sagged on a deep mold to 1400F and didn't drop much.  When redone, the cross piece was added, but it still has not touched bottom, so is rounded and will not sit up.  The one at right (C) was result of initial failure of the one below it. (E)  I planned on legs drooping down on 3 corners, but they broke on the mold, so I trimmed it to the mold shape and re-fired.  Then I took a brunch of fragments and put them in the mold for (D), with a trimmed center bottom piece and the others leaning.  The tip at the top was actually supported on the kiln wall. 2003-02-11

Funky round bottom fused bowl with accent for Empty Bowls 2003B

Fused from fragments of table top glass for Empty BowlsC

Simple sagged bowl from table top glass for Empty Bowls 2003D

Rough cut bowl with triangle accent, sagged and fused for Empty Bowls 2003E

More than one oven or a really big one? -  I have been working on making several one foot long curved blades for whirlies and have had to deal with the time delays of having just one annealer/kiln/oven, so I have to take the time to run a sheet and mold up, then down, then anneal.  One solution would be to make a big flat sagging/fusing kiln with lots of insulation, but I don't really have a use for it.  An alternative is to have two controlled kilns.  By taking all the sheets I want to sag up to the annealing point in one kiln, I could then transfer them one at a time to the mold in another kiln, run that up to 1300 or so, cool it to about the annealing point, move the stiffened glass to the holding kiln and then continue until time for slow annealing.  By stacking or edging, a lot more glass could be gotten in the annealing oven than in the openness of the mold kiln.  2004-08-21

I have often collected glass from curbside, usually thicker pieces of broken table tops that make up the fused stuff above.  Over the last several months, I have collected tempered glass from some doors that were put out at the curb intact and broke before pickup and just recently thin window glass that apparently fell off a vehicle on to the median and then was driven over, perhaps by the vehicle that it fell from.

Screener/separator for glass, etc.When glass is picked up from the curb, it is usually dirty and may have stones and grass in it.  The broken window glass was many different sizes and, in fact, what sent me back to pick it up was the hundreds of middle sized random pieces.  I needed to sort by size and get rid of all of the small dirt and glass.  I had a couple of failed attempts for experience.

The frame at the right has 1" hardware cloth (welded wire screen) held in place by wooden strips.  The wood strips are lightly nailed but are really held by Gorilla Glue.   The corner joints were drilled and dowelled with GG also.  GG expands while setting to form a water proof bond which can be seen at the corners and which I wanted to take advantage of between the wire.  Inside the box is an added 1/2" hardware cloth piece held in place by the bent up edges.  After the first sort through the 1",  added grids of other sizes can be inserted for smaller sorting.  I have a similar box with smaller screen (1/4") as the primary with thinner wood strips, that is used for sifting casting sand of metal bits.

My previous failures taught me three things:  Don't get too big and have bigger catch trays and have several.  The box above is 11"x11" (55mm x 55mm) and is one of the module sizes in my storage system.  Previously I had built a metal box 12x24 (30x60) which is another module.  This was too big to handle with a moderate amount of glass in it and when shaken, it dropped glass over the sides of the catch boxes.   For this 11" box, I have been using 12"x 18" cafeteria-style trays, of which I have several. Having several, or at least two, is necessary because after the first sort, and emptying the frame of larger stuff, it is much easier to put a second tray below the frame, add the smaller screen, and pour the smaller stuff off the first without trying to hold it in a bucket or something.  I also found it easier to knock all the sorted glass out of the frame onto a tray (which has sloping edges) then from the tray put it in storage (ZipLoc freezer bags.)  With the thickness of the wood frame, the stuff is dropping from an opening 10.25" square which allows shaking to work the glass.  By the way, making it of wood means no lower lip, which caught glass and trash in the metal box.

For cleaning, I built a bucket out of 1/4" hardware cloth (no picture yet) that I can slosh up and down in a 5 gallon bucket to wash.


This is a fused layout from the broken window glass.  It is based on a number of larger pieces with medium pieces on top of them.  It is fused (1455F) on a shallow clay mold formed on a small dinner plate. The clay is white (or gray) potters clay sold in 25 pound boxed bags at ceramics/pottery supply outlets and some big craft stores.  After shaping it was fired to about 1500-1550F, higher than fuse temperature.
All of the examples above and the tempered below were fused into bowl form by placing the glass and allowing it to sag and then fuse.  But with the thinner glass and smaller pieces, I wanted to do more placement, so I went to the technique commonly used in the warm glass community of fusing the pattern flat and then sagging to shape.  The sagging (to 1310F) is cooling as I write. 2004-12-19
Flat fuse of broken window glass
The tempered glass bowls are very fragile as there is little contact between the pieces as they don't overlap.  The clay mold, which has fired kiln wash on it, is painted with corn syrup (like Karo) to make the surface sticky.  The nuggets are then placed and pushed around to form a fairly even layer.  Without the syrup, the walls, even as sloped as these are, tend to slide out of the way and pile up at the bottom.  Here some of the pieces rolled away onto the lip while heating up.  The syrup burns away during firing, leaving just enough tack to hold a nice shape. Fused bowl from tempered glass nuggets.


Obviously, neither of these bowls is functional in the sense of holding water although fruit, nuts in the shell, dry arrangements, potpourri, etc. are possible.

Four bowls fused for Empty Bowls project from broken thick window glassThese are bowls being cast almost in a production mode from broken pieces of 1/4" window glass in various clay molds including a long oval and heart.  Usually fused glass is taken up to the temp and a soak done then a rapid vent to about 1000F, but I tried doing a ramp from room temp to 1450F over three hours then a shut down without venting (all unattended) which produces a glossy finish with slightly rounded corners while retaining the look of the broken edges.







Rolled sheet clay molds before firing and applying kiln wash.

These are two more of the clay molds I made for doing bowls, here in a hard dry state, but before firing and adding kiln wash.  These two were made by using stiff cardboard/tagboard to make a flat plate with the cutout opening and surrounding flange support.. The the flat plate was raised with a strip of  the same cardboard about 1.5" wide shaped to fit around the cutout and hold up from a counter.  The clay was sunk into to the opening and smoothed, the flat bottom being formed on the counter.  The edge of the cardboard tended to catch and cut the clay, so repair work had to be done on the back. Air vent holes were pushed in the clay while drying. 2005-01-30, 2010-03-11
clay mold forming plate and support


Dark water feature on clay form in kiln Clay formed water features
I am making from a carefully cut 1/4" plate glass and broken shards of the same glass, a series of glass water features that can be used as sun catchers to be sold on eBay.  The clay form plate was made by rolling out white clay 1/4" thick, shaping wet sand in the casting box to make undulations and sloping edges with a paper form as guidance, then laying the clay down, smoothing into the undulations and letting dry. Mixing grey glass shards on clear glass plate produced strain cracks. (image, link) 2007-09-18, 2010-03-11

Light water feature view of grid and water in basin.

Glass water feature layout of pieces and plateOn a light colored  flat 3/4" plywood board covering a rough workbench, the cut panel and a selection of fragments have been laid out.  All have been washed.  I have dark fragments in two colors, so selection must be made.  Various arrangements are tried.  Generally I use a couple of long pieces to trend the water toward the center (the mold has sloping sides also) and smaller pieces to break up the flow.  A medium piece partly diverts the center flow as some flows over it.  2007-09-22
Mold for water feature panel2007-09-25
Clay molds for sun catchers, a coordinated pair made by sawing a 1/4" plywood disk into 3/4" bands, shellacking, and gluing to paper backing to align.  Clay was rolled out about 1/4" thick, laid down and gently worked into the grooves (and center) formed by the plywood.  When the clay firmed up some by drying, the pieces were turned over and the paper and wood gently removed.  Then the molds were turned upright again on newspaper to even drying and slight touch up was done.  When fully dry, assisted by being outside on 90+F days, they were fired on a sagging run.     2007-09-25
Molds for suncatchers, thin clay, circular

The glass for the pieces below came from the glass walls of a pop corn popping stand that was very hard and would not break along straight lines.  So making lemonade from lemons, I broke it into further pieces and fused on the mold above. 2007-10-30

Fused window glass photographed in a windowFused window glass photographed on outdoor stand against glass.

Contact Mike Firth