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.