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Frequently Asked Questions

Kiln Shelf Questions

Devitrification Questions

Float Glass & Compatibility Questions

"Tin-Side" and "Air-Side" Questions

Miscellaneous Questions

Kiln Shelf Questions

Q: Will cordierite (ceramic) shelves be sufficient to use in glass fusing?

A: 

Yes, cordierite shelves are good for glass fusing, but make sure to properly kiln wash your shelves to prevent the glass from sticking to the shelf.

Q: How do I keep the glass from sticking to my kiln shelves?

A: 

Use Fusing Farm's Ultra Primer, to prevent glass adhesion to the shelf during fusing.

In fusing, the texture of the shelf is very important, especially in transparent and translucent pieces, because the texture transfers to the piece, and becomes very visible. Ceramic artist may not be familiar (or comfortable) with the thinness of the layers of kiln wash used in fusing. The kiln wash is often watered down to the consistency of milk (I like whole milk consistency), and is applied in three or four thin layers in alternating directions, to make sure that the body of the kiln shelf is covered and that there is minimal texture.

Some artists will pre-fire their shelves (with the kiln wash on them) prior to using them the first time. Other artists will scrape the surface clean and re-apply kiln wash before each firing. You methodology will depend upon the desired results on the finished pieces and the type (top temperature) of firing that you are doing.

Q: Will silicon carbide shelves work when fusing glass?

A: 

Yes, but if you touch two different elements simultaneously with the shelf, you will short out your kiln and receive a shock. Be sure your elements are safely tucked in their channels before using these conductive shelves.

Devitrification Questions

Q: How does thermal mass affect my firing (create devitrification)?

A: 

The amount of stuff (ceramic and glass) that you have in the kiln, is called the thermal mass. The more thermal mass you have in the kiln, the more time (and energy) it will take to get up to temperature. This can become an issue not only for the pocketbook, but also for the devitrification of the glass if the rate of climb between 1300 to 1400 degrees Fahrenheit, is slower than 250 degrees Fahrenheit per hour. This also becomes more of a problem on the way down, if the rate of fall is not greater than 250 degrees per hour (more thermal mass, the longer it will take to fall through this range, especially if the kiln is not vented).

Q: How do I prevent devitrification?

A: 

There are four simple steps to preventing unwanted devitrification: Make sure that your glass is clean. (Do not use a commercial glass cleaner such as Windex, because they contain polymers that create devitrification during the fusing process. Handle your glass only by the edges once it is clean. (Use white cotton gloves to prevent the transfer of oils to the surface or edges of the glass.) Make sure that you are using a non-devitrifying surface up. Most art glasses are formulated to help prevent devitrification. When firing with float glass (window glass) place the tin side up (see tin-meter).

Make sure that you are going faster than 250 degrees Fahrenheit per hour through the devitrification range (from about 1300°F to 1400°F). Be aware of thermal mass if you have an underpowered kiln, if you are kiln casting large objects, and/or if you have multiple shelves in a side fired kiln.

Use Fusing Farm Freedom Flux as an anti-devitrification surface modifier.

Float Glass & Compatibility Questions

Q: Is all float glass compatible?

A: 

Most of the commercially available float glasses are fairly similar to one another. The COE of any given float glass will depend on which plant it came from, and who tests it (which is typically not the glass manufacturer). Some glass distributors call float glass 82 COE, others call it 85 COE, and still others call it 88 COE. I have tested glasses labeled all three of the above COEs, against the standard that I use, and found them all to be compatible. In another circumstance, I have also tested two samples that were both marked the same COE, and found them to be incompatible with each other. Always test all of the glass against a known quantity before embarking on a major fusing project. For more information on testing compatibility and COE see Kiln Firing Glass: Glass Fusing Book One, by Boyce Lundstrom.

Q: Are your frits compatible with float glass?

A: 

Our frits are recycled from an art glass that is tested, using the dilatometer method to be 88 COE. This is COE is compatible with most commercial float glass manufactured in America (Canada, USA, and Mexico). I have used the stressometer method to test this against many of the float glasses manufactured in America, and have found it to be a good fit. I have also tested it against float glass that is distributed with the "82 ± 3 COE" label and have found it to be compatible. Always test your glass against a known quantity before embarking on a major fusing project. For more information on testing compatibility and COE see Kiln Firing Glass: Glass Fusing Book One, by Boyce Lundstrom.

Q: How are your frits different from float glass?

A: 

Our frits are manufactured from an art glass that is made to be the same COE as most American (Canada, USA, and Mexico) float glass. Although the COEs are similar, the formulations are very different. The art glass frits are "soft" in comparison to the "hard" float glass. This means that the frits will soften and fuse at a lower temperature than the float glass. Depending on the configuration of your kiln, the arrangement of your project, and your firing schedule; you will typically see about 25 to 75 degrees Fahrenheit difference between the two glasses in order to achieve the same state of fusion.

Q: How do I anneal float glass with Fusing Farm Float Glass Compatible Frits?

A: 

The difference between the two glasses will also require some alteration when annealing after fusing the two together. Different glasses will have different annealing points, ranging from 1100°F to 900°F. When fusing different glasses together it is advisable to go slowly through this entire range. The annealing schedule that I use when firing recycled glasses, with art glasses, and with window glass is:

Annealing point float compatible frits, 1070°F. Annealing point for float glass is approximately 975°F.

Q: Your float glass is 88 COE, is that compatible with 90 COE?

A: 

Although authorities have stated that a difference of two points in COE is close enough to fuses together, this is not always true. I have known artist that have successfully fused 90 COE frits to the surface of 88 COE float glass, but their rate of success is often less than 75%. When fusing float glass over 90 COE their success rate falls to less than 25%. The difference not only in expansion rate (COE effects the annealing point), but also in physical & chemical composition makes fusing these different glasses together very difficult. Always test your glass against a known quantity before embarking on a major fusing project. For more information on testing compatibility and COE see Kiln Firing Glass: Glass Fusing Book One, by Boyce Lundstrom.

"Tin-Side" and "Air-Side" Questions

Q: How do I detect the Tin-Side and Air-Side on my float glass?

A: 

The way that float glass is manufactured, is by floating the molten glass on a bath of molten tin. During this process, small particles of metal are imparted to the surface of the glass that faces the bath of tin. The other side is exposed to the open air of the batch chamber. Although this tin surface is not visible to the unaided eye, it can be detected using a tin-scope (tin-meter, or high powered UV light in a low to mid range spectrum). When light is shined through the surface of the glass the tin-side will appear to appear foggy, while the air-side will not become heavily clouded.

Q: How can I detect the tin-side without a tin-scope?

A: 

Another method of detecting the tin-side of the glass is by feel. This method is somewhat subjective, and can be difficult for the inexperienced and the callused of finger. The tin-side of the glass will feel rougher than the air-side of the glass. When undertaking this method it is important that both the glass and your hands are free of oils. Of course it will be important to clean the glass afterwards in order to prevent your hand oils from devitrifying the glass during the fusing process.

Q: What is this about Tin-Side of the float glass and why does it matter?
Q: What is the non-devitrifying surface of float glass?

A: 

The way that float glass is manufactured, is by floating the molten glass on a bath of molten tin. During this process, small particles of metal are imparted to the surface of the glass that faces the bath of tin. This tin side of the glass is less likely to devitrify during the fusing process. It is advisable to have the tin side up (exposed to the air of the kiln) when fusing the glass that you do not want to devitrify.

Miscellaneous Questions

Q: Can I fuse together my own recycled glass?

A: 

Every glass is different. Even within a fusing system there are frequently variations in Coefficient of Expansion (COE), heat adsorption rates, stiffness, chemical and physical properties; that make for ‘difficulties’ in fusing. Simple answer is that you should always do a compatibility test on any glass that you plan on using against the base clear glass that you plan on using. For more information on testing compatibility and COE see Kiln Firing Glass: Glass Fusing Book One, by Boyce Lundstrom.

From my personal experimentation I have found that a lot of wine bottle glass (green) is mostly compatible with most window glass. I have found particular Italian greens do not work. I have found that Skyy Vodka bottles (blue) do not work. It has been hit or miss when dealing with Japanese Sake bottles. You should always do a compatibility test on any glass that you plan on using against the base clear glass that you plan on using for your project.

Q: Can I fuse glass in my ceramic kiln?

A: 

Yes. Glass fusing process temperatures are considerably less than those used in most ceramic processes. That being said, firing glass in a “side-fired” ceramic kiln is a bit different than firing in a “top-fired” fusing kiln. Because the glass surface does not receive the direct radiant heat from the elements, it is necessary to fire slower than you typically would in a fusing kiln, to allow the heat to soak to the center of the project/kiln shelf. One of the advantages of fusing in a ceramic kiln is that the large volume of the kiln will allow you to stack several shelves, and therefore be able to fire several projects at once.

Q: What is COE and why do I need to know it?

A: 

Coefficient of Thermal Expansion (COE, CoE, CTE, or ECTE) is a number that denotes the rate at which something expands and contracts in a particular temperature range. In glass fusing, when two glasses have the same COE it is said that they are compatible, meaning that they will be likely to fuses together without creating major stresses that will tear them apart. Although COE is a major indicator of compatibility, it is not the only factor; physical and chemical properties of the constituent parts also play a roll in compatibility, as well as the way that the fused whole is treated (see annealing).

Q: What is annealing, and how do I do it?

A: 

Simply put, annealing is the controlled cooling of the glass to minimize the stresses between the constituent parts of the fused whole (inside vs. outside, center vs. edge, one color vs. another color, etc.). Annealing is a complicated subject, that entire volumes are devoted to. There is a lot of theory about what annealing is and how to go about it, but it boils down to being more of an art than a science.

Each glass has its own specific annealing point; this is the temperature that will allow the most amount of stress to be released with the least amount of time dwelling there. Different glasses will have different annealing points, ranging from 1100°F to 900°F. When fusing different glasses together it is advisable to go slowly through this entire range.

The annealing schedule that I use when firing recycled glasses, with art glasses, and with window glass is:
  • 999°F/hr to 1100°F and hold for 15 to 30 min. (depending on size/thickness of piece)
  • 100°F/hr to 1000°F and hold for 0 min.
  • 50°F/hr to 900°F and hold for 0 min.
  • 150°F/hr to 600°F and hold for 0 min. (past this point natural cooling takes over for most kilns)
  • 400°F/hr to 250°F and turn kiln off.

  • When the kiln temperature reads that it is at 250°F it typically means that the glass temperature is at or below 300°F. When the glass temperature is below 300°F it is much more resistant to thermal shock, and the kiln can often be cracked at this point to allow for rapid cooling. Of course if your ambient air temperature is cooler than room temp (72°F), or your glass piece is thicker than half an inch, it is advisable to allow the kiln to naturally cool.

    Q: How do I use Fusing Farm SS Primer as a cold treatment mold primer?

    A: 

    1) Clean the stainless steel. I would suggest using a turpenoid, or paint cleaner to remove any coating that came on the stainless steel from the factory. Barring that, you can use a mixture of soap and water, with borax (as a grit) to clean the surface of the mold.

    2) Once clean and dry, you can begin applying the SS Primer to the surface of the mold with a brush. I use a sponge brush in order to get a nice smooth surface with minimal texture.

    3) Let air dry. A single coat should be sufficient to release glass. Second coat not necessary, but use your own judgment based on experimentation.

    4) If there is too much texture, use an old nylon, or old used up fine grit sand paper, to remove any rough surface texture or brush marks.

    5) You can now use the mold to slump in. If you are going to full fuse temperatures (1400°F+), I would suggest firing the mold in the kiln to 900°F to make sure the SS Primer is dry and to burn out some of the binding agents (NOTE: this will make the primer somewhat less durable, but still tougher than regular shelf primer on a stainless mold).

    6) After using the mold, remove from kiln and inspect for blackening, or bubbling, of the surface.
  • If any is spotted, use soap, water, and a wire brush to remove coating and go to step #2.
  • If not the mold is already prepped and ready to go for the next firing.
  • Q: What is the right temperature for tack, fuse and full melt for the Fusing Farm 88 COE glass Frit?

    A: 



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