Lamination

Lamination in kiln forming is the adhering the glass pieces together without changing the shape of the glass.  On a laminated piece the edges of the glass will still be sharp but cannot be pulled apart.  Each manufacturer's glass will have slightly different lamination temperatures and it will be affected by the length of soak.

How do I find the lamination temperature?

Observe

Make your chosen layup in clear glass.

Peek at your glass at frequent intervals from 550C.  The rate of advance should be slow, say 150C or less. When it is observed the edges are just  beginning to round, you know you have the high-profile tack temperature for that rate of advance.

On another firing of the same setup and rate of advance, soak at 20C lower than the previous temperature for 60 mins. You need to keep peeking during the soak to ensure the edges remain sharp. 

When you see the edges begin to round, you need to advance to the cool and record the length of the soak used.  You will need to shorten that soak by the interval of your peeking.  If you were peeking every 10 minutes, reduce the length of the actual soak given by 10 minutes for the next firing.

These three firings will give you a schedule for laminating glass together for your chosen layup.  Other layups will require slight variations which will require observation to determine how much change from the original schedule is required.

Anneal and cool carefully

Do not forget to schedule the anneal soak and cool for at least twice longer than for a tack fuse each time.  This may make the soak four or more times longer than for a full fuse.

The reasons for the longer soak and slower cool are:

·         The glass pieces are not incorporated with each other. 

·         The pieces will react to cooling in different ways. 

·         Different colours have different viscosities and different contraction rates.

·         The shading effect of one piece on top of another is intensified. 

    There are right angles between the base and the stack of glass above.

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All these factors make it important to ensure all the glass is at the same temperature (the anneal soak) and that the pieces, that make up the whole, cool at the same rate even when shaded.

Further information is available in the e-book: Low Temperature Kilnforming.

The relative order of kiln forming events

When preparing for multiple firings of elements onto a prepared piece, you need to consider the order and temperatures of events so that you do not harm an earlier stage of the project.  This blog entry will not give definitive temperatures, as that varies by glass and by kiln.  Instead, it indicates what happens in progression from highest to lowest temperatures in approximate Celsius degrees.  

ca. 1300C  -  Approximate liquid temperature 

ca. 850 – 1000C  -  Glass blowing working temperature

ca. 950C  -  Raking and combing

ca. 850C  -  Casting

ca. 810C  -  Full fuse

ca. 790C  -  Large bubble formation

ca. 770C  -  High tack, low contour fuse

ca. 760C  -  Tack fuse

ca. 750C  -  Fire polish

ca. 700C – 760C  -  Devitrification range

ca. 700C  -  Lamination tack

ca. 600C – 680C  -  Slump and drape

ca. 650C  -  Vitreous paint curing temperature

ca. 600C  -  No risk of thermal shock above this temperature 

ca. 540 – 580C  -  Glass stainers enamel curing temperature

ca. 520 – 550C  -  Silver stain firing temperature

ca. 550C  -  Glass surface beginning to soften

Slow rates of advance needed from room temperature to ca. 500C

These temperatures are of course, affected by the soak times. The longer the soak time, the lower temperature required. The rate at which you achieve the temperature also affects the effective temperature.  Slower rates of advance require lower temperatures, than fast rises in temperature.  These illustrate the effect of heat work.

The table shows for example you need to do all the flat operations and firings before slumping or draping.  It also shows you can use vitreous glass paints at the same time as slumping and draping.  This emphasises that the standard practice is to plan the kind of firings you will need for the piece and do them in the order of highest temperature first, lowest last.

In general, you do need to do the highest temperature operation first and lowest last.  But there are some things you can do with heat work.  For example, if you needed to sandblast a tack fused piece, but did not want to risk reducing the differences in height there things you can do.  From the list above, you can see the glass surface begins to soften around 500C.  It is possible to soak the glass for a long time around 500C to give it a fire polish, instead of going to a much higher temperature.  You will need to experiment to find the right combination of temperature and soak length, but it can be done.

This article is to show that knowledge of what is happening to the glass at different temperatures, can help in “fooling” the glass into giving you the results you want without always following the “rules”.  This may also be what it is to be a maverick glass worker.  Use the behaviour of glass to your advantage.

Tack Fusing Considerations

1 – Initial Rate of Advance

Tack fuses look easier than full fusing, but they are really one of the most difficult types of kiln forming. Tack fusing requires much more care than full fusing.

On heat up, the pieces on top shade the heat from the base glass leading to uneven heating. So you need a slower heat up. You can get some assistance in determining this by looking at what the annealing cool rate for the piece is. A very conservative approach is needed when you have a number of pieces stacked over the base layer.  One way of thinking about this is to set your initial rate of advance at approximately twice the anneal cool rate. More information on this is given in this entry. 



2 – Annealing 

The tacked glass can be considered to be laminated rather than fully formed together. This means the glass sheets are still able, partially, to act  as separate entities. So excellent annealing is required.

Glass contracts when it's cooling, and so tends to pull into itself. In a flat, symmetrical fuse this isn't much of a problem. In tack fuses where the glass components are still distinct from their neighbours, they will try to shrink into themselves and away from each other. If there is not enough time for the glass to settle into balance, a lot of stress will be locked into the piece that either cause it to crack on cool down or to be remarkably fragile after firing. In addition, in tack fusing there are very uneven thicknesses meaning it is hard to maintain equal temperatures across the glass. The tack fused pieces shield the heat from the base, leading to localised hot spots on cool down.

On very difficult tack fuses it's not unusual to anneal for a thickness of four to six times greater than the actual maximum thickness of the glass. That extended cool helps ensure that the glass has time to shift and relax as it's becoming stiffer, and also helps keep the temperature more even throughout.

So in general, tack fused pieces should be annealed as though they are thicker pieces. Recommendations range from the rate for glass that is one thickness greater to at least twice the maximum thickness – including the tacked elements – of the whole item. Where there are right angles - squares, rectangles - or more acutely angled shapes, even more time in the annealing cool is required, possibly up to 5 times the total thickness of the piece.

It must be remembered especially in tack fusing, that annealing is much more than the annealing soak. The soak is to ensure all the glass is at the same temperature. The anneal cool over the next 110ºC is to ensure this piece of different thicknesses will all react together. That means tack fusing takes a lot longer than regular fussing.

3 – Effects of thicknesses, shapes, degree of tack

The more rectangular or pointed the pieces there are in the piece, the greater the care in annealing is required. How you decide on the schedule to use varies. Some go up two or even four times the total thickness of the piece to choose a firing schedule.

A simplistic estimation of the schedule required is to subtract the difference between the thickest and the thinnest part of the piece and add that number to the thickest part. If you have a 3mm section and a 12mm section, the difference is 9mm. So add 9 to 12 and get 17mm that needs to be annealed for. This thickness applies to the heat up section as well.

Another way to estimate the schedule required is to increase the length the annealing schedule for any and each of the following factors:

·         Tack fusing of a single additional layer on a six millimetre base

·         Rectangular pieces to be tack fused

·         Sharp, pointed pieces to be tack fused

·         Multiple layers to be tack fused

·         Degree of tack – the closer to lamination, the more time required

The annealing schedule to be considered is the one for at least the next step up in thickness for each of the factors. If you have all five factors the annealing schedule that should be used is one for at least 21mm thick pieces according to this way of thinking about the firing.

4 – Testing/Experimentation

The only way you will have certainty about which to schedule to choose is to make up a piece of the configuration you intend, but in clear. You can then check for the stresses. If you have chosen twice the thickness, and stress is showing, you need to try 3 times the thickness, etc. So your annealing soak needs to be longer, if stress shows. You can speed things by having your annealing soak at the lower end of the annealing range (for Bullseye this is 482C, rather than 516C).

You will need to do some experimentation on what works best for you. You also need to have a pair of polarisation filters to help you with determining whether you have any stress in your piece or not. If your piece is to be in opaque glasses, you need to do a mock up in clear.