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Tested Compatible

Compatibility is a feature of glass especially important when combining multiple glasses together into one piece in a kiln. Fusing is the heat bonding of glasses at high temperatures (Bullseye will fully fuse in most kilns at temperatures above 1450ºF). Simply stated, two glasses are said to be compatible if, after fusing and cooling to room temperature, they are free of undue stress. Such stress can immediately or eventually lead to cracking.

Bullseye's testing method for compatibility is still unrivaled in the industry. It is based on a standardized base clear testing ("T") glass that has been monitored for consistency for over 15 years. It is against this "T" glass that all other Bullseye glasses are measured. Those, which test as compatible to the standard, will be compatible with each other and with all Bullseye glasses so tested and so labeled in all years past.

Tested Compatible glasses are popularly called "fusible" and are designated by an "F" after the 6- or 8-digit style code.

 

Coefficients of Expansion

Although Bullseye glasses are popularly referred to as being of a "90" expansion, Bullseye does not encourage the use of this designation in describing its glasses.

The "linear expansion coefficient" is determined by a laboratory test, which expresses the average expansion rate from room temperature to 572ºF (300ºC). It ignores the more important range of expansion for determining compatibility for fusing is the expansion through the annealing and softening ranges. It also ignores viscosity, an important element in determining whether glasses will "fit" each other on fusing. All "90" expansion glasses are not compatible.

 

COMPATIBILITY OF GLASSES: EXPANSION
DOES NOT EQUAL COMPATIBILITY

© Daniel W. Schwoerer Bullseye Glass Co., 1997

A misunderstanding that the compatibility or "fit" of two glasses is solely a function of the expansion properties of those glasses has led to an overemphasis on "expansion" and the numerical value of the COE (coefficient of expansion) of glass. Studio artists constantly ask for the COE of a glass - hoping to predict whether it will "fit" another glass such as other fusing glasses or their own furnace glass. Matching COEs is simply not an accurate measure of compatibility.

The viscosity characteristics of a glass are equally as important as its expansion characteristics. Together, these two properties determine whether one glass will fit another. But it will be useful to first discuss each individually as it pertains to this subject.

Expansion affects compatibility throughout the full temperature range (from the annealing point to room temperature). This is because by nature all materials - whether solid or liquid - expand upon heating and contract upon cooling. It is commonly assumed that if they expand and contract similarly they will "fit" or be compatible once fused together.

 

Measured and Calculated COEs

The expansion of a glass may be determined by calculation or by measurement. The most common laboratory test (using a dilatometer) measures the actual expansion properties of a glass over the temperature range of 0 - 300ºC. (A COE number must always be accompanied by the temperature range over which it was measured or it has no value.) Unfortunately the equally important range in this measurement - from 300ºC to the annealing point - is ignored. It is a well known fact that the expansion properties of a glass change significantly through the transition range.(1) Therefore it is obvious that this measured COE number is not intended to describe the expansion characteristics of a glass for compatibility purposes. In actuality there is no one number that can describe the expansion properties of a glass through the full temperature range since it is not constant (linear).

To further confuse the issue many manufacturers publish a "calculated COE." This so-called calculated COE is a meaningless number in comparing the COE of different glasses in the context of studio usage The calculated number (2) should only be used to compare projected relative changes in expansion of a given glass with changes in composition of the same glass or in comparing very similar glasses to each other - such as one soft soda lime glass to another soft soda lime glass. It should never be assumed to represent a real COE. It is a tool that a glass formulator can use to predict changes in expansion when making raw material changes such as substituting magnesium for calcium or sodium for potassium. But I would encourage glass and batch suppliers as well as educators not to publish this number (unless they provided considerable explanation as to its use) because it is very misleading to the user in the studio glass community, implying for his/her furnace melted glass a meaningful COE which it clearly is not.

 

Why A Measured COE Alone Does Not Insure Compatibility

As mentioned above, the fitting of two different glasses is a function of both viscosity (resistance to flow) and expansion. Whereas expansion affects the compatibility predominantly in the lower temperature range - below the strain point, the viscosity properties affect compatibility predominantly in the middle temperature range - from the strain to the annealing point. Differences in viscosity between two glasses will cause compatibility problems. If one glass is stiffer than the other they will strain each other as they cool through the annealing range.

 

Compatibility via Compensating Error

For glasses of different viscosities to be compatible (which is frequently the case) their expansions must be different. What happens in actuality is a process of compensating errors. Two different glasses will be compatible if the strain set up by the mismatch in viscosity is cancelled out by the strain introduced by the mismatch in expansion. (once cooled to room temperature and assuming, of course that proper annealing has occurred). For instance, if the viscosity differences result in tension between the two glasses and the expansion differences result in an equal amount of compression between the two glasses, the two stresses cancel each other out. This is the critical phenomenon that results in compatibility of two glasses with different expansion/viscosity properties. This explains why glasses of very different viscosity/expansion characteristics actually fit (such as a hard opal with a soft blowing crystal). If you were to have samples of these two types of glasses measured for expansion you would find that they could have COEs (3) differing by as much as 5 or more points.

This, furthermore, is why the only practical test for compatibility is one that takes both phenomena into account - tests such as the chip test for fusing and the ring test for blowing. Looking at the COE alone is very misleading and cannot accurately predict compatibility.

 

Controlling The Expansion / Viscosity Properties During Furnace Melts

Accepting that expansion and viscosity both contribute to the compatibility of glasses, and given that one of those glasses may be your own furnace glass, how do you control its expansion/viscosity properties?

In insuring compatibility the melting cycle of a glass is as important as the composition. Melting the same composition glass using different melting cycles may produce a glass of different expansion/viscosity properties, leading to compatibility problems that may not have occurred with glass from a prior melt. Several factors must be considered in a melt cycle:

 

1.      size of the melt (e.g., 500 lbs of batch)

2.      temperature to which the furnace is heated prior to first charge (e.g., 2500ºF for 1 hour)

3.      melting time and temperature (e.g., 12 hours at 2500ºF)

4.      charging rate (e.g., 3 charges of 100 lbs each at 1 hour intervals)

5.      rate of temperature recovery after each charge (e.g. recovered to 2500ºF after 45 minutes)


Based on the examples cited above we might establish a typical melt cycle as "500 lbs melted at 2500ºF for 12 hours, charged in a furnace preheated to 2500ºF in 3 equal charges spaced 1 hour apart."

Since your goal is to produce glass with the same viscosity/expansion characteristics each time you melt, it is imperative - if you wish to avoid compatibility problems - that you follow the same cycle in order to insure the same results. (This is, of course, hoping that your supplier of color is doing the same.)

Changes to your cycle may alter the results and lead to problems of incompatibility. For instance, if your typical melting cycle is designed for a 500 lb melt but you melt only 300 lbs with the same cycle, that melt will very likely yield a lower expansion / higher viscosity glass. Consistency of procedure in melting your furnace glass is critical in maintaining compatibility with your color source (whether bar, frit or sheet). Once a melt schedule has been established as yielding acceptable results - do not vary it in subsequent melts.

It would be better if we in the glass community had never focused so much attention on the coefficient of expansion. We need to stop talking about it as if it defines compatibility. The only measure of compatibility is testing a sample appropriate for the type of forming - whether blowing, fusing, pa’te de verre - and measuring the results. Unfortunately books and manufacturers and teachers continue to print this misguided concept and information. So - please - let's quit talking about COE and talk about the real issue: all the factors that contribute to compatibility between glasses and how we can understand and control them.

NOTES

1.      F.V. Tooley, The Handbook of Glass Manufacture, Vol 2, 1974, pp 906-907.

2.      Of the many calculation methods (among them: English and Turner, OI, Winkelman and Schott) all utilize an expansion factor for each raw material, assume an additive mathematical result, and do not take into account the melting cycle of the glass.

3.      Assuming that all measured COE's were measured from 0-300ºC.

 

FIRING SCHEDULES

 

In heating and cooling Bullseye (or any other) glass in a kiln many variables will impact the choice of a firing schedule. The heating pattern of the individual kiln is critical. The accuracy of the temperature-recording device is also important. Whether the work has been previously fired; whether it was previously taken to a tack or a full fuse; whether the kiln is top fire or side fire; the proximity of glass to the heating elements, the circulation of air within the kiln chamber--all of these factors, and more, will determine the schedule you eventually choose to fire your work.

The charts we have provided for heating and annealing are based on theoretical schedules, modified by experience in the Bullseye factory studio. They are fairly conservative schedules, allowing more than adequate time in heating and cooling for most projects. However, they should be used as a starting point only and adjusted to the specifics of your kiln and your project.

 

HEATING

 

The two primary concerns on heating are 1) avoiding thermal shock and 2) preventing devitrification. Thermal shock - breakage due to excessive heat differential within the glass body - can be prevented by a slow rate of heating below the strain point. Devitrification - a crystalline scum on the glass surface - is rarely a problem with Bullseye glasses manufactured in recent years. Using an overglaze or rapidly heating the glass through that temperature range where devitrification may occur (1300 ° - 1400 ° F) will prevent it.

The following chart is a rough schedule for heating Bullseye glasses. It assumes a first time firing in which none of the individual pieces of glass in the lay-up is greater than 25% of the total glass mass. It further assumes a kiln with top-firing elements.

 

·         For a top-fired slumping of a pre-fused piece, double the initial heating times.

·         For a side-fired slumping of a pre-fused piece, increase the initial heating by at least 2.5 times.

 

HEATING A TYPICAL (12" DIAMETER) COMPOSITION OF BULLSEYE GLASS
Thickness
(inches) 		Initial Heating
(Room temp. to
1000°F) rate (°F/HR) 		Rapid Heating Rate (A)
For Full Fuse
(From 1000° to 1500 °F
& above) (B)
(°F/HR) 		Soak Time
At Full Fuse
(1500°F) 		Cooling Rate*
From Full Fuse
(or Slump)
To 960° (°F/HR)

1/8"

600

(C)

(C)

AFAP**

3/16"

525

(C)

(C)

AFAP**

1/4"

450

1000 ° F

10 mins

AFAP**

3/8"

375

1000 ° F

10 mins

AFAP**

** AFAP = As fast as possible


* Most kilns will not cool this rapidly due to residual heat in the refractories. Allowing the kiln to cool at its own rate between 750 ° F and room temperature is usually adequate for the final cooling stage. This may result in an actual cooling rate slower than that shown above. Cooling the work by opening the kiln door or large vent hole, however, risks thermal shock.

  (A) If you are having problems with bubbles, try slowing the rate of heating up to full fuse. Inserting a half-hour soak at 1250 ° F may also help "squeeze" air from between the glass pieces before the edges seal and trap bubbles.

  (B) A "full fuse" - generally considered to have occurred when the surface of the glass is completely smooth and free of bumps - is dependent on both temperature and time. Some kilns will achieve a full fuse of Bullseye at 1450 ° F with a 45 minute soak. The same kiln may achieve full fuse with a much shorter (5 minutes or less) soak when taken to 1540 ° F. At the factory we typically program a full fuse at 1500 ° F with a 10 minute soak.

  (C) Glass lay-ups of this size (12" diameter) which are less than 1/4" thick should not be taken to a full fusing temperature. They will distort in shape and are extremely prone to bubbles. They may, however, be successfully slumped to a shallow depth or tack fused at ~1375 ° F. See Note (A, SLUMP FIRINGS) above.

 

ANNEALING

 

Annealing, the controlled cooling of a glass, is critical to its longevity. Glasses which are not properly annealed will contain stress which may result in breakage before or at any time subsequent to their removal from the kiln.

The following chart represents a simple schedule used routinely in Bullseye's factory studio. Annealing schedules for thicker pieces (up to 8") are available from the factory upon request.

 

* Most kilns will not cool this rapidly due to residual heat in the refractories. Allowing the kiln to cool at its own rate between 750 ° F and room temperature is usually adequate for the final cooling stage. This may result in an actual cooling rate slower than that shown above. Cooling the work by opening the kiln door or large vent hole, however, risks thermal shock.

 

ANNEALING FAQ

 

What does an annealing break look like?

On a flat fused project (such as the hypothetical 12" disk suggested above) annealing breaks are most typically S-shaped. If the break runs along the interface of two different glasses, it is more likely due to incompatibility. If the broken glass appears to have moved apart (often with some force) during the firing, it has probably broken due to thermal shock. If the glass is broken into a web-like pattern of many small pieces, it has most likely stuck to the shelf or mold. Proper annealing cannot prevent incompatibility; incorrect heating or cooling in lower ranges, or improperly prepared contact surfaces.

 

Do tack-fused projects require less annealing than full-fusing?

Absolutely not. A tack-fused work has many more edges than a fully fused shape. Annealing is always more easily achieved with a glass body of uniform mass. A small glass sphere (the marble) is the easiest object to anneal. As the number of edges increase, so too does the annealing time.

 

Do slump firings require less annealing than full fusing?

No, especially if the glass does not come into uniform contact with the slumping mold (e.g. a dropout mold). Or if the mold itself is not of uniform thickness. In these instances you will need longer annealing times.

 

Can I over-anneal my work?

Theoretically no. In actual fact, if the heat distribution within your kiln is extremely uneven, yes. If you anneal soak your glass in a space with uneven heat distribution, you can increase the difficulty of cooling that work uniformly. An extremely critical but often overlooked factor in annealing is the quality of heat distribution within the kiln. Check yours. Make sure that all elements are firing properly and that you do not have drafts or cold spots within the kiln chamber.

 

Extending the annealing time hasn't seemed to help, what now?

Try insulating the edges of your work. A fiberpaper, blanket or board wall around the outer edge of the glass will reduce the rate of cooling at the edges, thus helping to stabilize the cooling rate throughout the glass body.

 

 

 

 

 

Additional information about Bullseye Glass may be obtained by visiting their web site: www.bullseye-glass.com