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Warm glass generally refers to kiln fired glass. It can be fusing, slumping, casting, or flameworking. (It is not stained glass which is not fired, and it is not blown glass which is done in a furnace at higher temperatures.)
Warm glass can be done in a normal ceramic kiln, with some limits. For large, complex, or thick pieces, you need a lid element to create even heating, especially when fusing. And you need to know the temperature inside the kiln. So you need an electronic controller or pyrometer. Most people doing glass use electronic controllers, because the heating and cooling rates are so critical to the glass process.
When fusing glass, the glass all needs to have the same COE (Coeficient of Expansion.) Otherwise the glass will crack as it expands and shrinks at different temperatures. Art glass is manufactured at various COE's. So that is the first decision you have to make, which COE glass to use. (Note, there is a lot of glass which is made for the stained glass market, and the COE's are not controlled. So you must use glass which is designated "fusible glass" and has a specified COE.
The lower the COE, the harder the glass is and it requires a higher temperature to cause it to melt.
COE 90: Historically the standard for art glass has been COE90. There is a huge range of colors at COE90.
COE 96: But now COE96 is offered by several manufacturers and also becoming very popular.
COE 82: And there is a new COE82 which has a unique advantage. Window glass is approximately COE82, so you can fuse this glass onto sheets of ordinary window glass you can pick up at a window store or home improvement center.
Glass is available in many colors and thicknesses. Usually it is sold "thick" which is 3mm, or "thin" which is 1.5mm. Thin glass is typically uses when building up many layers.
There are several types of glass.
Standard glass is a single color throughout.
Iridized glass, where the glass is coated by a metallic coating and gets a mother-of-pearl effect.
Dichroic glass, where the glass is coated with patterns of brilliant metallic colors. Dichroic glass is amazing looking; it comes in a wide variety of patterns and colors, and the color appears different from different angles. It is expensive so is usually used sparingly. Dichroic glass has a deep glossy finish when fused with a clear piece of glass covering it, and a rich satin finish when fused without a clear piece of glass on top.
Note that glass is also available in various textures, but these will normally flatten out when fused so aren't used in most cases.
There are many options for decorating glass.
Sheets of glass
Frits (crushed glass pieces)
Glass paints (be careful, there are glass paints made just for painting glass, and there are glass paints for firing, you want the ones made for firing.
Similar decorating techniques as ceramics can be used, such as masking and slip trailing.
Fusing is taking various pieces of glass and melting them together into one piece.
Slumping is taking a single sheet of glass (either a solid color or a previously fused piece) and melting it into a mold to take a shape.
Casting is taking chunks of glass and melting them into a mold.
Flameworking is using a flame (from a propane torch) to melt the glass into beads, figurines or other pieces. These are then annealed (cooled slowly) in a kiln.
There are other techniques such as pouring hot glass, or having glass melt into a crucible.
Note, these are approximate depending on the type of glass used, thickness, etc.
Initial Heating Phase This stage, which should take anywhere from 90-120 minutes, is the one during which the glass pieces are heated from room temperature to their strain release point, which ranges from 930 degrees F to 1000 degrees F (500-540 degrees C.)
Rapid Heating Phase The glass may then be brought to the desired fusing temperature very quickly (within 15-60 minutes, depending on the kiln you're using.) Depending upon the size and thickness of the glass pieces, this temperature is sometimes maintained for 5 to 15 minutes, especially during a first, full fuse firing.
Rapid Cooling Phase The kiln is now opened temporarily to cool it, but the temperature shouldn't fall below 930 degrees F (500 C) Opening the kiln helps speed up the work. If you aren't pressed for time, you may also allow the closed kiln to return to the strain release point, or you may open the peepholes.
Annealing Soak Phase At the strain release point of 930-1000 degrees F the kiln temperature is maintained for 15-60 minutes, depending on the size of the piece of glass. This "soaking" time, during which teh fused glass pieces are tempered by allowing their temperatures to equalize throughout, can never bee too long. Always allow yourself sufficient time for this stage.
Cooling Down Slowly cool to room temperature before opening kiln.
Average Temperature Ranges
1040 degrees F (560 degrees C) The point at which glass is heated to its strain release point; glass paints, lusters and metallics mature in this range
1200-1380 degrees F (650-750 degrees C) Slumping of glass in a mold. The steeper the sides of the mold, the higher the temperature must be.
1450-1540 degrees F (790-840 degrees C) Glass fuses. At the lower temperatures in this range the pieces of glass will fuse together but still be distinct (this is referred to as a tack fuse) and at the higher temperatures they fuse completely together (referred to as a full fuse)
Remember, the best molds for slumping glass into are ceramic molds. Since you already know how to work in clay, you can make your own custom molds. In addition to traditional slump molds you are used to, you can also use drop out molds. This would be, for example, a ring. When a flat sheet of glass is placed on a ring mold, the glass will melt through the open area creating its own bottom.