High Fast Slumps

What are the possible effects of fast rises to a high temperature for a slump?

Some of the possible effects of fast rises to a relatively high temperature slump are these:

Uneven slumps can occur. 

·         This largely due to differential heating of thicker/thinner parts. 

·         It can also emphasise anything off level.

·         Any unevenness in the heat across the kiln can also be emphasised by the rapid rise in temperature.

Uneven slumps can be promoted by contrasting colours. Dark and light colours heat at different rates, leading to one area of the glass slumping before another.

A dark/light contrast can lead to stress fractures in fast firings.

In a fast firing the top heats faster than bottom leading to the possibility of splits on the bottom of the piece

The edges of the piece heat faster than centre, increasing the possibility of spikes at the edge.

Fast slumps require higher temperatures to achieve the slump.  This means there will be more marking of the bottom surface.  It often includes stretch marks especially at the rim.

The Alternative to Fast High Temperature Slumps

Slow and Low

Slow rises in temperature means the slumps can be done at lower temperatures. Lower temperatures usually mean more control and fewer marks from the mould.  It does mean that you will need to observe at intervals to get the soak time you need, but this is required for all variations in rates and layups, as well as new moulds.

Combining Black and White

Black and white are at almost opposite ends of the viscosity spectrum in glass terms. Black is the runniest, and white is the stiffest. Black transmits heat more quickly to the lower layers than white.  White is the glass that absorbs and transmits heat most slowly.

an example from Pintrest

A lot of care is required when combining the two.

If the white is on top of black, the white shades the heat – more than other colours - from the black underneath, so a lot of stress can build up in the black.

You need to give a lot of time for the two to adjust to each other. A slower rate of advance than normal is advisable. A significantly longer soak at annealing temperature is required. The annealing cool needs to be much slower than for other glass of the same dimensions.  Consider slowing the rates to half your normal firing rates.  Also double your soak times.  After some experience you will be able to alter these cautious rates to those more suitable for you.

Soak

Kilnformers seem keen to reinvent terminology and then wonder about imprecise language being used in the field. Much of the terminology for kilnforming is already available from ceramics. It makes sense to continue to use that terminology where it applies.

A soak at a stated temperature is the same as "hold" at the same temperature.

The concept of soak is more useful than the term “hold”.  “Soak” implies the temperature is held at temperature to allow the heat to soak into the glass. And that is the purpose of a hold.  Using the term “soak” brings this purpose into the thinking about scheduling.  It is related to the concept of heat work. 

Using the concept of heat work allows you to use a slow rise to a temperature for a short time to get the effect you want.  Or to rise to a temperature in the normal way but with a long soak.

This is how you can get a tack fuse at 750C with a long soak – say 30 mins - as at higher temperature for a shorter soak – say 780C for 5 minutes. This the concept of heat work in practice.

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

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 also 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 2.5 times longer than for the thickest part.  

The reasons for the longer soak and slower cool are:

• The glass pieces are not incorporated with each other, but only at the interfaces.
• 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.

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.

    See also Sintering

revised 20.2.26