Elevation of Moulds

Putting slump and drape moulds on kiln posts is not necessary.  If you think you need air circulation under the mould, pieces of thick fibre paper are all that is needed.

The widely accepted notion that all moulds need to be elevated from the shelf is not backed up by evidence.  There is a lot of anecdotal discussion which supports the idea that it is necessary, though.  The idea is that air circulation under the mould will keep the lower part of the mould the same temperature as the upper part. That may or may not be the case. But it is the glass that needs attention.

Heating glass on moulds

Slumping

The glass is suspended either at the edges in slumps or at the centre in drops. The suspended glass will heat independently of the mould. Any difference in temperature between the glass and slumping mould will only become important at the end of the slump when it touches down onto the mould. If slow ramp rates and long soaks are used, the mould will be heated through by the time the glass touches down to the mould. But there could be an argument, based on the use of cold moulds in glass blowing, that a cool mould will impart less texture on the glass than a hot one.

Draping

In draping, the glass is supported in the centre on the mould and the differential temperature between mould and glass in the early stages of the firing may be a cause of a break. The supporting surface of the mould is being heated through the glass and, to a lesser extent, by conduction from the lower part of the mould. This requires slower heating than a slump mould does, to ensure the heat is not drained from glass by the mould which would allow the temperature differential between the supported and the unsupported parts of the glass to become great enough to break the glass.

Air Circulation

The idea that air circulating under the mould will even out the temperature between the bottom and top of the mould is not supported by two pieces of evidence.

The main effect to heat glass is infrared radiation. This is often described as the glass “seeing” the heat. Glass is only secondarily heated by the ambient temperature of the kiln. How heating the mould in the slumping process will protect from breaks or uneven slumping is not clear, and is supported only by analogies.

The idea that convection will help to heat the glass as well as the mould is countered by the general agreement that there is very little movement of air in the closed kiln. What convection of air there may be from the differential air temperature between the bottom and the top of the kiln is minor. If it were major, there would not be the large air temperature differential experienced – in deep kilns in particular, and to a lesser extent in shallow kilns – between the top and bottom of the chamber.

Temperature Differences

We all agree that there are temperature differentials from top to bottom in kilns.

The evidence is that the temperature difference is much greater between air and glass, than between mould and glass. The average temperature difference (∆T) between raised moulds and on-the-shelf moulds in tests ranged from about 50°C to 40°C (90°F to 72°F) on the rise to top temperature, depending on the ramp rate.  (a ramp rate of 150°C/ 270°F averaged a 50°C/90°F differential vs a 120°C/216°F ramp rate which gave the average 40°C/72°F differntial.)  It is also clear that by the time the slump temperature is reached, the ∆T between the two mould arrangements is about 25% less at the end of a 30 minute slump than at the beginning. So, yes, there is a difference in the temperatures under the mould versus an on-the-shelf mould.   Is it significant?

The ∆T between the raised mould and the air is approximately three times any difference between the on-the-shelf and raised moulds. This is consistent at ramp rates of 120°C, 150°C and 240°C (216°F, 270°F and 432°F). This indicates there is a greater risk to the glass with rapid ramp rates than there is from the difference in mould elevation. Slower ramp rates produce smaller differences between air temperature and mould temperatures. They also produce smaller differences between the mould bottom and top temperatures, making the argument for slow ramp rates up to the slump temperature, regardless of whether the mould is elevated or not.

Ramp Rates

The more rapid ramp rates create significant ∆T between the air and the mould. This much larger ∆T than between moulds raised or not, has the greater potential for causing breaks. The slower the ramp rate the less difference. These two elements on their own make the argument for slower ramp rates clear.

Distance from Elements

The distance between glass and heating elements is important in slumping.
The closer the glass is to the heat source, the more uneven the heat reaching the glass will be. This blog post indicates the distance required to even out the temperature. 

Where the elements are located is significant too. Side fired kilns need to use slower rates in going up in temperature, because the heat is concentrated on the edges of the glass unless baffles are in place. The uneven heating risks breaks, and this can be reduced by using rates for one to two layers thicker than actual thickness.

Moulds and Shelves

It can be argued that care in heating up ceramic moulds is required because of the quartz inversion.  If the ramp rate is fast enough to cause a mould to break, it is also too fast for the safety of the glass. The same caution in relation to the shelf needs to be employed for the glass.  Any large heat differential across the shelf will cause a break. 

Heavy, wet, damp, or large casting moulds must be elevated to keep the shelf from cracking due to a large ∆T between mould and shelf which has the real potential to break the shelf. The remedy for these kind of moulds is to elevate them on short kiln posts to maintain a relatively even temperature across the whole of the shelf.

Caution – the temperature differentials are averages and individual moulds will differ. But the principle remains that slow rises to top temperature are less risky than fast ones.

Data for these figures is available in table form and in graph form

Mould Elevation

 

The expansion characteristics of glass and ceramics components

Many people advocate the elevation of moulds.  Mainly for air flow to equalise temperatures above and below the mould.  But also, to prolong the life of the mould.  My observation on these reasons for elevating the mould are that they are not harmful, but not necessary, except for investment moulds.

My experiments have showed insignificant differences in temperatures above and below whether elevated or not.  Since the air temperature under the mould is much the same whether elevated or not, it indicates that elevation of the mould has no significant effect.  But, of course, elevation of the mould does no harm either. 

More important than elevation of the mould, is consideration of the nature of the ceramic mould.  Ceramics have two expansion/contraction temperatures called inversions.  The first is at 226˚C/439˚F, and the second around 570˚C/1058˚F.  The ceramic expands rapidly at these temperatures.  There is a 2.5% increase in volume at 226˚C and a slightly more gradual 1% increase around 570˚C.

This a main reason to use slow ramp rates up to at least 570˚C/1058˚F.  Slower rates ease the ceramic expansion speed and reduces the risk of breaking.  So, slower rates will lengthen the life of ceramic moulds. The cool down for annealing and cooling is slow enough that it presents no risk for the ceramic.

There are occasions when the mould must be elevated, though.  These are when the mould is large, heavy, or damp.  This is to protect the shelf rather than the mould or glass.

 

Drying Kiln Washed Moulds

A question about kiln wash. Do you have to let each coat dry while applying before applying the next coat?

 There seems to be a popular notion that newly kiln washed moulds must be cured before use.  I'm not sure where the information comes from, and no reasoning is given.  It is suggested that that quickly heating newly kiln washed moulds to 550°F (290°C) is important.

 If you want to make sure the mould is dry, this may not be the best way to do it.  All ceramics have a cristobalite inversion at around 225°C/437°F.  This a very rapid increase in volume of 2.5% that often leads to cracks and breaks in ceramics when the rate of advance is quick.  The mould will react better and last longer if the rate of advance is slow until that inversion temperature is passed.  But also note there is a quartz inversion at around 570°C/1060°F that is significant.

 

 This is another reason to advance the temperature slowly when slumping or draping with a ceramic mould.  A further reason to heat slowly is to avoid steam formation within the ceramic body.  If the steam is created over a short time, the force can be great enough to break the ceramic.  To ensure the water evaporates, a soak at 95°C/203°F for a significant amount of time is a better, safer option.

 But in addition to all these precautions, it simply is not necessary to cure kiln wash on slumping and draping moulds made of ceramics.  The glass does not begin to move until after 540°C/1000°F. Therefore, the kiln wash will be dry long before the glass gets near slumping temperatures.  Any vapor caused by evaporating water will escape through the vent holes in the mould or under the glass at the rim, as it will not form a seal until higher temperatures.

 

Newly kiln washed mould beside others already fired

 If you want to be sure your kiln wash is dry before you put the mould in the kiln, you can leave it in a warm ventilated space, or even on top of your kiln while it is being fired.  Using either drying method will dry the kiln wash sufficiently before the glass is placed on the slumping mould.

 The other part of the question was about drying the kiln wash between applying coats. It is not necessary to dry between coats of kiln wash.  In fact, a better result is obtained by applying all the coats at one time. It is not like painting wood. The result of applying all coats is a smoother surface.  There is no dragging of the dry powder along with the wet kiln wash as it is being applied over the existing coats.

 Kiln drying ceramic slumping and draping moulds is not necessary. It only adds another, unnecessary step in kilnforming preparations.  There are exceptionally good reasons to avoid rapid firing of damp moulds. 

 Some extra care could be taken with texture moulds and those intended for casting.

Simultaneous Fusing and Slumping

“I sometimes slump at the same time as I do a tack fuse. Is slumping at this higher heat bad for the mould? “

Image credit: Creative Glass

Mould

 It is possibly not bad for the mould, but it does depend on your temperature and heat work.  Ceramic moulds are typically fired to 1200° or 1300°C so higher kilnforming temperatures are unlikely to affect the moulds.  The speed at which the target temperature is reached is of concern though.  Ceramics have what is called quartz inversions.

 Two of the constituents of ceramics – cristobalite and quartz – have significantly large expansions at 226°C and 570°C / 440°F and 1060°F.  Rapid rises through these two temperatures risks breaking the ceramic mould.  This is not the case with steel moulds, of course.

Glass

 There may also be effects on the glass.  Slumping typically ranges between 620°C to 677°C (1150°F to 1250°F).  Tack fusing typically is done in the 740°C to 790°C (1365°F to 1455°F)range.  This is a significant difference even at the higher end of the slumping range and the lower end of the tack fusing range. 

 Some of the effects are:

·        The marking of the slumped glass will be greater at tack fusing. 
·        The glass will slip down the mould more. 
·        Any pieces applied to the base are likely to slide during the slumping process.
·        There is a risk of creating an uprising or bubble at the bottom as the glass slips down the side of the mould. 
·        There is more risk of creating needle points at the edges.

 Performing two processes at the same time risks difficulties.  Inevitably, compromises will need to be made between slumping and tack fusing.  Eventually, it will come to a time when the two process won't work together.

  

A slump taken to tack fusing temperatures is at risk from uprisings at the bottom, needling at the edges, excessive marking on the back, slipping down the mould and thickening

Drying Kiln Washed Moulds

There seems to be a popular notion that newly kiln washed moulds must be cured before use.  I'm not sure where the information comes from, and no reasoning is given.  It is suggested that that quickly heating newly kiln washed moulds to 550°F (290°C) is important.

If you want to make sure the mould is dry, this may not be the best way to do it.  All ceramics have a quartz inversion at around 225°C.  This a very rapid increase in volume of 2.5% that often leads to cracks and breaks in ceramics when the rate of advance is quick.  The mould will react better and last longer if the rate of advance is slow until that inversion temperature is passed.

This is a reason to advance the temperature slowly when slumping or draping with a ceramic mould.  Another reason to heat slowly is to avoid steam formation within the ceramic body.  If the steam is created over a short time, the force can be great enough to break the ceramic.  To ensure the water evaporates, a soak at 95°C for a significant amount of time is a better, safer option.

But in addition to all these precautions, it simply is not necessary to cure kiln wash on slumping and draping moulds made of ceramics.  The glass does not begin to move until after 540°C (about 1000°F). Therefore, the kiln wash will be dry long before the glass gets near slumping temperatures.  Any vapor caused by evaporating water will escape through the vent holes in the mould or under the glass at the rim, as it will not form a seal until higher temperatures.

Newly kiln washed mould beside others already fired

If you want to be sure your kiln wash is dry before you put the mould in the kiln, you can leave it in a warm ventilated space, or even on top of your kiln while it is being fired.  Using either drying method will dry the kiln wash sufficiently before the glass is placed on the slumping mould.

Kiln drying ceramic slumping and draping moulds is not necessary. It only adds another, unnecessary, step in kiln forming.  There are exceptionally good reasons to avoid rapid firing of damp moulds. The exceptions can occur with texture moulds and those intended for casting that do not have vents.

Thermal Shocking Ceramics

When firing glass in ceramic moulds, and especially ceramic pots for pot melts, you should be aware of the temperatures at which the ceramic material quickly expands and contracts.

There are refractory ceramics which are not as sensitive as the kind of ceramics we are using in most kiln work.  The ceramics we use as moulds are not refractory materials and contain, among other things, quartz and crystobalite. These two elements are important, as they have considerable effect on the survival of the pot or mould during the firing.

The effects are called inversions.  This is because the rapid expansion experienced upon the heating is reversed as rapid contraction on the cooling of the ceramic.

The first element to be affected by the heat up is crystobalite.  This element has a sudden expansion of 2.5% at 226°C.  This does not seem to be much, but compare it to the expansion of glass at this temperature - .0085% - almost 300 times that of glass at the same temperature.  And of course, the ceramic contracts by that amount when it reaches 226°C on the cooling.

The second element affecting the heat up is quartz.  There is quite a bit of this in clay.  The critical temperature for this is in the 570°C to 580°C range.  The expansion and contraction is not so great here – only 1% - but it is still more than 100% that of the glass, and in a critical range for the glass on the cooling.   

More information on the quartz and crystobalite inversions are given here.

The importance of these inversions for us are to remind us to be careful at these temperatures of about 225°C and 570°C - 580°C to prolong the life of the ceramic pots and moulds that we use.  

It is probable that 150°C per hour is as quickly as we should increase the temperature when using ceramic moulds or pots.  Some thought should be given to the cooling of the moulds too.  They should not be taken from the kiln while hot nor subjected to draughts of relatively cold air.

Quartz Inversions and Conversions

You need to know about this in both casting and when using ceramic pots in the kiln.

Quartz
Crystalline solids are rather temperamental and quartz is no different. Quartz is a crystalline form of silica in that it has a three dimensional regular pattern of molecular units. These form naturally in nature because lengthy cooling times allow arrangement. Quartz is made of a network of triangular pyramid (tetrahedron) shaped molecules of silicon combined with four oxygens.

Unfortunately, the quartz delights in changing the orientation of the tetrahedron shaped molecules with respect to each other, thus loosening or tightening the whole mass (and thus changing its total size). It exhibits twenty or more “phases”. A change to another phase is called a “silica conversion”. The most significant phases are quartz, tridymite, crystobalite, and glass.

Inversions
Changes which occur between these are reversible, that is, the change which occurs during heat-up is inverted during cool down. These changes are thus called “quartz inversions”. These inversions, unfortunately, often have associated, rather sudden, volume changes. That means that quartz conversions are something to consider when optimizing the fired properties; quartz inversions are something to consider when firing to prevent cracking losses. There are two important inversions you need to know about because of their sudden occurrence during temperature increase and decrease.

Quartz
The first is simply called ‘quartz inversion’ and it occurs quite quickly in the 570°C range (1060°F). In this case, the crystal lattice straightens itself out slightly, thus expanding 1% or so. This is therefore an important temperature in casting as it is an expansion on the heat up and a contraction, “grabbing” the glass on the way down. This is the reason for various modifiers when silica or flint is used as the strengthener.

Crystobalite
The second is crystobalite inversion at 226°C. This is a little nastier because it generates a sudden change of 2.5% in volume. This material has many more forms than quartz, so it is complex to say the least. However, while all bodies will have some quartz, you won’t have a problem with crystobalite inversion unless there is crystobalite in your body. Crystobalite forms naturally and slowly during cooling from above cone 3 (1104-1149°C). It forms much better if pure crystobalite is added to the body to seed the crystals or in the presence of catalysts (e.g. talc in earthenware bodies). Thus, this element exists in most ceramic moulds and moving slowly around 226°C should be observed when firing containers made of ceramic materials.