Glasses at Risk of Compatibility Shift

Many people take their fusing glasses beyond the tested parameters of the manufacturers in pot and screen melts and combing and casting operations. It has been speculated that there are compatibility shifts of hot colours and of opalescents.

Reading, and some experience, lead me to the belief that is the colouring minerals that are the key to which glass will shift in compatibility. Colours made with sulphur and selenium are more likely to opalise and also change their compatibility at extended times at high temperatures. Extended time is in the region of an hour or more. High temperatures are those over 850ºC

The colours at most risk of compatibility shift seem to be:

Reds

Oranges

Browns

Ambers

and a few bright and olive greens, but not dark greens.

http://www.warmtips.com/20070207.htm

Of course testing, using polarising light filters, is required to determine which will remain compatible after long, high temperature firings.  A method of testing is given here.

High temperature compatibility shifts are discussed here.

Compatibility Shift at Higher Temperatures

People experience breakages of their pot and screen melts that do not seem to have anything to do with annealing or glass sticking to the shelf. The common suggestion is that there has been a compatibility shift of the glass. This view is re-enforced by the opalisation of the transparent hot colours experienced by most.

Bullseye indicates in their glass notes that some colours are not suitable for high temperature work. This probably applies to other fusing glasses too. My experience leads me to believe that this compatibility shift occurs with all the opalescent glass colours as well as the hot ones. Further work will appear soon. is required to determine if there are any general indicators of the kinds of glass that are likely to develop incompatibility at high temperatures.

If you are concerned about the lack of durability of your piece due to possible incompatibility, you need to include tests with the firing. To make this test, place a piece of each colour used in the melt on a double layer of clear. If you are using a single base piece, ensure you leave space between the colours. It is best to place each colour on its own stack of clear. Also place a stack of clear glass as thick as your blank along side the other test pieces. Put all those pieces somewhere within the kiln out of the way of the area the melt will occupy and fire the lot together.

When cool, take all the pieces from the kiln and check the test pieces for compatibility. Do this check with a polarising filter to determine whether there is any incompatibility by looking for the halo showing the degrees of incompatibility.

If any or all, of the the pieces show stress, check the clear stack for stress. If the clear also shows stress, the annealing has been inadequate, rather than just the compatibility shift. Ideally, this process should be conducted in every firing.

Performing these tests will give you confidence in the durability of your piece, as it will show the levels of stress in the finished piece.

Annealing Unknown Glass

Sometimes you may want to use a glass in kiln forming when its characteristics are not known, such as for bottle slumping. It is possible to determine the approximate annealing point of this glass in your own studio. This tip on slump point testing gives you the information to do the test and calculations.

If you do not want to go to that detailed effort for a one-off process, you can adopt the shotgun annealing approach. This does require some observation of the glass, of course.

You need to observe when the glass has reached the temperature for the process you are performing. This will enable you to compare the behaviour of this unknown glass with what you normally use. This will give some idea of the relative annealing temperature to use. If a higher temperature is required for this glass than your normal glass, a higher annealing point can be assumed. The difference in top temperature can be added to the annealing point of your known glass.  If the top temperature is lower, you subtract the difference from the known glass' annealing point.

Set the annealing temperature to be 10C to 20C above the predicted annealing temperature and soak there for 30 to 60 minutes. This will help ensure the glass is all at the same temperature throughout. Set the annealing cool to be at about 30C per hour for pieces up to 6mm for the first 55C. The next segment should be about twice that to 110C below your chosen annealing temperature. The final segment can be around 150C per hour to 100C.  For thicker glass, the annealing cool should be proportionately slower.

This may seem an excessive, overly cautious process, but as you get to know the characteristics of the glass, you will be able to alter the schedule. This is a conservative and safe process to ensure your glass is well annealed.  And to be certain, you should check the cooled glass with polarised light filters.

amended 22.12.18

Plating

Objective

The object of plating is to modify the original colour, either by changing the tone or the intensity. This will, for example, darken a piece of glass where it would otherwise be to bright; or it will modify the colour to better blend with the surrounding pieces.

A further use of plating is in conservation, where the additional detail is placed on a separate piece of glass and placed in front or back of the original.

Leads

In leading, you normally use high heart lead. This is lead with a heart of 7mm or 10mm instead of the usual 5mm. Other heights are available, of course. The 7mm heart will accommodate two 3mm pieces, but if you are using thick hand made glass, you may require the 10mm high heart.

Comparing the Arrangement

Try the glass combination with each piece on top. Often there is a difference in tone or texture. Choose the one that suits your composition best.

Cleaning

Before finally fixing the glass together, make sure they are very clean as there will be no opportunity to clean the inside again. Try to avoid finger prints on the insides while you do further work with the glass.

Sealing

Make sure the glass fits the cartoon lines. You will be sealing the two pieces of glass together, so there is no opportunity to change the shape later. There are a variety of traditional methods of sealing the glass, but the easiest modern approach is to copper foil the edges to ensure that no cement creeps between the pieces.

Fitting

You then fit the glass into the came as for thiner pieces. Where you have a combination of heart heights, you can simply slip the ends of the lower heart cames inside the leaves of the high heart leads. The differences in height are small enough that no special support is needed for the thinner glass unless you feel better with the single layers of glass supported above the work surface.

Solder Alloys, 1

Common Alloys of Solder with Melting Ranges:

% tin    % lead    % silver    Melting range

20        80                            183-275C    361-527F

30        70                            183-255C    361-491F

40        60                            183-234C    361-453F

50        50                            183-212C    361-414F

60        40                            183-188C    361-370F

63        37                            183-183C    361-361F

62        36            2              179-189C    354-372F

45        54            1              177-210C    351-410F

This shows the solder compositions of lead and tin only have a solidification temperature of 183C.  The proportions of the two metals alter the the melting point and at 63/37 the melting and solidification temperature are the same, making for excellent solder beads.

The addition of silver can reduce the solidification point but the melting point can vary significantly.  Other solder alloys can make significant alterations in the melting and solidification temperatures.

revised 3.12.24

Defining the Glass Transition Phase

We often treat glass as a simple material. However it is a very complex and as yet not fully understood material. One of the most curious aspects is the transition between plastic and solid states. This is the temperature range of glass annealing – called the glass transition by scientists. This note comes largely from "Glass Properties" produced by Schott. The text in brackets [ ] is my additional explanation.

The glass transition comprises a smooth but very large increase in the viscosity of the material. Despite the massive change in the physical properties of a material through its glass transition, the transition is not itself a phase transition  of any kind [in this case from a liquid to a solid] and involves discontinuities in thermodynamic and dynamic properties such as volume, energy, and viscosity.

Below the transition temperature range, the glassy structure does not relax in accordance with the cooling rate used. The expansion coefficient for the glassy state is roughly equivalent to that of the crystalline solid. [Thus the CoE, which is taken as an average of expansion per degree Celsius over the range of 0C to 300C, is an inadequate guide to how the glass will behave at the glass transition and higher temperatures.]

Glass is believed to exist in a kinetically locked state, and its entropy, density, and so on, depend on the thermal history. Therefore, the glass transition is primarily a dynamic phenomenon. Time and temperature are interchangeable quantities (to some extent) when dealing with glasses.

[Viscosity shows a relatively regular change with temperature changes.] In contrast to viscosity, the thermal expansion, heat capacity, shear modulus, and many other properties of inorganic glasses show a relatively sudden change at the glass transition temperature. Any such step or kink can be used to define T_g [the transition phase of glass].  To make this definition reproducible, the cooling or heating rate must be specified.

Plating in Copperfoil

Plating is used to modify the colour, or intensity of local areas in a window or panel. Plating for leaded glass is normally putting two pieces of glass in the same came, although there was a common practice at the turn of the 19th into the 20th century to have the plate cover several pieces of leaded glass. In principle, the plating of copper foil panels is the same as for leaded glass, except there is no came to fit the glass into. So there are some variations.

An example where the fruit and leaves are all plated

Build the flat, single thickness window first. This provides a solid panel to work on. It also enables you to see whether you really need the plating, and if so the exact areas where it will be applied.

You should solder the whole panel except where the plate is to be soldered. In this/these areas just lightly tin the back, although you will have already put a solder bead over the whole of the front.

Patina the back of the panel, except where the plate is to go. Allow this to dry and clean up any spills, especially in the neighborhood of the plating.

Foil the plate with a backing to match the colour of the patina. So use copper-backed foil where the panel is in copper patina, but black-backed where the patina is black.

Tin the foil on the plate with solder. If the piece is to cross a number of the base pieces, you need to patina the tinned face that will be placed toward the viewer with the same colour patina. You need to make sure this is absolutely dry before proceeding.

Clean the plate and the base glass where the plate is to cover very well. Make sure there are no oils or tarnish on the solder, and that everything is dry.

Solder the plate to every seam that it contacts with no flux and a small amount of solder. This is to insure there is no leakage of flux - by not using any - or solder between the two pieces of glass.

Put a small amount of clear silicone between the edge of the plate and the base glass where you were not able to solder. Just lightly fill the gaps to ensure a seal against moisture and insects.
When the silicone has cured, carefully patina the plate so no fluid seeps between the glasses.

Protect the uneven back when handling by placing a soft foam pad, or a polystyrene sheet with cutouts for the plating, on the back to protect the panel from the carrying board.

Temperature Conversions

Temperature conversions from Celsius to Fahrenheit for some common temperatures in kiln forming:

Temperatures
100C  =  212F
200C  =  396F
300C  =  577F
400C  =  759F
427C  =  808F
482C  =  908F
500C  =  941F
600C  =  968F
650C  = 1123F
677C  = 1263F
760C  = 1414F
780C  = 1450F
800C  = 1487F
850C  = 1577F
900C  = 1668F
950C  = 1759F

The formula for temperature conversion is:
ºC divided by .555 plus 32 (for the freezing point of water)

Conversion of rates of advance is different (the freezing point of water does not need to be taken into account):
25C   =   45F
50C   =   91F
75C   = 136F
100C = 182F 
150C = 273F
200C = 364F
250C = 455F
300C = 545F
350C = 636F

The formula for rate of advance conversions is:
ºC divided by .555 only

Volume Calculations

When creating a casting, pot melt or other object from glass cullet or billet, you need to be sure you have a large enough volume of glass to fill the area. You can do it by measuring the volume or by calculating the weight. This note is about calculating the weight.

Filling a damed area with enough pieces of glass provides an illustration of volume control. To help make sure you have enough glass to fill the space, measure in centimetres to determine the area. For a rectangle, measure length by width in centimetres. For a circle multiply the radius by itself (radius squared) times 3.14 (pi) to get the area.

To determine the minimum volume required, multiply the area by 0.6 cm. This is the approximate thickness that glass takes up at full fuse. As the amount of time and heat that we normally give to the process is insufficient to allow the glass to fully flow, the glass will tend to be thicker in the middle when using pieces of glass rather than sheets. So you may wish to multiply the area by 0.7 cm (to make sure you have enough).

To get the weight of glass required for the space, multiply the calculated volume by 2.5 (specific gravity) to get the weight in grams. Divide by 1000 to get kilograms. If you must use pounds, multiply the kilos by 2.2, the number of pounds in a kilo.

Tack Fuse Temperature

The tack fuse range is around 730C – 780C. This will give a graduation in profile from the very sharp, almost barely laminated, to one very rounded almost flat. Choosing the right heat for the right profile is one of balancing several elements: temperature, time, speed.

Low temperature, high tack fuse

If there were no other considerations, you could go slowly up in temperature and peek in at infrequent intervals until the right profile had been achieved. However this tack fusing is happening in the devitrification range, so slow rises in temperature are not advisable.

Medium temperature, mid tack fuse

So an alternative strategy would be to go quickly through the devitrification range (700C to 760C) and soak for a bit longer above that range. However, often the desired profile may has disappeared by the time you get to 770C.

High temperature, rounded tack fuse

It would seem that you can attempt to balance the temperature, time and speed equation by firing quickly (such as 330C/hr) to your desired temperature and soak there for 10 minutes only.

To ensure you get the profile that you want you should begin to observe from at least 10C below your chosen temperature. If you do not get the profile you want, you can extend the soak until the desired effect is achieved. On a subsequent firing, you can set the top temperature a bit higher, but with the 10 minute soak and again observe. This can be repeated until the desired combination is achieved.

Each of these attempts needs to be completely recorded so that the results can be used in later firings if slightly different profiles are needed.

Also look at this entry for annealing of tack fusings.