Float Annealing Temperatures

Float glass annealing temperatures vary quite a bit from one manufacturer to another; and even within one manufacturer’s product line.

Comparisons of various float glasses

Some companies are more informative that others.  Pilkington are one of the more open of European glass manufacturers on various bits of information.

Pilkington Float

CoLE 83 *^10-5

Softening point:  715°C

annealing point:  548°C

strain point: 511C

Pilkington Optiwhite ™

Softening point:  ca. 732°C

annealing point:  ca. 559°C

strain point:  ca. 526°C

There is a difference of 11C between two of the Pilkington product lines for the annealing points.  The softening and strain points are slightly wider.

Glaverbel, a Belgian company, restricts their information to CoLE and the softening point.

CoLE 91 * 10^-5

Softening point: 600°C

Saint-Gobain, a French company, shows some more of the variation in the product lines, although they do not give specific annealing points for the different products.

CoLE 90 * 10^-5

annealing range:  520 - 550°C

Low E glass

softening – 840°C

strain - 617°C

R glass (sound reducing)

softening – 986°C

strain - 736°C

D glass (decorative)

softening point – 769°C

Compatibility

Even this small sample of float glasses shows there is a significant difference between manufacturers for the softening, annealing and strain points.  This means that, unless you are sure of the glass merchant’s source of glass, you will need to test each batch of glass for compatibility with previous batches, if you are combining from different suppliers.

I included the CoLE numbers (which all the manufacturers specified as an average change in length for each degree C increase in temperature from 0 to 300°C) to show the variation and to challenge anyone to find Bullseye and Saint-Gobain or Glaverbel compatible with each other.  My experience has shown that the Optul coloured frit and confetti is more likely to be compatible with Pilkington than the other two.

Annealing

I have been beginning my annealing of float glass at 525°C.  This little bit of literature research shows that my annealing soak should be starting higher, possibly at 540°C, certainly no lower than 530°C.  Other areas of the world may find their float glass has significantly different annealing ranges.

Frit by thermal shock

Frit can be created by thermal shock.  You will still need to do some manual breaking up. The principle is that you heat the glass and then cool it rapidly, causing the glass to break into pieces from the thermal shock.

There at least two approaches depending on the type of kiln.

Clamshell and front opening kilns

Place the glass in a stainless steel bowl and heat as fast as possible to 300C – 400C. Turn the kiln off and pull out the bowl, using heat resistant gloves and dump the hot glass into a large bucket of water. 

Lidded and deep kilns

Place the glass, in a stainless steel colander, into the kiln.  Retrieve the hot glass with a hooked pole to lift the container out of the kiln and lower it into a bucket full of water.

Once the glass is cool, pour off the water and dry the glass.  When dry, you can break the crazed glass into smaller bits just as you would with other glass.  Note that pouring water over the glass has two disadvantages – one, it does not completely thermal shock the glass, and two, the large amount of steam released is very dangerous.The advantages of this quenching method of obtaining frit are that you can create frit with less effort.  You also get less fines and powder with this method. And less effort is required to smash up the glass.

Some indicate that ice cold water to quench the glass is a good idea.  This is because warm water will not provide enough of a shock to the glass to craze it throughout.  But if you have a large bucket of water, there is no necessity, as the volume of water will cool the glass quickly enough.  Of course, if you are planning another quenching, you need to renew the water, as it will not be cold enough to thoroughly craze the glass.

You can, in part, control the size of the resulting frit.  Firing at 300C/573F results in larger frit than firing at 400C/753F.  However, firing at 500C/933F does not provide even smaller frit.  The best results are between 300-400C/573-753F, although frit can be made at 200C/392F as well.  Experiment with temperatures to get the frit you want.

Once you have dried the frit, you can begin to break it up. Some can be done by hand, but the pieces are often sharp, so gloves are essential.  The other standard methods of breaking up glass to make frit are applicable. But it does not take as much effort as breaking from cullett.

revised 7.2.25

Hard Spots in Moulds

Hand pouring of slip into a mould

Some ceramic moulds have small areas where the kiln wash does not seem to adhere as well as on the rest of the mould.  This comes from the manufacturing of these slip cast moulds and this blog post explains how it occurs.  The question is what to do to make the mould separate from the glass after firing.

Coat the mould as usual, which shows up the area where it seems no kiln wash is sticking.  There is some coating the area, but not in the same amount as the rest of the mould.  You can add a little extra kiln wash to the area once first layer has dried, but be careful to avoid creating a ridge against the rest of the kiln wash. If one does appear gentle smoothing with a finger can disguise the transition.

Another approach is to abrade the spot a little to make a more textured surface for the kiln wash to attach.  This needs to be done carefully and by hand to avoid creating a shallow divot in the mould.

The safe approach is to coat as usual and slump a sacrificial piece of glass to ensure the glass does not stick to the hard spot.  If it does not, the spot has enough separator to be useable, although I would continue to add kiln wash to that spot for several firings.

 

Tack Fusing Considerations

Initial Rate of Advance

Tack fuses look easier than full fusing, but they are really one of the most difficult types of kiln forming. Tack fusing requires much more care than full fusing.
On heat up, the pieces on top shade the heat from the base glass leading to uneven heating. So you need a slower heat up. You can get some assistance in determining this by looking at what the annealing cool rate for the piece is. A very conservative approach is needed when you have a number of pieces stacked over the base layer.  One way of thinking about this is to set your initial rate of advance at approximately twice the anneal cool rate. 

Annealing 

The tacked glass us loosely attached rather than fully formed together.  So, the glass pieces are still able, partially, to act as separate entities, meaning excellent annealing is required.

Effects of thicknesses, shapes, degree of tack

1. Tack fusing of a single additional layer on a six millimetre base

1. Rectangular pieces to be tack fused

1. Sharp, pointed pieces to be tack fused

1. Multiple layers to be tack fused

1. Degree of tack – the closer to lamination, the more time required

Glass contracts when it's cooling, and so tends to pull into itself. In a flat, symmetrical fuse this isn't much of a problem. In tack fuses where the glass components are still distinct from their neighbours, they will try to shrink into themselves and away from each other.  If there is not enough time for the glass to settle into balance, a lot of stress will be locked into the piece that either cause it to crack on cool down or to be remarkably fragile after firing.  In tack fusing there also are very uneven thicknesses, making it is hard to maintain equal temperatures across the glass.  The tack fused pieces shield the heat from the base, leading to localised hot spots during the cool down.

On difficult tack fuses it's not unusual to anneal for a thickness of two to three times greater than the thickest part of the glass.  That extended cool helps ensure that the glass has time to shift and relax as it's becoming stiffer, and keeps the temperature more even throughout.

In general, tack fused pieces should be annealed as though they are thicker pieces. Recommendations range from the rate for glass that is one thickness greater to at least twice the maximum thickness of the whole item.  Where there are right angles - squares, rectangles - or more acutely angled shapes, even more time in the annealing cool is required.

It must be remembered, especially in tack fusing, that annealing is much more than the annealing soak.  The soak is to ensure all the glass is at the same temperature, but it is the anneal cool that ensures the different thicknesses will all react together. That means tack fusing takes a lot longer than regular fusing.

  

The more rectangular or pointed the pieces there are in the piece, the greater the care in annealing is required.  Decisions on the schedule to use varies - some go up two or even four times the total thickness of the piece to choose a firing schedule.

A simple way to determine the schedule is to subtract the difference between the thickest and the thinnest part of the piece and add that number to the thickest part. If you have a 3mm section and a 12mm section, the difference is 9mm. So, add 9 to 12 and get 17mm that needs to be annealed for. This thickness applies to the heat up segments too.

Another way to estimate the schedule required is to increase the length the annealing schedule for any and each of the following factors:

The annealing schedule to be considered is the one for at least the next step up in thickness for each of the factors. If you have all five factors the annealing schedule that should be used is one for at least 21mm thick pieces according to this way of thinking about the firing.

 

4 – Testing/Experimentation

The only way you will have certainty about which to schedule to choose is to make a mock-up of the configuration you intend in clear.  You can then check for the stresses.  If you have chosen twice the thickness, and stress is showing, you need to try 3 times the thickness, etc., which can be done on the same piece.  You can reduce time by having your annealing soak at the lower end of the annealing range (for Bullseye this is 482C, rather than 516C).

You will need to do some experimentation on what works best for you. You also need to have a pair of polarisation filters to help you with determining whether you have any stress in your piece or not. If your piece is to be in opaque glasses, The mock-up in clear will be useful.

First published 18.12.2013

Revised 29.01.25

Over Annealing

 I hear the comment "you can't over anneal" all the time. Is it true?

My response to this may be controversial, and I do expect there will be some dispute with aspects of what follows.  My view of the statement “you can’t over anneal” is that it results from a lazy approach to thinking about the process.

The short answer is, in my view “yes, you can over anneal”.

• ·         Lengthy anneal soaks can induce stress in certain circumstances. More later.
• ·         Excessive annealing soaks waste time and money.
• ·         Annealing is more than the soak.  It is a combination of equalisation of the heat within the glass (not just temperature) and the gradual cooling of the glass to below the lower strain point to ensure the glass does not incorporate differences of temperature of plus or minus 5°C.

There is both tradition and research to assist in determining the length of the anneal soak.  The tradition seems to embrace 30 minutes anneal soak for each 3mm-layer of glass. The research has been done by Bullseye and they have developed a table to assist in accurately determining annealing soaks for thick glass. 

It informs users of the relationship between thickness and annealing soaks and cooling.  The table starts at 6mm/0.25" thick, and goes up to 200mm/8" thick.  The annealing soak temperature used needs to be altered for glass other than Bullseye, but the soaks, rates, and temperatures remain valid for all fusing glasses. Use the research, rather than tradition.

Other considerations include the nature of the kiln.  If your kiln has significant temperature differentials across the shelf, long annealing soaks will incorporate those differences during the annealing cool and result in a stressed piece. You do know the temperature distribution within your kiln, don’t you?  This Tech Note #1 from Bullseye will give you the information to test for the temperature distribution. Using this information will enable you to avoid the cool spots when placing your pieces and utilise the areas where the heat is even.

Economy is another reason that it is possible to over anneal.  Soaking at the annealing temperature uses a significant proportion of the electricity consumed in a firing.  This means an overly long temperature equalisation soak will use more electricity than necessary.  It also uses more kiln time than necessary, by delaying the anneal cooling and the following natural cooling rate of the kiln.

It is possible to under anneal, of course.

You need to learn about the effects of your project on annealing requirements, because it is possible to under anneal.  The research on annealing is based glass of uniform thickness. The most popular style of kilnforming appears to be tack fusing of one degree or another.  This is unfortunate for the novice, as it is the most difficult of styles to anneal adequately. There are a lot of factors to consider when setting the annealing schedule for tack fusing. 

I feel this is the origin of “can’t over anneal” thinking.  Instead of thinking about the specific annealing difficulties, many seem to just add more time in a generally random manner.  The post on tack fusing considerations (the link above) is designed to help in thinking about the requirements of the lay-up of your piece. The cumulation of factors can easily triple the annealing soak time and slow the rates by three times. 

What is the anneal?

Another problem is that most often annealing is thought of as merely a soak at the annealing point of the glass.  It is much more than that.  The annealing point is usually the temperature at which the heat within the glass is equalised in preparation for the anneal cool.  This is because the annealing temperature is that at which the glass will most quickly anneal.  Since the anneal is temperature sensitive, the equalisation of the temperatures within the glass will be most successful at getting a good anneal throughout the cool.

For two-layer flat fused items, the annealing point can be used as the heat equalisation temperature.  The soak is to get the glass within 5°C/10°F throughout the piece.  The annealing, especially with thicker or more difficult pieces, is done closer to the lower strain point. The reasons for this is to save time in the annealing cool, it is easier to maintain the small difference in temperature, and it has been shown to produce a more dense (therefore stronger) glass.  If you look at the Bullseye annealing chart, you will see how slowly thick pieces need to be cooled, so starting 35°C/ below the annealing point can save many hours of cooling.

Once the glass has equalised in temperature, the object is to cool the glass at a rate that ensures the internal temperatures do not vary more than plus or minus 5°C/10°F across and through the piece.  The rate can increase by 1.8 times the initial cool rate after the lower strain point has been reached.  This second stage rate should take the glass to around 370°C/700°F, where the rate to room temperature can be doubled as much as six times the initial cool rate.

Difficult pieces

Tack fused and other pieces with uneven thicknesses require more care in the annealing to ensure even cooling of the whole without a greater variation in temperature than +/- 5°C.  As said above, tack fusing is one of the most difficult of styles to anneal adequately and the blog entry indicates some factors requiring more careful annealing.

As an example, a piece 6mm thick, with two layers of rectangular and pointed pieces that are just barely rounded.  This adds five factors of complications for the fusing - two levels of tack fusing, rectangular pieces, pointed pieces, laminated tack fusing.  This number of complications increases the practical thickness to 21mm – 6mm of flat base, 3mm each layer of tack (6mm), 3mm for rectangles, 3mm for pointed pieces, 3mm for laminated fuse.  Because this is tack fused, the next practical step up in the table needs to be used. That is the one for 25mm, which requires a four-hour temperature equalisation soak, and 15°C per hour initial anneal cool rate.

Of course, a simpler method can be used, as it has been researched and practiced by many people.  That is simply double the thickest part of the piece and use that thickness for the anneal soak and cool.  Sharp tack profiles need to be annealed as though 2.5 times thicker, and contour profiles only need 1.5 times the thickness.

Glass other than Bullseye

It is possible to apply these times and rates to any glass of which you know the annealing point.  The annealing soak can be set above the lower strain point, which to be safe, can be taken as a point 35°C/63°F below the annealing point.

E.g., if you are annealing a 12mm slab of float glass, the annealing point of which (in the UK) is 540°C, you chose a temperature of 505°C to do your two-hour soak, followed by a cool rate of 55°C/100°F to 427°C and then 99°C/178°F per hour for the second stage cool to 370°C/700°F.  The final cool of 330°C/595°F.  So, you can see the soak times, rates and target temperatures remain the same regardless of the glass type.  

More information on long annealing soaks in a video at 13:00 minutes.

More discussion on annealing and cooling is given in the ebook Low Temperature Kilnforming from Bullseye and Etsy.

Long Annealing Soaks

You Can’t Anneal Too Long.

Can you anneal too long?

Yes, you can.

It’s not just the possible temperature differences in the kiln.  If you have temperature differentials across your kiln, any piece that crosses those boundaries will have temperature differences locked into the glass.  If you know you have temperature differentials and your glass by circumstance must be in both the cooler and the hotter regions, you need to do a standard length of soak only.  Then reduce the rate of cooling a little more than normal, so that a slower cool occurs.  This should avoid most of the stress that can be induced by very long soaks in a kiln with hot and cool spots.

The other factor against annealing too long has been revealed by Bullseye research on annealing.  This video at about 13:00 minutes into the film explains.  This complicating factor in annealing is about the difference in temperatures of the surfaces of the glass.  The research shows that the longer you anneal the greater the differential in temperature becomes between the upper and lower surfaces of the glass.  This means that you can introduce stress across the whole piece, rather than just a section as in an unevenly heated kiln.

This comes from the recording of a typical long annealing cool.

What is more, the longer you soak, the cooler the bottom becomes in relation to the top.  The reported research is described in this video at about 13:00 minutes.  It can be assumed that the air temperature differences are the cause.  Even during cooling the air is hotter on top of the shelf than under.  This would allow the bottom surface to cool more than the top. This assumption is borne out by the fact that the effect is reduced or eliminated by having elements under the shelf.

There are two reasons to avoid long soaks. Uneven temperatures across the surface are locked into the glass.  And long soaks at annealing induce an unwanted temperature differential between the top and the bottom of the piece.

More information is available in the ebook Low Temperature Kilnforming; an evidence based approach to scheduling.

Revised 29.1.25

How Much Frit is Too Much

Scheduling for powder and frit.

 

Bullseye  pumpkin orange medium frit 00321.0002
Cedit: Bullsye Glass Company

“How much frit is too much for thickness calculation?”

There are differences between powder and frit effects on calculations for scheduling.

Powder needs to be about 2mm thick to provide strong colour, and will thin to 1mm or less during firing, so there is unlikely to be any significant effect for scheduling.

Fine frit sizes for Bullseye are between 0.2 and 1.2mm, so a single thickness layer will not affect the firing.  However, several layers thick over a portion of the area will make up to a 3mm layer and will need consideration in the scheduling.

Medium (Bullseye) frit is 1.2 to 2.7mm, So, a concentrated layer of medium and larger frits needs to be treated as an additional layer when they cover significant areas of the glass.

Scattered frits of any size with proportionate spaces between the frit will not need separate consideration in the scheduling.  Frits used to fill spaces between pieces of glass will have no effect on the scheduling either.

 

Polishing Edges by Hand

 This post is about hand polishing edges, although the most common method seems to be a fire polish.  But the other, less considered, method is to polish by hand. 

Advantages of cold working

• Hand polishing edges does not need to take long, as the area to be polished is very small in relation to the whole piece.
• The effort of manual polishing is rewarded by kiln time saved for additional pieces that can be produced while refining the edges of the current piece.
• There is much less risk of anything going wrong in hand work than in re-firing the piece.

Equipment

Handheld smoothing pads and water are all that is required. 

The pads are normally diamond ones and should start with 60 grit, if a lot of glass needs to be removed, but 100 grit will be good to start with for smoothing a ground edge.  Then double the grit number (which is a halving of the particle size) to remove the coarser scratches and finally a 400 grit.

Then move to a 220 grit resin smoothing hand block.   These hand pads with diamonds encased in resin, are similar to this from HIS Glassworks.  

Credit: HIS Glassworks

They give the edge a satin finish, and that may be enough to be so pleased with the appearance that you do not need to do any further work.

In all these stages you need to have the surface damp.  When a white paste appears around the grinding area, it indicates that more water is needed.

If you want to go further toward an optical finish, you can use a cerium impregnated hand pad such as this. 

Credit: HIS Glassworks

For cerium impregnated pads you need less water than previously, to be able to generate the heat required to cause the chemical reaction between the cerium and glass.

You, of course, can use machines such as a handheld rotary tool.  You can get small diamond and cerium pads for these from many suppliers such as HIS Glassworks or Eternal Tools.  You will need to turn the speed down to almost the minimum to do the work needed without generating too much heat, or spraying water all over the workspace.  Most importantly you need eye and breathing protection against glass particles and dust when using rotary tools with no guards on them.

 

 

Multiple Firings of Kiln Wash

Many people report that they fire multiple times on kiln wash that has not been renewed.  Most add coats over existing kiln wash.  They only remove all the kiln wash when it begins to crack, stick to the glass or gets divots.

We all know that kiln wash fired a second time to full fuse is likely to stick to the glass.  We also know that kiln wash fired to slumping temperatures lasts almost indefinitely.  The kaolin in the kiln wash that allows easy spreading, undergoes a gradual change from platelets to crystals with increasing temperature.  This begins at around 600C/1115F and is complete by 900C/1655F.  The crystalline version of kaolin sticks kiln wash to glass, but as the transition from platelet to crystal is so slow at the lower end of the range, kiln wash on slumping moulds does not exhibit the sticking behaviour even over very many firings.  But, as the temperature rises, the risk of there being enough crystals to stick the kiln wash to the glass also increases.  By full fuse temperatures the proportion of crystalline kaolin is high and becomes complete on the next firing.

. 

credit: Immerman Glass

It is possible to fire several times to tack fusing temperatures without experiencing the sticking behaviour of kiln wash.  However, the more times and the higher temperature used, the greater risk of kiln wash sticking.

Some people continue firing without adding additional layers of kiln wash until cracks, divots, or sticking occurs.  This leads to creating a fix after the failure of the kiln wash. This requires both finding a means of cleaning the kiln wash residue from the glass, and fixing the firing surface.

Others paint a layer of kiln wash on top of the existing separator before high temperature firings. This continues each firing with a fresh layer of kiln wash.  However, the same cracks, divots, and sticking occurs at some point, requiring a complete re-coating of the shelf, and getting the kiln wash off the glass.

credit: Sue McLeod Ceramics

Re-coating of a shelf takes a couple of minutes and can be done with simple tools.  A broad scraper will remove most of the kiln wash.  This can be followed by rubbing with an open weave sanding sheet as used for plaster board or other dry walling.  If you are worried about the dust – which has less risk than fibre papers – you can dampen the surface before beginning the cleaning process.

If the kiln wash has been on the shelf for many firings, it is more difficult to remove, requiring more effort than a single firing.  High temperature firings as for melts also make the kiln wash more difficult to remove. But the same process is used in these cases.

       

Kiln wash in firings at slump and low temperature tack fuses can be reused as many times as it remains smooth and undamaged since the temperature is not high enough to cause the chemical changes.

The ultimate benefits of renewing kiln wash are that not only less effort is required to clean and re-coat, than to fix pieces, and also the cost of kiln wash is significantly less than fibre papers.

Removing kiln wash

Applying kiln wash

Revisde 18.1.25

Specific Gravity of Unknown Glass

(warning: lots of arithmetic)

Knowing the specific gravity of a glass can be useful in calculating the required amount of glass needed, e.g., for casting, and screen and pot melts, where a specific volume needs to be filled.

Most soda lime glass – the stuff kilnformers normally use – is known to have a specific gravity of approximately 2.5.  That is, one cubic centimetre of glass weighs 2.5 grams. 

If you have glass that is of unknown composition for your casting, you will need to calculate it.

Calculating the specific gravity of unknown glass.

Specific gravity is defined as the ratio of the weight of a substance to the weight of water (in simple terms).  This means first weighing the item in grams.  Then you need to find the volume.

Calculating the specific gravity of regularly shaped items

For regularly shaped item this is a matter of measuring length, width and depth in centimetres and multiplying them together. This gives you the volume in cubic centimetres (cc).

As one cubic centimetre of water weighs one gram, these measurements give you equivalence of measurements creating the opportunity to directly calculate weight from volume. To calculate the specific gravity, divide the weight in grams by the volume in cubic centimetres.

An example:

To find the specific gravity of a piece of glass 30cm square and 6mm thick, multiply 30 x 30 x 0.6 = 540cc.  Next weigh the piece of glass. Say it is 1355 grams, so divide 1355gm by 540cc = s.g. of 2.509, but 2.5 is close enough.

Calculating specific gravity for irregularly shaped objects.

The unknown glass is not always regular in dimensions, so another method is required to find the volume.  You still need to weigh the object in grams.

Then put enough water in a measuring vessel, that is marked in cubic centimetres, to cover the object.  Record the volume of water before putting the glass in.  Place the object into the water and record the new volume.  The difference between the two measurements is the volume of the submerged object.  Proceed to divide the weight by the volume as for regularly shaped objects.

Credit: study.com

Application of specific gravity to casting and melts.

To find the amount of glass needed to fill a regularly shaped area to a pre-determined depth, you reverse the formula.  Instead of volume/weight=specific gravity, you multiply the calculated volume of the space by the specific gravity.

The formulas are:

v/w = sg to determine the specific gravity of the glass;

v*sg = w to determine the weight required to fill a volume with the glass.

Where v = volume; w = weight; sg = specific gravity.

You determine the volume or regular shapes by deciding how thick you want the glass to be (in cm) and multiply that by the volume (in cc). 

For rectangles

volume = thickness * length * depth (all in cm)

For circles

Volume = radius * radius * 3.14 (ϖ) * thickness (all in cm)

For ovals

Volume = major radius * minor radius * 3.14 (ϖ) * thickness (all in cm)

Once you have the volume you multiply by the specific gravity to get the weight of glass to be added.

Calculating weight for irregularly shaped moulds.

If the volume to be filled is irregular, you need to find another way to determine the volume.  If your mould will hold water without absorbing it, you can fill the mould using the following method.

Wet fill

Fill the measuring vessel marked in cc to a determined level.  Record that measurement.  Then carefully pour water into the mould until it is full.  Record the resulting amount of water. Subtract the new amount from the starting amount and you have the volume in cubic centimetres which can then be plugged into the formula.

Dry fill

If the mould absorbs water or simply won’t contain it, then you need something that is dry.  Using fine glass frit will give an approximation of the volume.  Fill the mould to the height you want it to be.  Carefully pour, or in some other way move the frit, to a finely graduated measuring vessel that gives cc measurements.  Note the volume and multiply by the specific gravity.  Using the weight of the frit will not give you an accurate measurement of the weight required because of all the air between the particles.

An alternative is to use your powdered kiln wash and proceed in the same way as with frit.  Scrape any excess powder off the mould.  Do not compact the powder. And be careful to avoid compacting the powder as you pour it into the measuring vessel.  If you compact it, it will not have the same volume as when it was in the mould.  It will be less, and so you will underestimate the volume and therefore the weight of glass required.

Irregular mould frames

If you have an irregular mould frame such as those used for pot and screen melts that you do not want to completely fill, you need to do an additional calculation.  First measure the height of the frame and record it.  Fill and level the frame with kiln wash or fine frit.  Do not compact it.  Carefully transfer the material to the measuring vessel and record the volume in cc.

Calculate the weight in grams required to fill the mould to the top using the specific gravity.  Determine what thickness you want the glass to be.  Divide that by the total height of the mould frame (all in cm) to give the proportion of the frame you want to fill.  Multiply that fraction times the weight required to fill the whole frame to the top.

E.g. The filled frame would require 2500 gms of glass.  The frame is 2 cm high, but you want the glass to be 0.6 high.  Divide 0.6 by 2 to get 0.3.  Multiply that by 2500 to get 750 grams required.

Regular mould frames

For a regular shaped mould, you can do the whole process by calculations.  Find the volume, multiply by specific gravity to get the weight for a full mould.  Measure the height (in cm) of the mould frame and use that to divide into the desired level of fill (in cm).

E.g. The weight required is volume * specific gravity * final height/ height of the mould.

The maths required is simple once you have the formulae in mind.  All measured in centimetres and cubic centimetres

Essential formulae for calculating the weight of glass required to fill moulds (all measurements in cm.):

Volume of a rectangle = thickness*length*width

Volume of a circle = radius squared (radius*radius) * ϖ (3.14) * thickness

Volume of an oval = long radius * short radius * ϖ (3.14) * thickness

Specific gravity = volume/ weight

Revised 18.1.25

Sticking Fiber Paper

People are reporting different behaviours of their thicker fibre papers such as small fibres sticking to the glass after a fuse, and a different smell from the burning binders.  These are most likely to be from a body soluble refractory fibre paper.

It seems more suppliers are selling the body soluble versions of fibre paper. It sticks to glass and it gives off a smell of volatile chemicals. I don't like it, but I may have to use it due to the unavailability of the more health risky refractory fibre that worked very well without so much sticking.

There are several ways to minimise the fibres sticking to the glass.  They all relate to adding a separate coating of separator to the fibre paper before firing.  Among the coatings that can be used are 

• shelf paper on top, 
• a kiln wash solution brushed on, 
• kiln wash powder dusted over, 
• sprinkled alumina hydrate, and 
• boron nitride (Zyp is one brand name).  

Others have found that simply soaking the fired glass in water overnight allows the fibres to be brushed off with stiff brushes.

It seems body soluble refractory fibre papers tend to stick to the glass at anything over low temperature tack fuses.  This requires an additional layer of separator to be applied over the paper.  It is each person’s choice, of course, but I will continue to attempt to get the older version of fibre paper.

Fused Glass in Dishwashers

“Can glass be put into dishwashers?”

image credit: very.co.uk

There are many recommendations to avoid placing fused glass into a dishwasher.

The main reasons given are:

·        Corrosion

·        Devitrification

·        Etching and

·        Breaking.

There are distinct differences between these effects.

Corrosion

Glass corrosion generally comes from constant contact with moisture and has a greasy feel.  As experienced by weather or washing, the wetting of glass is not constant, and it dries between wettings.  No visible corrosion is present on window glass and, although float glass is a little different from fused glass, the same effect applies.

Devitrification

Devitrification occurs at much higher temperatures than those created in a dishwasher, and therefore is not a risk.

Etching

The main risk is etching from the washing process.  This can be mechanical or chemical, and dishwashers combine both. Over time, the glass will be etched just the way lead crystal is in a dishwasher.

Breaks

Glass breaks can occur in the dishwasher because of the shock of hot water.  Most dishwashers rinse while heating the water, so the glass experiences only slow rises in temperature.  Float glass of 4mm can withstand 140˚C differentials according to manufacturers.  Full and tack fused glass is not as homogenous as float glass and will be affected by smaller temperature differentials.  So, there is a small risk of breaks in dishwashers.

Additional risks relate to the layup of the glass. 

• ·   Tack fused glass has a variety of thicknesses that make it more prone to breaks from temperature differentials.
• ·   Contrasting colours can react differently and split at the contact lines.
• ·   Large internal bubbles can cause difficulties, which may arise from the insulating element of the contained air, or simply because of thickness.