Differential Cooling of Transparent and Opalescent Glass

A statement was made on a Facebook group that transparent glass absorbs more heat than opalescent glass. And it releases more heat during cooling. The poster may have meant that the transparent heats more quickly than the opalescent, and cools more quickly.

Yes, dark transparent glass absorbs heat quicker than most opalescent (marginally), and it releases the heat more quickly (again marginally) than opalescent. The colour and degree of transparency do not absorb any more or less heat, given appropriate rates. They gain the same heat and temperature, although at slightly different rates due to differences in viscosity.

An occasional table

The rate of heating and cooling is important in maintaining an equal rate of absorption of heat. The temperature of both styles can become the same if appropriate lengths of heating, annealing, and cooling are used. The slightly different rates of heat gain can give a difference in viscosity and therefore expansion.  This slight mismatch during rapid ramp rates, might set up stresses great enough to break the glass. This can occur on the quick heat up of glass during the brittle phase (approximately up to 540ºC/1005ºF). In fact, most heat-up breaks occur below 300ºC/540ºF.

The main impact of differential heat gain/loss is during cooling. Annealing of sufficient length eliminates the problem of differential contraction through achieving and maintaining the Delta T = 5C or less (ΔT≤5C). It is during the cooling that the rates of heat loss may have an effect. The marginally quicker heat loss of many transparents over most  opalescent glass exhibits different viscosities and rates of contraction. The stresses created are temporary. But they might be great enough to cause breaks during the cooling. Slow cooling related to the thickness and nature of the glass takes care of the differential contraction rates by maintaining small temperature differentials.

Significance of Differential Heat Gain/Loss

Uneven thicknesses and the tack fusing profile both have much greater effects than the differential cooling rates of transparent and opalescent glass. It may be that strongly contrasting colours (such as purple and white) are also more important factors in heat gain and loss than transparent and opalescent combinations.  Cooling at an appropriate rate to room temperature for these factors will be sufficient to remove any risk of differential contraction between transparent and opalescent glasses.

Ceramic Drape Moulds

Characteristics of materials

One of elements you need to consider in selecting a mould for draping is the characteristics of the ceramic material in relation to the glass being draped.  

Ceramic

Ceramic materials have what are called inversions. These are  points at which the ceramic has a quick change in expansion both on the heat up and cool down.  The two major ones are cristobalite inversion temperature at around 225°C and the quartz inversion at about 570°C.  The Crystobalite inversion is a sudden change of about 2.5% and the quartz is a sudden change of 1%.  These are very sudden and dramatic changes in comparison to the average of around 0.1% over the temperature range of 570°C to 800°C.  The crystobalite inversion does not occur until ca. 225°C.  This means that the whole structure of the ceramic is contracting less than the glass – exhibiting a CoE of ca. 66 rather 90 to 96.

Ceramic drape mould from Creative Glass Guild

Glass

We are used to saying glass expands and contracts at a standard rate, depending on the glass this may be a CoE of 83 to one of 104.  This is not the case.  The coefficient is an average calculated between 20°C and 300°C.  If you change the temperature range, the coefficient will also change.  And if you look at the range 570°C to 580°C you find the CoE is around 500.  This means that as the glass cools into the annealing range, it is contracting about 7 times faster than the ceramic. 

This dramatic difference in contraction means that the glass is attempting to crush the ceramic by enclosing it tightly.  Sometimes it does it so strongly that the strength of the glass is exceeded, and it breaks.

Possibilities

It is possible to drape over ceramic in certain conditions.

Influence of draft

The term “draft” indicates the slope of the sides of the form.  The steeper the sides, the more likely the glass is to trap the ceramic mould.  To be useful, the draft of the mould needs to be sufficient for the glass to slide upwards on the mould as it cools. This means the mould needs smooth sides and be well covered with a separator.

Compensations

You can compensate for steep drafts by wrapping the ceramic form in 3mm refractory fibre paper.  You can bind this with high temperature wire to ensure it stays throughout the firing. The fibre paper can be compressed and so provides a cushion between the rapidly contracting glass and the slowly contracting ceramic.

These need a circle of 3mm fibre paper over the open top of the kiln posts that have no draft at all before use. Of course, they need to have a circular piece of fibre paper over the hole in the post.

The use of ceramic forms to drape over requires care about the draft of the ceramic or addition of a cushion to avoid the greater contraction of the glass than the ceramic grabbing the mould so tightly it cannot be removed.

Texture Moulds and Glass Sizes

I had an overhang [on a texture mould] and I heard a pop and opened kiln and saw it cracked off the mold. … [The piece] is 2 layers Bullseye irid placed face down and Tekta [on top]; the mold was sprayed 3-4 times with zyp and Thinfire; and I put mold on kiln posts. [I] fired to 1440[F].

Diagnosis 

The overhang of the glass caused the break. As the glass heats it expands. The ceramic does not expand as much as the glass.  This means even more glass will hang over the edge than at the start.  As the glass reaches slumping temperatures, it begins to drape over the edge. At the same time the glass on the interior is beginning to slump into the textures.  When the glass has fully taken up the texture, the overhanging glass will be touching the outer sides of the mould. This means at the end of the heating and soaking part of the firing, you have the ceramic mould partially and tightly encased in glass.  The glass has formed around the ceramic.

Credit: theavenuestainedglass.com

The physics of the two materials are that glass expands more than ceramic. On cooling, the glass grips the sides of the ceramic mould tightly. This is because it shrinks more than the ceramic.  In this case, the ceramic was stronger than the glass and the strain caused the glass to break.  Upon occasion the opposite can happen.

Two other notes.

The temperature of 781°C is higher than needed.  You will need to do a bit of experimentation to find the right combination of temperature and time for each mould.  You could consider 630°C as an initial temperature with a 90-minute soak.  Bob Leatherbarrow (p.161) describes a method of scheduling a sequence of slightly higher temperatures with soaks.  If the texture is not forming (as determined by observations), you can advance to the next segment with a higher temperature and see how that goes.  When the appropriate amount of texture has been achieved, advance to the cooling to anneal segment.

Iridised surfaces provide a very good separator.  With the iridisation down against the mould, it may be unnecessary to use Thinfire, especially when you already have used boron nitride as the separator.

Further information is available in the ebook: Low Temperature Kiln Forming.