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.

Orientation of Iridised Glass

Iridised Glass – Which Side Up?

“One rule in using iridised glass is iridised down to fuse, up to slump”

You need to understand the conditions for the “rule" before applying it. Bullseye iridescent and Spectrum System 96 (made by the Uroboros method - not the stained glass) iridescent are not temperature sensitive in the way the other fusing ranges are. Both Bullseye and System 96 can be fired face up.  It is not yet (June 2019) known how Oceanside fusing compatible iridescent will perform when it becomes available.

The Wissmach luminescent and the other fusing glasses seem to be more temperature sensitive and so need to be fused iridised side down.  Avoid using Bullseye Thinfire in contact with the iridescence, as it sometimes reacts with the iridescence and creates pits in the glass.

Use the lowest practical temperature when forming with the luminescent face up. This avoids the partial or complete disappearance of the metal film that creates the iridescence.

More information on what causes the iridescence.

http://glasstips.blogspot.com/2019/06/iridescence.html

Iridescence

What is it?       How permanent is it?

“Many special effects can be applied to glass to affect its colour and overall appearance. Iridescent glass … is made by adding metallic compounds to the glass or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere.” 

Older glass can appear iridised because of the light reflection through the layers of weathering.

“Dichroic glass is an iridescent effect in which the glass appears to be different colours, depending on the angle from which it is viewed. This effect is caused by applying very thin layers of colloidal metals (e.g., gold or silver) to the glass.”

Source: https://www.thoughtco.com/

A rainbow iridescent appearance caused by an oil film on water is seen by light being reflected from both the top oil surface and the underlying water surface.  The light reflected from these two surfaces or boundaries have slightly different wave times and so interfere with each other to create this colourful pattern.

This is also observed in soap bubbles.  Here the light is reflecting from both the inner and outer surfaces of the film.

This iridescent appearance is termed thin-film interference.  It is an occurrence in nature where there is a thin film through which light can penetrate and so reflect off the surfaces of the film.  These surfaces are termed boundaries where the light can reflect. 

The thickness of the film can enhance or reduce the iridised effect. 

At a certain thickness the light waves reflected can cancel each other out.  This is described as a destructive interference pattern as it reduces the reflection.  The phenomenon can be used to provide non-reflective surfaces.

At other thicknesses there is an iridised effect.  This is caused by the reinforcement of the recombination of the two light waves reflecting in phase or nearly so.

Control of the thickness can give the silver or the gold iridised appearance, as in the Bullseye iridised glasses, in addition to the rainbow and other effects.

The nature of the light affects the colours of the iridescence.  If the light is daylight or similar it is a combination of many wavelengths.  The different wavelengths reflecting from the “boundaries” or surfaces provide the multiplicity of colour.  If the film has variations in thickness, there will be variations in the colours created.

A diagram from Wikipedia shows how the reflections work at the microscopic level.

https://en.wikipedia.org/wiki/Thin-film_interference

The permanence of the film causing the iridisation appears to be dependent on the metals used and the way in which they are deposited.