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.

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.

Melts, Apertures and Height Effects

The effects on the pattern of melts are a combination of several factors. The normal pattern is of spirals as a thread of glass moves down to the shelf and begins to spiral just as any other viscous fluid around the high spot of the drip.  The specific effects centre around three main elements.

Aperture size

The size of the holes determines the diameter of the thread of flowing glass.  Also, the larger the diameter, the quicker the flow.

relatively small apertures

large, long apertures

Height from shelf

The height from the shelf has the effect of determining both the thickness of the thread at touch down and the degree of spiralling.

Relatively low screen

Relatively high screen

Heat

The temperature and time determine the heat work.  The amount of heat (as well as top temperature) influence the flow of the glass.

These three elements interact

Aperture

Aperture size determines the maximum diameter of the thread.  You can thin the threads by having smaller grids or holes. 

The height affects two things. 

Height affects the relative thickness of the flowing thread. Higher makes for thinner strands. The reduction in size can be lessened by placing the apertures closer to the shelf.

Height also affects how the thread behaves on touching the shelf.  More spiralling occurs with height.  A low height will reduce the spiralling to just moving outwards.

Note that when talking about height, it is relative to the aperture sizes.

Heat affects how the glass flows

The higher the heat or the greater the heat work, the faster the glass flows.  Lower heat gives slow moving threads.  Faster flowing glass promotes thicker threads.  Slower moving threads can take up patterns other than spirals.

These factors give you three interacting elements

You could have, for example, a high screen with large openings and low heat to give thin threads with eccentric spiralling.

You could have low height with small apertures and high heat to give thick threads with minimum spiralling.

In theory, you could have at least twelve main combinations by using the extremes of each element, with multiple variations of dimensions in each case.

Experimentation Required

This is to illustrate the interactions are complex and require significant experimentation to be able to predict the probable outcome.  The outcomes will always be only probable, even though you can come to control more aspects of the process and you develop experience.

Seedy base glass

Sometimes your clear base has bubbles, or as the trade calls it, seeds.  When capped with opalescent glass, in certain circumstances, these tiny bubbles can become larger and rise to the surface, pushing the opalescent aside as it rises.  This leaves a clear spot in the midst of the opalescence.

Clear cap

One way of reducing this problem, is to avoid it altogether.  This can be done by placing the clear on top of the opalescent as a cap.  This way the bubbles, if any, are rising through the clear.

Flip and Fire

If you can’t, for one reason or another, cap the piece with clear, you can fire upside down. Again, the bubbles are rising through the clear.  When the firing is complete, you can flip it over to the right side.  You will need to clean thoroughly and take to a fire polish temperature to get the shiny surface back.

Heat Work

Another way is to fire to a lower top temperature with a longer soak.  This means the glass can take up the profile you want without becoming so soft that the bubbles can rise through the glass.  You will need to observe to determine when the glass has the right profile, and then advance to the cooling and anneal phases.

Low and Slow

This last way of reducing the possibilities of bubbles rising through toward the top is based on the characteristics of glass.  As glass becomes hotter, it becomes less viscous and so allows the air to rise toward the top of the glass surface.  Using a low temperature gives a more viscous glass to resist the bubble movement.  The long soak at the chosen lower temperature allows the surface of the glass to take up the profile you want, as the surface is hotter than the bottom of the glass, therefore reducing the possibilities of bubbles rising.  It does take a longer soak at the top temperature, but it also reduces the marking on the bottom of the piece.

This low temperature process is using the principles of heat work.  The effect on the glass is a combination of temperature and time.  The higher the temperature, the less time is required.  The longer the time, the less heat is required.  The heat work put into the glass to achieve the effect you desire is determined by the combination of temperature and time used in firing the glass.  This principle of heat work is why you can achieve the same effect at very different temperatures, depending on the length of time a piece is soaked.

Striking glass

Yes, much glass is striking in its effect.  But the term is used in a technical sense to indicate the glass has not reached its intended colour without further firing.

A striking glass is one that changes to its true colour. Not one which takes up a different colour.  There seem to be differing ideas on how striking works, but it is an intentional process.

Several glasses coloured with copper or silver strike to Their final colour when heated.  It seems that copper when used to make red (rather than blue or green) can undergo a chemical change during the heating.  The copper oxide used is normally Cu₂O.  When heated the copper and oxygen molecules can separate and form bonds with other molecules.  The rapid cooling that is used in glass prevents the copper and oxygen from combining in the Cu₂O formation.  The extent of this dissociation determines the degree of colour change.  Thus, the colour is affected by the heat work given to the glass – assuming the starting proportions of materials are the same.  This can occur with some other colouring metals too.

Another form of striking is caused by the growth of crystals within the glass. In these cases, usually in silver bearing glass, the metals separate from the silica and form small crystalline structures which are also fixed by the rapid cooling required for glass.

There is another theory that the colour change is due to the orientation of the colouring molecules within the glass matrix.  The idea is that the molecules will change from the clearer state to the struck colour due to the orientation caused by reheating and cooling.

The actual process seems to be unknown in a definitive sense.  What is known is that temperature, a reducing or oxidising atmosphere, and heat work will vary the intensity of the strike in colour.  This means that where the project is especially sensitive, you must undertake experiments to help predict the colour that will be achieved with the conditions you choose to use.

Tack Fuse vs Fire polish

Are tack fuse and fire polish the same thing?

Maybe

They both occur in the same temperature same range, depending on the degree of tack fuse you want.

What you are doing in the fire polish process is heating the top surface enough to appear polished. Very little time is needed in a fire polish at top temperature as opposed to a tack fuse.

In a tack fuse, you want the bottom of the upper pieces to be hot enough to stick to the bottom layer. This requires a higher temperature or longer soak than a fire polish.

At around 730C, depending on your kiln, you will be softening the upper surface of the glass enough to give a polished appearance.  To determine whether the polished surface has been achieved, you can peek into your kiln at the chosen temperature to see if the polish is complete.

This is also the temperature at which sintering, or a lamination of the glass pieces occurs.  The edges will still be sharp, but cannot be pulled apart.  This kind of fusing needs careful annealing – long soaks and slow cools.

Tack fusing of various degrees occurs in the temperature range from 730C to 770C.  To determine which temperature and soak time will give you the result you desire will require experimentation and observation.  Generally, you can achieve the desired level of fuse with lower temperatures and longer soaks, as you can at higher temperatures and longer soaks. 

It is also possible to give a fire polish to your glass at a really low temperature, such as 550C, with a very long soak. This will avoid significantly flatening the surface of your piece.  This is the effect of heat work.

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