Wednesday, 25 February 2015

Calcium Silicate for Moulds

I have recently discovered calcium silicate board. It is light weight, with no fibrous material. It is workable with wood carving tools and can be smoothed with sanding papers. It is manufactured in 25 mm thick boards, and so is suitable for shallow forms.

It requires no hardeners, but it does require kiln wash to ensure clean separation of the fired glass. It also is durable, but needs careful storage and handling.  It does not appear to warp with repeated heating.

It is uncertain that the board will perform adequately above 800C, so it is best to support the shelf well and consider it a mould for temperatures below 720C where it is known to perform very well.

An additonal factor in its favour is that it is significantly cheaper than either ceramic fibre board or vermiculite board.

Wednesday, 18 February 2015

Flattening stringer

Placing stringer is often difficult. Not simply to put it into place, but to keep it there. People tend to hold the pieces with glue. However, the glue burns off before the stringer is anywhere near even tack fusing temperature. This allows the stringer to roll. Also an excess of glue will boil off during the heating and so move the stringer even more than gravity will. Two methods are effective in reducing the ability of the stringer to roll, by flattening one side.

One method used by Bob Letherbarrow is to hold a stringer that is much longer than needed against the glass grinder bit. Use your thumb to hold the stringer against the bit. Slowly pull the stringer between the bit and your thumb or finger, using light pressure, to hold the stringer against the spinning grinder bit. This will slightly flatten one side of the stringer.

Another method is to place the stringers on a prepared kiln shelf, making sure they do not touch. Take the temperature quickly up to a tack fuse, soak for a few minutes and turn off. This will take only an hour or so and enables you to prepare a number of stringers with a minimum of effort.

In both cases, cut the stringer to the length needed and place on the glass with the flat side against the glass. Glue it very lightly to hold it in place while moving the piece to the kiln if it is absolutely necessary. When the glue burns off the stringer should not roll around. It is best, of course and if at all possible to place the stringers in the kiln, so no glue is needed. 

Wednesday, 11 February 2015

Equalisation temperature

In my view a schedule has the following stages.
  • Initial rate of advance to bubble squeeze,

  • Rapid increase in temperature to target, or working temperature,

  • Quick fall to temperature equalisation (often called the annealing point),

  • Slow decrease in temperature - to keep internal stresses at a minimum - to 110C below that temperature equalisation point,

  • Faster cool to 100C or less.

Of course, some of my firings have up to 10 segments, so don't mistake the stages as equivalent to schedule segments. The following graph is a generalised version of these stages.  The times and temperatures are for illustration only.

The equalisation temperature is what is most often called the annealing point. This is a mathematically determined temperature at which the glass most quickly anneals - has stress relieved. However, the way kiln formers work, annealing does not occur at one temperature point on the controller output, because of the inherent inaccuracy of our kilns and controllers. The soak at the annealing point has the purpose to equalise the temperature throughout the glass before proceeding to the anneal cool.

There is little point in soaking above this temperature, only to have another, lower temperature soak at the published annealing point. The soak at the annealing temperature will negate any effect of a soak at a higher temperature. So, a soak above the annealing temperature will simply slow the whole cooling process.

Of course, the soak at the equalisation temperature must be long enough to get the whole substance of the glass to the same temperature. The thickness of the glass will determine the length of this equalisation soak. Fortunately Bullseye have published a table to help determine the time required.

The slow decrease in temperature is to keep all the substance of the glass to within 5C difference on the cooling. Thus, the rate of cooling is related to the thickness of the glass. It will be increasingly slower with increasing thickness. The cooling to around 110C below the equalisation temperature is all part of the annealing process. The more rapid cooling after that is to control the rate of temperature fall to avoid thermal shock.

Wednesday, 4 February 2015

Thermal Shock

Thermal shock is a term for a break caused by a too rapid change of temperature within a piece of glass.

"Glass tends to be 
1) very brittle, 
2) expand and contract quickly when subjected to temperature changes, and 
3) is an insulator (when solid) and therefore does not readily conduct heat. 
That is why glass is highly susceptible to thermal shock"

This can occur on both an increase or decrease in temperature. Glass conducts heat poorly.  The ideal is to keep the temperature differentials within the glass to 5C or less.  This is the purpose of the anneal cool.  The risk of thermal shock can be increased by different thicknesses across the piece. Greater care is required in cooling these pieces than those of uniform thickness.

A piece showing large differences in thickness and so at greater risk of shock


The break normally is straight through the glass without following the edges of the various pieces of glass.

This shows the break crossing multiple colours of glass

The line of the break will be rounded if it parted on the heat up. In some cases, the glass will have stuck back together if it was dammed or the break was gentle enough to avoid pushing the glass apart.

If the shock occurred on the cool down, the edges will be sharp.  

The edges will also be sharp in a slump whether the break occurred on the advance or the reduction in temperature.  If the pieces fit together perfectly the break is likely to be in the down phase.  If the pieces are slightly different shapes the break likely occurred in the rise in temperature phase.

Other kinds of breaks are possible and are described elsewhere.

Wednesday, 28 January 2015

Glasses at Risk of Compatibility Shift

Many people take their fusing glasses beyond the tested parameters of the manufacturers in pot and screen melts and combing and casting operations. It has been speculated that there are compatibility shifts of hot colours and of opalescents.

Reading, and some experience, lead me to the belief that is the colouring minerals that are the key to which glass will shift in compatibility. Colours made with sulphur and selenium are more likely to opalise and also change their compatibility at extended times at high temperatures. Extended time is in the region of an hour or more. High temperatures are those over 850ºC

The colours at most risk of compatibility shift seem to be:
and a few bright and olive greens, but not dark greens.

Of course testing, using polarising light filters, is required to determine which will remain compatible after long, high temperature firings.  A method of testing is given here.

High temperature compatibility shifts are discussed here.

Wednesday, 21 January 2015

Compatibility Shift at Higher Temperatures

People experience breakages of their pot and screen melts that do not seem to have anything to do with annealing or glass sticking to the shelf. The common suggestion is that there has been a compatibility shift of the glass. This view is re-enforced by the opalisation of the transparent hot colours experienced by most.

Bullseye indicates in their glass notes that some colours are not suitable for high temperature work. This probably applies to other fusing glasses too. My experience leads me to believe that this compatibility shift occurs with all the opalescent glass colours as well as the hot ones. Further work will appear soon. is required to determine if there are any general indicators of the kinds of glass that are likely to develop incompatibility at high temperatures.

If you are concerned about the lack of durability of your piece due to possible incompatibility, you need to include tests with the firing. To make this test, place a piece of each colour used in the melt on a double layer of clear. If you are using a single base piece, ensure you leave space between the colours. It is best to place each colour on its own stack of clear. Also place a stack of clear glass as thick as your blank along side the other test pieces. Put all those pieces somewhere within the kiln out of the way of the area the melt will occupy and fire the lot together.

When cool, take all the pieces from the kiln and check the test pieces for compatibility. Do this check with a polarising filter to determine whether there is any incompatibility by looking for the halo showing the degrees of incompatibility.

If any or all, of the the pieces show stress, check the clear stack for stress. If the clear also shows stress, the annealing has been inadequate, rather than just the compatibility shift. Ideally, this process should be conducted in every firing.

Performing these tests will give you confidence in the durability of your piece, as it will show the levels of stress in the finished piece.

Wednesday, 14 January 2015

Annealing Unknown Glass

Sometimes you may want to use a glass in kiln forming when its characteristics are not known, such as for bottle slumping. It is possible to determine the approximate annealing point of this glass in your own studio. This tip on compatibility testing gives you the information to do the test.

If you do not want to go to that detailed effort for a one-off process, you can adopt the shotgun annealing approach. This does require some observation of the glass, of course.

You need to observe when the glass has reached the temperature for the process you are performing. This will enable you to compare the behaviour of this unknown glass with what you normally use. This will give some idea of the relative annealing temperature to use. If a higher temperature is required for this glass than your normal glass, a higher annealing point can be assumed. The difference in top temperature can be added to the annealing point of your known glass.  If the top temperature is lower, you subtract the difference from the known glass' annealing point.

Set the annealing temperature to be 10C to 20C above the predicted annealing temperature and soak there for 30 to 60 minutes. This will help ensure the glass is all at the same temperature throughout. Set the annealing cool to be at about 30C per hour for pieces up to 6mm for the first 55C. The next segment should be about twice that to 110C below your chosen annealing temperature. The final segment can be around 150C per hour to 100C.  For thicker glass, the annealing cool should be proportionately slower.

This may seem an excessive, overly cautious process, but as you get to know the characteristics of the glass, you will be able to alter the schedule. This is a conservative and safe process to ensure your glass is well annealed.  And to be certain, you should check the cooled glass with polarised light filters.

Wednesday, 7 January 2015

CoE Varies with Temperature

Information from Bullseye shows that the Coeficient of Linear Expansion changes rapidly around the annealing range.

The following is from results of a laboratory test of Bullseye clear (1101F)
Temperature range.......................COE
20C-300C (68F -­ 572F).................90.6
300C-400C (572F - ­752F).............102.9
400C-450C (752F - 842F).............107.5
570C-580C (1058F-1076F)............502.0

Bullseye glass is probably typical of soda lime glasses designed for fusing.

This shows why it is not helpful to refer to CoE without also mentioning the range of temperature.

In addition, here is an illustration of the effect. 

(If the owner of this illustration comes across this, please let me know, as I have lost the source)


The object of plating is to modify the original colour, either by changing the tone or the intensity. This will, for example, darken a piece of glass where it would otherwise be to bright; or it will modify the colour to better blend with the surrounding pieces.

A further use of plating is in conservation, where the additional detail is placed on a separate piece of glass and placed in front or back of the original.

In leading, you normally use high heart lead. This is lead with a heart of 7mm or 10mm instead of the usual 5mm. Other heights are available, of course. The 7mm heart will accommodate two 3mm pieces, but if you are using thick hand made glass, you may require the 10mm high heart.

Comparing the Arrangement
Try the glass combination with each piece on top. Often there is a difference in tone or texture. Choose the one that suits your composition best.

Before finally fixing the glass together, make sure they are very clean as there will be no opportunity to clean the inside again. Try to avoid finger prints on the insides while you do further work with the glass.

Make sure the glass fits the cartoon lines. You will be sealing the two pieces of glass together, so there is no opportunity to change the shape later. There are a variety of traditional methods of sealing the glass, but the easiest modern approach is to copper foil the edges to ensure that no cement creeps between the pieces.

You then fit the glass into the came as for thiner pieces. Where you have a combination of heart heights, you can simply slip the ends of the lower heart cames inside the leaves of the high heart leads. The differences in height are small enough that no special support is needed for the thinner glass unless you feel better with the single layers of glass supported above the work surface.

Saturday, 3 January 2015

Strain Points

A critical range is the temperature around the annealing point. The upper and lower limits of this range are known as the strain points. The higher one is the highest temperature at which annealing can begin. The lower one is the lowest point at which annealing can be done. Soaking at any lower temperature will not anneal the glass at all. This temperature range is a little arbitrary, but it is generally considered to be 45C above and below the annealing point. The ideal point to anneal would be at the annealing temperature, as annealing occurs most rapidly at this temperature.

However, glass kiln pyrometers are not accurate in recording the temperature within the glass, only within the kiln. The glass on the way down in temperature is hotter than the recorded kiln atmosphere temperature. So a soak at the annealing temperature is required. If you do a soak at 515°C for example, the glass is actually hotter, and is cooling and equalising throughout to the 515° point during the soak. The slow cool to below the lower strain point constitutes the annealing, the soak at the annealing point is to ensure that the glass is at the same temperature throughout, before the annealing begins.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken. But no further annealing will take place. If you do not anneal properly the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

The glass is brittle below the upper strain point temperature, although it is less and less likely to be subject to thermal shock as it nears that point.  It is after the upper strain point that you can advance the temperature as fast as you like without shocking the glass.  So, if you have a glass that gives its annealing temperature as 5125C, you can safely advance the temperature quickly after 560C (being 45C above the annealing point).

Friday, 2 January 2015

Solder Alloys, 1

Common Alloys of Solder with Melting Ranges:

   % tin    % lead    % silver     melting range
20 80        183-275C
30 70     183-255C
40 60        183-234C
50 50     183-212C
60 40     183-188C
63 37     183-183C
62 36 2     179C
45 54 1     177-210C
62 36 2     179C

Solder Alloys, 2

This is an updated version of a table on various possibly useful solders.
Solder Alloy  Composition  Solidus  Liquidus Uses
25/75 Sn/Pb 183C 266C general plumbing, car radiators
30/70 Sn/Pb 183C 256C general plumbing, car radiators
30/50/20 Sn/Pb/Zn 177C 288C economical solder for aluminium, Zinc and Cast iron
40/60 Sn/Pb 183C 238C brass, plumbing, car radiators
50/50 Sn/Pb 183C 216C general purpose, plumbing, not for gold, silver
50/48.5/1.5 Sn/Pb/Cu 183C 215C reduces copper erosion on irons
60/40 Sn/Pb 183C 190C electronics, good wetting, duller surface than 63/37
63/37 Sn/Pb 183C 183C eutetic, eletrionics, stainless steel, bright joints
62/37/1 Sn/Pb/Cu 183C 183C similar to 63/37 and reduces erosion on irons
90/10 Sn/Pb 183C 213C
95/5 Sn/Pb 238C 238C plumbing and heating
96.5/3/0.5 Sn/Ag/Cu 217C 220C recommended lead free for electronics 
95.8/3.5/0.7 Sn/Ag/Cu 217C 218C wave and dip soldering
95.6/3.5/0.9 Sn/Ag/Cu 217C 217C eutectic
95.5/3.8/0.7 Sn/Ag/Cu 217C 217C European preference for wave and dip soldering
96.5/3.5 Sn/Ag 221C 221C wide use, poor wetting, strong lead free joints, stainless steel
95/5 Sn/Ag 221C 254C strong, ductile joints on copper, stainless steel
94/6 Sn/Ag 221C 279C strong, ductile joints on copper, stainless steel
93/7 Sn/Ag 221C 302C strong, ductile joints on copper, stainless steel

Ag = Silver
Cd = Cadmium
Cu =Copper
PB = Lead
Sn = Tin
Sb = Antimony

Wednesday, 31 December 2014

Defining the Glass Transition Phase

We often treat glass as a simple material. However it is a very complex and as yet not fully understood material. One of the most curious aspects is the transition between plastic and solid states. This is the temperature range of glass annealing – called the glass transition by scientists. This note comes largely from "Glass Properties" produced by Schott. The text in brackets [ ] is my additional explanation.

The glass transition comprises a smooth but very large increase in the viscosity of the material. Despite the massive change in the physical properties of a material through its glass transition, the transition is not itself a phase transition  of any kind [in this case from a liquid to a solid] and involves discontinuities in thermodynamic and dynamic properties such as volume, energy, and viscosity.

Below the transition temperature range, the glassy structure does not relax in accordance with the cooling rate used. The expansion coefficient for the glassy state is roughly equivalent to that of the crystalline solid. [Thus the CoE, which is taken as an average of expansion per degree Celsius over the range of 0C to 300C, is an inadequate guide to how the glass will behave at the glass transition and higher temperatures.]

Glass is believed to exist in a kinetically locked state, and its entropy, density, and so on, depend on the thermal history. Therefore, the glass transition is primarily a dynamic phenomenon. Time and temperature are interchangeable quantities (to some extent) when dealing with glasses.

[Viscosity shows a relatively regular change with temperature changes.] In contrast to viscosity, the thermal expansion, heat capacity, shear modulus, and many other properties of inorganic glasses show a relatively sudden change at the glass transition temperature. Any such step or kink can be used to define Tg [the transition phase of glass].  To make this definition reproducible, the cooling or heating rate must be specified.

Thursday, 25 December 2014

Plating in Copperfoil

Plating is used to modify the colour, or intensity of local areas in a window or panel. Plating for leaded glass is normally putting two pieces of glass in the same came, although there was a common practice at the turn of the 19th into the 20th century to have the plate cover several pieces of leaded glass. In principle, the plating of copper foil panels is the same as for leaded glass, except there is no came to fit the glass into. So there are some variations.

An example where the fruit and leaves are all plated

Build the flat, single thickness window first. This provides a solid panel to work on. It also enables you to see whether you really need the plating, and if so the exact areas where it will be applied.

You should solder the whole panel except where the plate is to be soldered. In this/these areas just lightly tin the back, although you will have already put a solder bead over the whole of the front.

Patina the back of the panel, except where the plate is to go. Allow this to dry and clean up any spills, especially in the neighborhood of the plating.

Foil the plate with a backing to match the colour of the patina. So use copper-backed foil where the panel is in copper patina, but black-backed where the patina is black.

Tin the foil on the plate with solder. If the piece is to cross a number of the base pieces, you need to patina the tinned face that will be placed toward the viewer with the same colour patina. You need to make sure this is absolutely dry before proceeding.

Clean the plate and the base glass where the plate is to cover very well. Make sure there are no oils or tarnish on the solder, and that everything is dry.

Solder the plate to every seam that it contacts with no flux and a small amount of solder. This is to insure there is no leakage of flux - by not using any - or solder between the two pieces of glass.

Put a small amount of clear silicone between the edge of the plate and the base glass where you were not able to solder. Just lightly fill the gaps to ensure a seal against moisture and insects.
When the silicone has cured, carefully patina the plate so no fluid seeps between the glasses.

Protect the uneven back when handling by placing a soft foam pad, or a polystyrene sheet with cutouts for the plating, on the back to protect the panel from the carrying board.

Wednesday, 24 December 2014

Temperature Conversions

Temperature conversions from Celsius to Fahrenheit for some common temperatures in kiln forming:

100C  =  212F
200C  =  396F
300C  =  577F
400C  =  759F
427C  =  808F
482C  =  908F
500C  =  941F
600C  =  968F
650C  = 1123F
677C  = 1263F
760C  = 1414F
780C  = 1450F
800C  = 1487F
850C  = 1577F
900C  = 1668F
950C  = 1759F

The formula for temperature conversion is:
ºC divided by .555 plus 32 (for the freezing point of water)

Conversion of rates of advance is different (the freezing point of water does not need to be taken into account):
25C   =   45F
50C   =   91F
75C   = 136F
100C = 182F 
150C = 273F
200C = 364F
250C = 455F
300C = 545F
350C = 636F

The formula for rate of advance conversions is:
ºC divided by .555 only

Wednesday, 17 December 2014

Volume control

When creating a casting, pot melt or other object from glass cullet or billet, you need to be sure you have a large enough volume of glass to fill the area. You can do it by measuring the volume or by calculating the weight. This note is about calculating the weight.

Filling a damed area with enough pieces of glass provides an illustration of volume control. To help make sure you have enough glass to fill the space, measure in centimetres to determine the area. For a rectangle, measure length by width in centimetres. For a circle multiply the radius by itself (radius squared) times 3.14 (pi) to get the area.

To determine the minimum volume required, multiply the area by 0.6 cm. This is the approximate thickness that glass takes up at full fuse. As the amount of time and heat that we normally give to the process is insufficient to allow the glass to fully flow, the glass will tend to be thicker in the middle when using pieces of glass rather than sheets. So you may wish to multiply the area by 0.7 cm (to make sure you have enough).

To get the weight of glass required for the space, multiply the calculated volume by 2.5 (specific gravity) to get the weight in grams. Divide by 1000 to get kilograms. If you must use pounds, multiply the kilos by 2.2, the number of pounds in a kilo.

Wednesday, 10 December 2014

Tack Fuse Temperature

The tack fuse range is around 730C – 780C. This will give a graduation in profile from the very sharp, almost barely laminated, to one very rounded almost flat. Choosing the right heat for the right profile is one of balancing several elements: temperature, time, speed.

Low temperature, high tack fuse

If there were no other considerations, you could go slowly up in temperature and peek in at infrequent intervals until the right profile had been achieved. However this tack fusing is happening in the devitrification range, so slow rises in temperature are not advisable.

Medium temperature, mid tack fuse

So an alternative strategy would be to go quickly through the devitrification range (700C to 760C) and soak for a bit longer above that range. However, often the desired profile may has disappeared by the time you get to 770C.

High temperature, rounded tack fuse

It would seem that you can attempt to balance the temperature, time and speed equation by firing quickly (such as 330C/hr) to your desired temperature and soak there for 10 minutes only.

To ensure you get the profile that you want you should begin to observe from at least 10C below your chosen temperature. If you do not get the profile you want, you can extend the soak until the desired effect is achieved. On a subsequent firing, you can set the top temperature a bit higher, but with the 10 minute soak and again observe. This can be repeated until the desired combination is achieved.

Each of these attempts needs to be completely recorded so that the results can be used in later firings if slightly different profiles are needed.

Also look at this entry for annealing of tack fusings.

Wednesday, 3 December 2014

Steel Pipe for Slumping

Steel pipe as opposed to stainless steel can be used for slumping. It will spall, so there will be a need to clean up the flakes of rust after firing. But since there is so much spalling, putting kiln wash or boron nitride is a waste of effort. Each firing will flake off any separator painted onto the metal. Cover the pipe with fibre paper instead - 0.5 mm at least.

You need to advance in temperature slowly as the pipe drains the heat from the glass where it rests. My practice is to advance the temperature at 100C/hr to 100C with a 20 minute soak, followed by 50% increases in rate to 250, and to 500 with 20 minute soaks before proceeding to the next segment. This probably is more cautious than necessary on all but the first segment.

Wednesday, 26 November 2014

Bubbles in Thin Pieces

Bubbles are often blown through frit castings and other thin pieces. This most results from insufficient volume of glass in the mould or on the shelf. Also the design can induce bubbles where there are thinner parts surrounded by thicker parts. As the glass softens, the surface tension of glass - from around 730 - causes it to pull up to equalise at about 6-7mm thick. This causes thinning in certain areas to allow thickening in other areas. This then leads to the risk of blowing bubbles through the glass where the glass has become thinner.

If thinner work is required, you can fire an over-sized piece to about 750C for a short time and then cut it back to the final size. If you want a flat thin sheet, you can also place the glass between two kiln shelves. You need to separate the shelves with a 3mm spacer to keep the upper shelf from coming completely down on the shelf, giving an extremely thin fragile piece of glass.

Wednesday, 19 November 2014

Diagnosing Fractures

What does the nature of the fracture tell about the reason for the break?
  • incompatibility
  • annealing
  • adhesion
  • splits
  • lamination

Fractures that follow the outline of a glass are normally indicators of incompatibility. The fracture starts at the incompatible glass and then - usually – goes directly to the nearest edge. Occasionally, the stress is not so great, so it only breaks around the offending glass without proceeding to the edge.

A sinuous break – often with a hook at the edge – across the whole of the piece is generally an indication of one caused by an annealing stress. Inadequate annealing builds up stress within the glass that breaks through the whole piece in a lazy “S” pattern, rather than a straight line or following outlines of glass pieces.

Another kind of fracture occurs that is most often seen in ceramics. It is a kind of crazing that leaves the glass in granules. I call these adhesion fractures. This is indicative of the glass having stuck to the surface it is resting upon. This can be ceramic, steel or any other rigid refractory material. This comes from inadequate amounts of separator, often at high temperatures.

Sometimes during slumps the piece can develop a tear or split in the lower surface without the upper breaking. This kind of split comes from heating the top of the glass more rapidly than the heat can penetrate the whole thickness. The weight of the relatively plastic upper surface overcomes the resistance of the lower surface by splitting it on the bottom face.

Occasionally, a break will have both of the characteristics of incompatibility and annealing stress. The break is relatively straight and goes through differing colours rather than skirting them. This seems to happen most often on tack fused pieces and so is likely to be inadequate annealing. The annealing requirements of tack fused glass are much greater than flat fused glass, as the pieces are to some extent still reacting separately. If the whole piece is not given enough time for each piece to settle with the others they will contain unrelieved annealing stresses, which may have be too great to be held within the whole.

Wednesday, 12 November 2014


It is the monitoring and observation of the effects firings as they progress that allows confidence in setting firing temperatures and schedules. Although we all have busy lives, planning the firings so you can watch at the forming temperatures enables you to develop your firing practice much more rapidly than firing and waiting to see what comes out the next day. It means that in a single firing you can pretty accurately determine the temperature you need for firing that type of piece, rather than an number of separate firings.

You set your schedule - for the best guess that you can make - at the required temperature, rate of advance, and soak to achieve what you need. At about 50ºC to 20ºC (depending on your certainty) before the set point, you begin peeking to see what the glass is doing. When the glass has achieved the desired result, you advance to the next segment. You of course, have already refreshed your memory on how to do that from your kiln manual.

There is a method of opening and closing kiln to be safe and avoid disturbing the contents. Any observation ports should be opened first. The lid/door should be opened slowly and only enough to see what you had already planned to look at, to determine whether it is ok or a decision is needed for some other action. This opening should be only a few seconds. The air temperature will change dramatically, but the glass temperature will lag behind significantly, so a few seconds with the door only cracked open will not damage the glass at most temperatures. The exception to this is the annealing range – generally around 520C to 400C. The kiln should not be opened at these temperatures so that there is no disturbance possible to the steady and even annealing of the glass.

At temperatures above the annealing, you need to have protective clothing. At the minimum you need natural fibres such as cotton or wool, and eye protection. It is important to check with your hand the amount of heat coming from an observation port before moving your face toward it to look into the kiln. When the kiln is being opened even for brief periods, you should protect you eyes from the infra red given off by the kiln's interior. You should have something to protect your arms and chest too.

Always when raising and lowering the lid – or opening and closing the door – do it slowly to avoid creating puffs or billows of air moving through the kiln which might disturb the pieces at low temperatures or move debris over the hot glass at the higher end of kiln forming.

If the glass has not achieved what you want by the end of your soak, just extend the hold until the effect is achieved. You will have reviewed how to do that from your kiln manual before starting the firing. When the glass has achieved the effect you desire, advance to the next segment of the schedule as the kiln manual directs.

You then record the schedule including temperatures, rates, times, effects, etc. You should include a description of the project and its dimensions and nature e.g. full fused, tack fused etc. You will also want to include what this was fired on, what kind of mould – include its description. This will give you the reference for that nature of project for the future without needing to guess.

Wednesday, 5 November 2014

Recognising Devitrification

The appearance of devitrification varies from mild streaks as a dirty appearance on the surface, to at worst a granular surface that breaks away in small pieces. The glass will often have raised sharp corners in cases of severe devitrification.

Avoiding devitrification relates to cleaningfiring rapidly through the devitrification rangeavoiding devitrification-prone glasses, and grinding edges as little as possible.

Repairing devitrification requires the removal of the devitrified surface. This can be done by sandblasting, sanding the surface by hand, using acid pastes to remove the surface. Then the piece needs to be fired again to a fire polish.

To ensure a polished surface a devitrification solution may need to be applied. It can be a commercial product or a borax solution.  Any devitrification solution should be applied evenly.

Wednesday, 29 October 2014

Spray nozzles

Quite a bit of the material we spray is solids in a colloidal suspension. This means that the nozzle can clog easily. Frequent agitation is needed to keep the material in suspension and not building up on the bottom which can clog the screen at the bottom of the tube.

You should not have long delays between spraying in one session, as the solids can begin to solidify within the spray head and so clog it.

When you have finished spraying, take the spray head off and clean it completely and thoroughly to ensure there are no solids left to harden. Then put it back together and it should be clear for the next use as well as sealing the container.

Wednesday, 22 October 2014

Stainless Steel Preparation

Preparing stainless steel rods and moulds for kiln work is done slightly differently from ceramic moulds.

Just to ensure that the steel is of the right grade, I fire it in the kiln to about 720C. This ensures that if the steel is not adequate for the high temperature work, you will find out that it spalls before the glass is put on top. It also has the advantage of removing any dirt and oils on the surface of the metal.

The separator that you need to put on the steel can be done cold if you use MR97 or other boron nitride coating. Its main advantage is that it can be put on cold and also that it has a very smooth surface. This should be put on thinly, or it will come off onto the glass.

You can also put standard kiln wash on the metal. The metal needs to be dry and clean. It could be sandblasted if desired for a bit of extra “tooth”, but is not normally necessary. Heat the metal to about 120C – 150C in the kiln. Remove it from the kiln with tongs or similar thing to grasp the hot metal. Spray or paint the kiln wash solution onto the hot metal. Return it to the kiln as necessary until you have a coating all over the metal. It does not have to be even all over, but noes need to have all of the metal covered.

If the kiln wash boils off the metal, it is too hot. So turn the kiln down a bit.
If the kiln wash runs off without sticking at all, the metal is not hot enough and needs to be returned to the kiln to heat up.
It is best to avoid applying the kiln wash to the metal in the kiln, as water and the hot elements do not mix well.