Wednesday, 31 August 2016

Scheduling for a New Slump Mould

Often you will see statements that imply a single temperature and time is suitable for all slumping or draping.  This is not so.  In fact, slumping is a delicate balance of layup, time, gravity, shape and temperature. This applies to draping operations too.

Factors in glass forming

The balance of colour arrangement has an effect on how the glass forms.  In an extreme example of white on one side and black on the other, the forming will begin on the black side first, leading to an uneven slump. Read on - there are ways to make this effect less severe.

The length of time you are willing to wait for the piece to slump is a factor in the temperature required.  Patience is rewarded.  Longer soaks mean that lower temperatures can be used. Lower temperatures lead to less marking on the back.

The mass (often thought of as the thickness) of the glass affects how quickly the glass will form. The greater the mass, the sooner the glass begins to form. This means with heavy glass lower forming temperatures can be used, because of gravity effects.

To get glass conform to a mould with complicated shapes takes longer soaks or higher temperatures than simple shapes.  This is because the glass requires to be more plastic to get into multiple shapes, small details or sharp angles.

General Principles

Since all these factors interact, any one schedule will not do for all occasions.  The general principles for a good slump are:
  • Use a steady rate of temperature increase (rate of advance).
  • Use the lowest practical temperature to get the forming done.

The reasons for using a single steady rate of advance in kiln forming are:
  • It is much simpler to program a single rate of advance all the way to slump temperature.  
  • Glass reacts best to steady inputs of heat, allowing the whole substance to be at the same temperature as it heats up.
  • It helps avoid uneven slumps.  Glass that is the same temperature across and throughout itself is more likely to begin to form all at the same time.
  • It helps ensure that the whole of the thickness of the glass is at the same temperature, thus avoiding splits on the bottom.
  • The slow steady input of heat means the glass can be formed at a lower temperature because of the heat work put into the glass on the rise in temperature.

The reasons for using the lowest practical temperature to slump and drape are:
  • It allows the glass to begin moving before it gets sticky, and so dragging on the mould producing stretch marks and sometimes needles.
  • A low temperature slump reduces the risk of uneven slumps.  At low temperatures the glass is less likely to react to colour variations that absorb heat more quickly than others.  Where there is uneven weight, the forming is more likely to be even as it cannot react so quickly to the differences in weight.
  • The glass will be less marked on the mould side at lower than at higher temperatures.  The glass, being less plastic, will take up less of the mould texture.

Calibration of Schedules

As each mould is different, there are as many schedules applicable as there are moulds.  Bullseye has recognised this by publishing suggested schedules for their moulds.  But there are lots more moulds than the Bullseye ones.  And even for the Bullseye moulds there are a variety of variables in the glass put on top.

The point is to find a way to determine the appropriate schedule for the mould and the glass it supports.  This involves the main variables - rate of advance, top temperature and soak time - although there are others such as lay up, degree of fusing, weight and its distribution, colour variation, etc.

The rate of advance will depend on:
  • The thickness of the piece.  Thicker glass needs slower rates of advance.
  • The degree of fuse.  A tack fused piece will require a slower rate than a full fused piece.

The top temperature depends largely on the complexity of the mould shapes, although it is very closely related to the soak time.  One of the principles of slumping given above is to use the lowest practical temperature. The reason for this is to get a good result with the minimum of mould marks.

The main means of determining forming temperature and time is observation. I determine my slump temperature (normally) by what temperature I have to use for the particular mould to get the glass fully slumped in half an hour.  For more complicated moulds such as a candle bridge I would use 1.5 hours as the soak time.

There are two main methods of doing this observation.  One is to set the “one size fits all” schedule and modify it. The other is to create a new schedule by working up from the lowest temperature to the practical temperature.

Modification of existing schedule

To prepare for the modified schedule, you need to do several things.  

Get your kiln log out ready to record the information about the firing.  Record the mould shape and separator (and add a picture of the set up if you can) and include the lay up of the blank to be formed.  Also record anything you think may be relevant to the forming process for this firing.

Set your single rate of advance all the way to the top (forming) temperature and record it in the kiln log.  Begin observing the progress of the slump from 60°C below the top temperature you have set.  This involves quick peeks at approximately five to ten minute intervals.  You may not see much movement at first, but at later peeks you will see the glass progressively forming.  When the glass appears to have just touched down at the bottom, you can use that as the top temperature.  Advance the schedule to the soak portion (read the controller manual if you do not already know how to do that).  Note the temperature and time in your log book when you do this.

Continue to observe the progress of the slump but at about ten minute intervals to check on the progress of the slump.  When the slump appears complete, advance to the next segment of the schedule and note the time.  Subtract the start of the slump soak from the present time and you know how long the soak needs to be for that layup in the mould. Record that in your log book. 

When cool, inspect the slumped piece to determine if it is fully formed. Record the results in your kiln log.  If it is not fully formed, you can decide if it is practical to add additional soak time or if you need to increase the top temperature.  Only you can determine what is a practical soak time.  If you are soaking while you are away or asleep, it does not really matter how long a soak you need at the chosen temperature.  However, there are times when you need to have a piece out of the kiln to be able to put in the next.  Somewhere between these two is the practical soak time.

You may find that the glass does not need as much time as you gave it.  Record this result too.  In this case, you can reduce the top temperature in future firings until you find the best combination of temperature and time. You will have experience from watching the forming (whether slump or drape) to give an indication of the lower temperature to choose.  A general guide would be to reduce the temperature by 10C, and extend the time by at least 50% more than  what you used in the higher temperature firing.  

Record each firing with the lay up, rates, temperatures and soak times, plus the results.  When you have determined the ideal combination of factors, record the determined temperature and soak time together with the layup in your log book and on something in your mould box.  I have also used vitreous paint on the underside of the mould to indicate my standard temperature and soak time so that I don't loose the information.

Development of a new schedule

This is not as difficult as might be imagined.  It does involve a lot of peeking into the kiln, though. You start with an appropriate rate of advance for the thickness and style of fusing.  Remember that thicker glass and tack fused glass require slower advances than thinner and flat fused glass.  Set this rate all the way to your predicted top temperature.  No rapid rises with short soaks are required or desirable. Set a predicted soak time. If you are not certain, use 30 minutes as a general average. Then set the anneal soak and cool rates.

As you observe, you will see when the glass on the mould begins to form. It will generally start at about 600°C.  Peek at about 10 minute intervals from that temperature onwards toward the target.  When you see the glass begining to change shape, Change the top temperature to be about 20C higher than the initial forming temperature, and then observe after 15 mins at the new temperature. If it hasn't moved much, add 5°C more to the temperature and observe. Repeat as necessary. When the glass has a significant curve, stop the rise and soak at that temperature with the 30 minute soak.  Continue to observe at 10 – 15 minute intervals to determine when the slump is complete.  Then proceed to the anneal cool. Record rates, temperatures and times in your log book.

When you remove the piece from the kiln, check it over.  If it is not fully slumped, you can add time or temperature.  Adding time is likely to give a better surface to the glass on the mould side.  Sometimes, but not often, adding temperature will be the choice. 

It is possible that the piece will show evidence of too high a temperature or too long in the mould.  This will be clear from extensive mould marking, sometimes needles at the edges, stretch marks, or uprisings at or near the bottom of the mould.  In these cases, the temperature needs to be reduced.  Reducing the time is not advisable, as quick slumps can often be distorted or unbalanced.

Glass Types

Remember that these tests for the best forming schedule for you and your mould are only relevant to the kind of glass you are using at that moment. There will be only minor variations between Bullseye, Uroboros and Wissmach. There will be major variations between these and float glass. Separate schedules will need to be worked out for it, remembering that there are a variety of manufactures of float and they do not all behave the same as each other.  Float and other glass that is not formulated for fusing will not provide such consistent results as fusing glass, but successful schedules can be determined in the same way as for the fusing compatible glasses.


Once you have calibrated the temperature and time for the mould and the layup, you will know how to schedule for that mould. Record it in your log book and also along with your mould, either in the box or on the mould.

It will be for you to decide whether you use longer times and therefore lower temperatures.  When making the decision remember the principles of slumping – steady rate of increase to the working temperature, and use of the lowest practical temperature.

These actions will give you the standard forming temperature for the mould.  It is a base from which to make variations when you use a different thickness, lay up, or degree of fusing.  

You should continue to record each of your firings with full details, because sometimes things change. This will give you a basis to diagnose what has become different. It will help avoid the cry of "this has always worked for me before."  It means you have the possibility of working back to see what, if anything has changed. If nothing has changed in your level of fusing, thickness, lay up, schedule and all those other things you record, then you can begin looking at your kiln to see what might be different.