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

Glass Volume for a Frit Mould

There are several ways to determine the volume of a mould. 

Calculation of the weight of glass needed

Calculate the amount in the metric system of measures, as that gives much easier calculations. Cubic centimetres of volume times the specific gravity of glass (2.5) will give you the number of grams of glass required.

This works best on regular geometric forms.  Rectangles and parallelograms are easy to measure the length, width and depth in centimetres.  Multiply together and you obtain cubic centimetres.  That times the specific gravity – 2.5 – will give the number of grams to fill the mould.  The frit will of course be mounded above the levelled surface, because of the air spaces between the frit pieces.

Example of a small frit mould

Irregular shaped moulds

The moulds which are irregular in shape or depth are more difficult to calculate. 

You can determine the volume by starting with a measured amount of water.  Quickly fill the mould to the surface, so that no water is absorbed into the mould. Empty the water from the mould into the drain so it does not become soaked. The difference between the starting and finishing amount of water is the volume of glass required to fill the mould. 

You can use that volume in cubic centimetres times the specific gravity (2.5) to get the number of grams of glass required.

However, it is much easier to put the frit into the water until the measure shows the same amount as before the mould was filled. Then you only need pour off the water and allow glass and mould to dry.  No calculation required.

This post gives some alternatives.

Revised 30.12.24

Scheduling for Thick Landscapes

Thick slabs often involve numerous firings of increasingly thick work.  I am using an existing example, with their permission, of the first stages of a thick landscape.  The initial concern was with bubbles in the first layup, then the strategy for firing the thick slab.

Plans

This is the first part of a landscape with depth.  It will be fired 5-7 more times.  This first piece will be inverted for the next firing with the clear facing up, to avoid reactions between the colours.  It is similar to an open face casting. There is a Bullseye Tip Sheet on open face casting that will give a lot of information.

Layup

Picture credit: Osnat Menshes

This work has a base of clear that is mostly overlaid with one layer of 3mm pieces, although in some places another layer, and there are some pre-fired elements as well.  It is fired on Thinfire shelf paper.

Bubbles 

There is concern about the number and size of the bubbles after the firing, and how to avoid them.  Will they grow over the multiple firings?

The many small bubbles are characteristic of kilnformed glass.  The few larger bubbles may result from the frit that is under the pieces that form the top surface.  And there are some overlaps of clear over colour that may form pockets where air can collect. I advise leaving the scattering of the frit until all the decorative pieces are in place.  The bubbles will migrate toward the top during the multiple firings.  They will not grow in size unless they combine during the upward migration.  A later suggestion about reducing the number of firings will reduce the bubble migration and risk of increasing in size.

Picture credit: Osnat Menshes

Schedule

Proposed Schedule (Temperatures in degrees Celsius)

1: 180 – 560, 30’    I would go to 610 for 30'

2: 25 – 680, 120’    I would use only 30'

3: 220 – 810, 15’    I would set the top temperature at 816, 15’.

4: 9999 – 593, 30’  Eliminate this segment. 

5: 9999 – 482, 120’ I suggest one hour soak

8: 55 – 370, off      83 – 427, 0’

7: 150 – 371, 0’

8: 330 – to room temperature, off.

 

Eliminate segment number 4.  Any temperature equalisation done at this temperature, is undone by the AFAP to the  annealing.  The temperature equalisation occurs at the annealing temperature. No soak at an intermediate temperature is required.  This blog post gives some information about annealing above and below the annealing point (Tg). 

Firing Incremental Layers

The plan is for five to seven more firings.  Continuing to build up the thickness on each firing, may have some problems.

• There is increased risk of compatibility problems when firing a piece to full fuse many times.
• There is a risk of more bubbles and of the existing ones becoming larger as they move upwards and combine with other smaller ones.
• With each firing the thickness is increasing and so becoming a longer firing.  This is because the heat up, annealing, and cooling each need to be longer.  For example - 6mm needs 3hour cooling, 12mm needs 5 hours, 19mm needs 9 hours. 

Multiple Slabs

These are the main reasons that I recommend firing a series of 6mm slabs separately and combining them in one final firing.  Firing a series of 6mm slabs and then combining them in a single long and slow final firing has advantages.

• The individual pieces do not need to go through so many full fuse firings, reducing the risk of compatibility problems.
• The small bubbles in each firing will not have the chance to rise through all the layers to become larger.
• The total time in the kiln for the combined pieces will be less than adding layers to already fired layers.

Examples

It is often difficult to convince people that firing by adding incrementally to an existing slab, longer firing times are required than by firing a group of 6mm slabs and a single combined firing of all the slabs.  I give an example to illustrate the differences.

Annealing

Assume there are to be a total of eight firings (existing 6mm slab and 3mm for each of seven more firings).  Also assume that each additional firing is of 3mm. This makes a total of 28mm.  Compare annealing and cooling times for each firing:

Firing      thickness       anneal and cool (hours minimum)

1            6mm                    3

2            9mm                    4

3            12mm                   5

4            15mm                   7

5            18mm                   9

6            21mm                   11.5

7            25mm                   14

8            28mm                   17

Total                                   70.5 hours annealing time (minimum)

To fire up 5 six millimetre slabs takes less time – 3 hours annealing and cooling time for each firing cumulates to 15 hours.  Add to that the final firing of 17 hours annealing time.  A total of 32 hours.  This is half the time of adding to the existing slab at each firing.  Multiple 6mm slabs can be fired at the one time if there is space in the kiln, which would reduce the kiln time for the 6mm slabs even further. 

An additional advantage of firing 6mm slabs and combining them, is that bubbles can be squeezed out more easily in the final thick slab fring because of the combined weight of the  slabs.  You could make the individual slabs a little thicker, but that would involve damming each slab.  Not an impossible task of course.  And it would change the calculations, by reducing the number of firings.

Heat Up

Another time saving is to use the second cooling rate from the Bullseye document Annealing Thick Slabs as the first up ramp rate. Take this rate up to a minimum of 540˚C. Although, this is an arbitrary temperature above the strain point to ensure all the glass is above the brittle phase.  It is possible to maintain this initial rate to the bubble squeeze.  But with the slow rises in temperature required for thicker slabs, it is sensible to increase the rate from 540 to bubble squeeze to reduce the firing time.  Once past the bubble squeeze a more rapid rate can be used to the top temperature.  

The heat up times could be about half the minimum cooling times.

A worked example (with certain assumptions) would be:

Firing      thickness       time to top temperature total time.

1            6mm             6.3               

2            9mm             7.1

3            12mm            8.4

4            15mm            10.7

5            18mm            15.9

6            21mm            19.4

7            25mm            25.1

8            28mm            29.1              ca.122 hours

But firing five times for 6mm equals 31.5 hours plus the final firing up of 29.1 hours equals a total of 60.6 hours.  Again about one half the time of progressively building up a base slab to the final thickness.

Savings

This example shows that approximately 90 hours of firing time can be saved by making a series of six millimetre slabs and combining them in a final firing.  There is the additional advantage of reducing the occurrence of bubbles between the layers in the final firing because of the weight of the combined slabs.

Home Made Billets

You can make your own billets from small pot melts.  But why should anyone go to the effort? Some reasons are:

• ·        You can make your own colour. 
• ·        You can use your cullet/scrap (avoiding buying or making frit).
• ·        You don’t have to buy and break billet to size 
• ·        You can reduce the clouding caused by many microscopic bubbles surrounding the frit pieces. 
• ·        You can make a size to fit your casting mould. 
• ·        Potentially, you will reduce needling.

 Now you are convinced of the advantages, you want to know how.

Preparation

• ·        Select the glass. Avoid iridised glass and any ground edges – they will cause haze in the final casting. Wash all the glass. Place the glass in a small flowerpot.
• ·        Weigh out the amount of glass cullet needed for the mould and add about 50gms to account for the glass that will stick to the pot.  Calculating the required weight is relatively simple and this post gives the information.

Dams

• ·        Arrange dams in such a way that the resulting billet will fit into the mould without overhang.  It might be quite a tall billet. In which case cast it horizontal with the height as the length of the billet.
• ·        Line the dams with Thinfire/Papyros at least. One mm fibre paper would be better. 
• ·        The dams can be on a kiln washed shelf or on fibre paper. The bottom of the glass will be fine either way.
• ·        Place the pot above the dams.  The higher, the fewer bubbles in the billet.  And any left in the billet will be reduced by flow in the casting firing.
• ·        Multiple billets can be made of different colours, sizes, etc., at the same time.

Firing

• ·        Fire to around 900ºC/1650ºF and soak for hours.  Observation will show when the pot is empty.  Clue: There will be no string of glass from the bottom of the pot.
• ·        Anneal as for the smallest dimension.  If you are doing multiple sizes, the dimension must be taken from the biggest piece.
• ·        When cool, remove and clean the separator off the pieces thoroughly.  A 15 minute soak in a 5% citric acid solution will speed the process.

Casting

• ·        Place billet in casting mould. The first ramp rate needs to be for the smallest dimension of the billet.  This may be a slower rate than when using frit for casting.
• ·        Do a long bubble squeeze in the 650ºC to 670ºC range – up to two hours, but a minimum of one.
• ·        Fire to your normal top temperature and time.
• ·        Anneal for the largest piece.

 

More information here 

Spikes on Frit Castings

Credit: The Crucible.com

It is frequent to have castings from frit with spikes, needles, or prickles around the edges. 

Causes

These spikes result from the glass touching the edge of the mould or separator during the hottest part of the firing. The glass particles first begin to compact as the glass rises toward the fusing temperatures. As the temperature increases toward the casting temperature it begins to expand both horizontally and vertically from that compact mass. As it cools, the glass sinks down and retreats from the edge. This movement leaves some small bits of glass stuck to the sides. The glass contracts as it cools, leaving the spikes as it contracts from its hottest state. 

Avoidance

The usual recommendation is to mound frit in the middle and let it flow to the outside. Still, the glass flows to the outside of the mould at casting temperature and it touches the sides. Leaving the risk of creating spikes. Accurate measuring of the amount of glass to charge the mould with is important. With the right amount of glass, the mould will not be overfilled and so, reduce the spiking. 

Measuring the weight of glass for the mould is not difficult. In many cases, the manufacturer of the mould has done the work for you. If you need to calculate the weight of glass required for the mould, it is not difficult. A method is given here. In short, you use a dry fill of the mould. Measure the volume (using the metric system) and multiply by the specific gravity to get the weight in grams. 

Larger chunks of glass tend to produce fewer spikes than smaller frit. Usually longer soaks at top temperature are required to fully form the glass with smaller frit. It is also possible to drip glass into the mould from a pot suspended above the mould. Accurate measurement of the weight will still be important. But add 100gms/4oz. to the amount to allow for the glass that will stick to the pot.

My view is that with dams, it is better to use a straight sided shape with fibre cushioning around the outside. When annealed and cool, clean it well. Then fire polish with a slow ramp to 540°C/1000°F followed by a quick ramp to the fire polish temperature. This will polish the sides of the piece that were in contact with fibre paper.

Fire Polishing of Frit Castings

Image credit: Obsession Glass Studio

 Fire polishing castings is relatively difficult.  Even though people may suggest temperatures for this kind of fire polish for castings from frit:

• ·        They are relevant to particular kilns. 
• ·        They are also dependent on the ramp rate. 
• ·        The presence or absence of a bubble squeeze is important. 
• ·        The size of the casting is relevant.

 The objective is to get a fire polish without distorting the shape of the piece.  The general procedure is to fire slowly to the softening point. This is to ensure the casting is of similar temperature throughout. The softening point for fusing glass is around 540°C/1000°F. You should soak at that point for a time to ensure the glass is all at that boundary between brittle and plastic.

 You may prefer to use a bubble squeeze soak to achieve the same thing.  This has a slightly higher risk of distorting the piece.  If you do use the bubble squeeze, it should be done at the lower end of the bubble squeeze after a slow rise.  The casting will not be subject to much change at 600°C to 620°C/1110°F to 1150°F, if the soak is short.

 The rates to be used are dependent on the size and thickness of the piece.  Larger and thicker pieces need slower rates than thin ones.  Fire at an initial ramp rate for twice the thickness to be sure of heating thoroughly.

 When the softening point is reached, or the slump soak is complete, proceed at a rapid rate to the tack fusing temperature. To get the result you want you will need to observe.  Peek at frequent intervals. Be prepared to advance to the next segment when the gloss appears on the surface.  Your controller manual will tell you how this is done.

 

Specific Gravity

This is an important concept in calculating the amount of glass needed to fill a pot melt, and in glass casting.  This will also help in the calculation of the amount of glass required to fill a given area to a defined thickness.

Specific gravity is the relative weight of a substance compared to water. For example, a cubic centimetre of water weighs 1 gram. A cubic centimetre of soda lime glass (includes most window and art glass) weighs approximately 2.5 grams. Therefore, the specific gravity of these types of glass is 2.5.  

If you use the imperial system of measurement the calculations are more difficult, so converting to cubic centimetres and grams makes the calculations easier. You can convert the results back to imperial weights at the end of the process if that is easier for you to deal with.

Irregular shapes

Water fill method
Specific gravity is a very useful concept for glass casting to determine how much glass is needed to fill an irregularly shaped mould. If the mould holds 100 grams of water then it will require 100 grams times the specific gravity of glass which equals 250 grams of glass to fill the mould.

Dry fill method
If filling the mould with water isn't practical (many moulds will absorb the water) then any material for which the specific gravity is known can be used. It should not contain a lot of air, meaning fine grains are required. You weigh the result and divide that by the difference of the specific gravity of the material divided by 2.5 (the specific gravity of soda lime glass). 

This means that if the s.g. of the mould filling material is 3.5, you divide that by 2.5 resulting in a relation of 1.4   Use this number to divide the weight of the fill to get the amount of glass required to fill the mould.   If the specific gravity of the filler is less than water, then the same process is applied.  if the specific gravity of the filler is 2, divide that by 2.5 and use the resulting 0.8 to divide the weight of the filler.  This only works in metric measurements.

Alternatively, when using the dry fill method, you can carefully measure the volume of the material.  Be careful to avoid compacting the dry material as that will reduce the volume.  Measure the volume in cubic centimetres.  Multiply the cc by the specific gravity of 2.5 for fusing glasses.  This will give the weight in grams required to fill the mould.  If you compact the measured material, you will underfill the mould. The smaller volume gives a calculation for less weight.

Regular shapes

If you want to determine how much glass is required for a circle or rectangle, use measurements in centimetres.  

Rectangles
An example is a square of 20cm.  Find the area (20*20 =) 400 square cm. If you want the final piece to be 6mm thick, multiply 400 by 0.6cm to get 240 cubic centimetres, which is the same as 240 grams. Multiply this weight by 2.5 to get 600gms required to fill the area to a depth of 6mm.

Circles
For circles you find the area by multiplying the radius times itself, giving you the radius squared.  You multiply this by the constant 3.14 to give you the area.  The depth in centimetres times the area times the specific gravity gives you the weight of glass needed.

The formula is radius squared times 3.14 times depth times specific gravity.   R*R*3.14*Depth*2.5
E.g. 25cm diameter circle:
Radius: 12.5, radius squared = 156.25 
Area: 156.25 * 3.14 = 490.625 square cm.
Volume: 490.625 * 0.6 cm deep =294.375 cubic cm.
Weight: 294.375* 2.5 (s.g.) = 735.9375 gms of glass required.  
You can round this up to 740 gms for ease of weighing the glass.

The relative order of kiln forming events

When preparing for multiple firings of elements onto a prepared piece, you need to consider the order and temperatures of events so that you do not harm an earlier stage of the project.  This blog entry will not give definitive temperatures, as that varies by glass and by kiln.  Instead, it indicates what happens in progression from highest to lowest temperatures in approximate Celsius degrees.  

ca. 1300C  -  Approximate liquid temperature 

ca. 850 – 1000C  -  Glass blowing working temperature

ca. 950C  -  Raking and combing

ca. 850C  -  Casting

ca. 810C  -  Full fuse

ca. 790C  -  Large bubble formation

ca. 770C  -  High tack, low contour fuse

ca. 760C  -  Tack fuse

ca. 750C  -  Fire polish

ca. 700C – 760C  -  Devitrification range

ca. 700C  -  Lamination tack

ca. 600C – 680C  -  Slump and drape

ca. 650C  -  Vitreous paint curing temperature

ca. 600C  -  No risk of thermal shock above this temperature 

ca. 540 – 580C  -  Glass stainers enamel curing temperature

ca. 520 – 550C  -  Silver stain firing temperature

ca. 550C  -  Glass surface beginning to soften

Slow rates of advance needed from room temperature to ca. 500C

These temperatures are of course, affected by the soak times. The longer the soak time, the lower temperature required. The rate at which you achieve the temperature also affects the effective temperature.  Slower rates of advance require lower temperatures, than fast rises in temperature.  These illustrate the effect of heat work.

The table shows for example you need to do all the flat operations and firings before slumping or draping.  It also shows you can use vitreous glass paints at the same time as slumping and draping.  This emphasises that the standard practice is to plan the kind of firings you will need for the piece and do them in the order of highest temperature first, lowest last.

In general, you do need to do the highest temperature operation first and lowest last.  But there are some things you can do with heat work.  For example, if you needed to sandblast a tack fused piece, but did not want to risk reducing the differences in height there things you can do.  From the list above, you can see the glass surface begins to soften around 500C.  It is possible to soak the glass for a long time around 500C to give it a fire polish, instead of going to a much higher temperature.  You will need to experiment to find the right combination of temperature and soak length, but it can be done.

This article is to show that knowledge of what is happening to the glass at different temperatures, can help in “fooling” the glass into giving you the results you want without always following the “rules”.  This may also be what it is to be a maverick glass worker.  Use the behaviour of glass to your advantage.

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.

Bubbles in Casting Mould Firings

There seems to be an increasing popularity for re-useable ceramic casting moulds. One of the common problems with these moulds is bubbles.  

Frit size 

It rather depends on the sizes of the frit and cullet used as to how many and what kind of bubbles are created. The converse of expectations is what happens.  You get more small bubbles with powders and fine frits than with coarser frits.  The small bubbles rise and coalesce to form larger bubbles which rise more slowly as they have to push through a greater mass of material (just as in a liquid). Since glass is viscous, these little bubbles usually do not have time to push their way through the glass at fusing temperatures.  But at casting temperatures, there is less resistance from the glass, as it is less viscous, and so the bubbles can clump together and form the larger bubbles that burst through the surface.

Temperature range and rate of advance

The amount and kind of bubble also depends on the speed of the ramp and the bubble squeeze you give it. If you proceed rapidly to top temperature, you will have to go to a higher temperature, allowing the surface to become more plastic and be pushed out of the way by the expanding air that almost certainly is in the mix. A slow rise will allow all the glass to become the same temperature throughout without using a high top temperature, so reducing the risk of the bubbles pushing through the more viscous glass to the surface.

Vents

All these problems would be reduced by having a vent or sprue to allow the air out from the bottom. Almost all purpose made casting moulds have these things. Sometimes they are as thin as a few hairs (from somebody with long hair) to as thick as a toothpick. As you have to do some cold work on the results from these moulds anyway, a few little strands of glass should be no problem to clean up. If the manufacturers won't do it, it is possible to take your Dremel or similar drilling tool and with a fine drill bit and make these tiny holes in appropriate places.  

I do not understand why these casting moulds do not have tiny air vents at the bottom of the depressions. Yes, there would be a tiny pimple on the surface of the final piece, but this can be cleaned away easily. The holes could be really small diameter ones. They just need to be opened after each separator coating with a fine wire. I'd be sending the ones without vents back to the manufacturer as not fit for purpose. If these moulds had vent holes, they would be a lot less bubble prone. 

Master moulds

If the mould continues to give trouble with bubbles, it might be best to take a negative of the mould that you can keep as a master.  Then make one-use investment moulds from this master positive as you need. Investment moulds usually allow air to move through the material pretty well, but you can add sprues if you want.

Reservoirs

A further possibility is to drip the glass into the mould.  To do this you need to place a ceramic pot, supported by kiln furniture, above the mould with the glass for the casting in it.  Take to a temperature between 850°C and 900°C, depending on how long you wish to wait for the glass to flow out of the pot and into the casting mould.  The action of the glass forming in the pot eliminates many of the bubbles caused by frits and powders.  A further advantage is that this forming in the pot eliminates the possibility of the edges of the original glass pieces being seen. It would also allow you to add a different colour causing swirls or wisps of colour to move through the main colour.

The main focus is to eliminate the bubble formation.  This can be done with vents, adjusting the schedule, modifying the method by melting the glass into the mould, or making a master and individual investment moulds.  You can also combine several of these methods in one firing if you wish.