Metal Strengths

Metal Strengths

The strength of metals is most often compared by their tensile strengths. These numbers are Newtons per square millimetre and represent the relative strength of each metal compared to another.  The range of numbers indicates the variations caused by various alloys.

Tin                      19

Lead                14 – 32

Solder 60/40        48
Zinc                120 – 246

Aluminium       120 – 246

Copper            220 – 270

Brass              340 – 540

Mild steel         500 – 750

Stainless steel  740 – 970

These figures may be of interest in considering what frame to place around a free hanging stained glass panel.

Panel Framing Options

Some framing options for free hanging stained glass panels are given here.  They are not exhaustive, of course, but do give some principles to be considered when making frames.  Wood and metal are the two traditional materials for framing panels to be hung.

Wood

A wood frame requires joints of some kind. These joints are important to the durability of the frame. The two main kinds of joints are glued and screwed.

Glued joints

Lap joints seem to be strongest. An odd element relating to the strength of this joint is that placing a wooden pin in the joint weakens, rather strengthens the lap joint.

Mortice and tenon is also a strong joint. It requires considerable skill to make a good joint.

A mitred is among the weakest, but can be strengthened with a biscuit or fillet in the joint.

A mitred joint with biscuit ready for glueing.

Screwed joints

These have a lot of movement before failure, but do give a lot of resilience to the joint as they can stretch rather than immediately give way. They also can be used with any of the glued joints if appearance is not of prime importance.

Frame style

The width and thickness of the frame are interrelated – thicker frames (front to back) can be narrower, thinner frames need to be wider. So the desired appearance of the frame width has a significant effect on the dimensions of the frame.

Metal cames or angle

Lead can be an adequate framing material, but if strengthening is required, you can use copper wire within the came and fold the leaves closed over it. You can also use steel rod within the came, as shown in the posting.

Zinc is a stronger metal than lead – about 8 times, but still has a weak tensile strength. I corrodes easily, but accepts solder as a joining method. It is more expensive than lead.

Some of the variety of zinc came available

Aluminium is a little stronger than zinc, but does not take solder. It has similar costs to zinc.

Some of the aluminium profiles available

Copper is about 1/3 stronger than zinc and also takes solder. It corrodes to a verdigris, but can be protected by clear varnish or paint. It is more expensive than zinc, but can be used as wire which is less expensive than other forms of copper.

Brass is over two times stronger than zinc and also takes solder. It resists corrosion well, and is a little cheaper than copper.

Some of the brass came options.

Mild steel is over 3 times stronger than zinc, but does not take solder at all well. It is relatively cheap and welds easily, making it a good framing material, although a method of fixing the panel into the frame is required.

Stainless steel is about 4.5 times stronger than zinc, but does not take solder and needs special welding. It resists corrosion very well, but is expensive in relation to zinc.

Hanging and fixing options

Two point hangings are the most common as they prevent twisting and distribute the weight to the sides of the panel.

The hanging material is straight up from the zinc framed sides to the fixing points

The hanging material whether line, wires or chains should be straight up from the sides to two separate fixing points. A triangle shaped hanging puts a bowing stress on the panel or frame.

A variation where the chain is taken to the corner of the window, is less secure, as it stresses the joint away from the sides

Loops or holes for screws should be placed in the frame rather than the panel.

The hanging is from reinforced corners directly to fixing points on the overhead beam

Ensure the fixing points for the hanging wires are sound and secure.

If the panel is fitted tight to the opening, consider ventilation requirements to reduce condensation between the primary glazing and the hung panel.

Grinder maintenance

There are several elements in maintaining one of the work horses of many glass studios.

Water

Ensure there is enough water to supply the pump or sponge that wets the grinding bit before starting any grinding. Too little water reaching the bit, fails to lubricate the diamonds and keep the glass cool. If you are getting a white paste or a powder on or near the glass, you need to increase the water supply.

Empty the reservoir daily. This keeps the water from producing a smell, and allows you to clear the glass residue from around the grinding bit. 

If you are changing to a finer grit, it is important to change the water, clean the resevoir, and thoroughly clean the sponge each time you make that change. Otherwise, you risk bringing coarser grit to scratch the finer grinding surface.

You can also buy a additive for the water – often called a diamond coolant – which is intended to provide a kind of lubrication for the diamonds. This may extend the life of the bit a little.

Bit maintenance

Periodic removal of the bit and lubrication of the shaft should be part of the regular maintenance of the grinder. You should make sure that the socket for the grub screw is clear of glass residues before attempting to turn it. I do this by using a needle or other thin sharp object to clear out all the glass powder. When the socket is cleaned, I push the key into the socket very firmly and hold it there while turning. Prevention maintenance is to fill the socket with vaseline or thick grease after tightening the screw.

Inspect your bit carefully for smooth areas showing that the diamonds have been worn away. Also look for dents, and other irregularities on the surface, indicating that the bit is damaged. In these cases, the bit should be replaced.

Before putting the old or new grinder bit back, ensure the shaft is smooth and without corrosion. Then coat the shaft with Vaseline or a proprietary anti seize-compound. This will ease the removal of the bit later. If the shaft is corroded, use a strip of fine wet and dry sandpaper to shine the shaft.

Sometimes bits need to be dressed – removing protruding diamonds, or cleaning and exposing new ones on a worn bit. To dress the bit you can grind some scrap glass, brick, or use a dressing stone to lightly grind some of the abrasive material away. This can extend the life of the bit.

Adjustment of height

If your grinder bit is too low or too high the diamond surface will not grind the whole of the glass edge. This can lead to chipping of the surface of the glass at the edges.

A good practice is to start with the bit as high as possible to allow for differing thicknesses of glass. As high as possible is with the bottom of the diamonds just below the platform of the grinder. This will ensure that you can deal with varying thicknesses of glass without immediate adjustment. You can then reduce the height of the bit as it wears.

Kiln maintenance

Before or after each use

Vacuum the inside of the kiln. Use a low suction setting, especially on fibre walls and ceilings. Stronger suction is possible when cleaning the brick floor.

Check on the kiln furniture – including shelves, boards, supports. Are they kiln washed and without scrapes, scratches, gaps? Has the kiln wash been fired to full fuse temperature?. In both cases, clean the used kiln wash off the shelf and renew.

Check that the shelves and other kiln furniture are without cracks.

Clean kiln furniture of dust and debris.

Check the level of any item placed in the kiln, e.g., mould, with a spirit level.

Example of a small 2-way spirit level

Monthly

Electrical parts: check the elements and their connections (normally at back or side). The screws on the connectors for the element tails should be tight. If they are badly corroded , they need to be replaced.

Any support pins or wires should be firmly seated in the brick work or supported by sound hangers.

Check the level of the kiln and internal shelves on a a regular basis and every time the kiln and its internal furniture is moved.

Stretch Marks in Slumping

Occasionally a slumped piece will develop faint lines beginning about half to two-thirds of the way from the centre and radiating toward the edge.

My experience leads me to think that these marks come from the glass moving too quickly at too hot a temperature. The glass softens as it reaches its slump point. If the temperature is taken above that, the glass conforms to the mould and then begins to slide downwards. The mould is by its nature not perfectly smooth and so the high points make marks on the glass as it moves.

This is re-enforced by the fact that the glass at the centre of these slumps does not have those marks. It deforms less than the edges of the piece and so (at whatever temperature) does not get so marked as the sides and edges.

To avoid these stretch marks you need to slump at the lowest possible temperature and ensure the glass is the same temperature throughout by the time it gets to its slumping point.

Temperature

Finding the lowest temperature for the slumps in a particular mould requires experimentation and observation. A simple curve – circular, oval or rectangular – requires less heat than one with a flat bottom and much less than one with angles. For a simple curve you can set your slumping temperature at say 620ºC with up to an hour soak. The important element to remember is that each shape and curve of mould will require different schedules. To determine this you need to make observations.

The glass for these two moulds requires different temperatures or  schedules. The back one will conform to the mould at a lower temperature than the front one due to the simpler shape and larger span of the back one.

From about 600ºC you need to make periodic observations of the progress of the slump. Note the temperature at which the glass begins to move – the reflections in the glass will begin to be curved. This is the minimum temperature you can use for this span and thickness of glass on this mould. The length of time required to get a complete slump may be so long as to make using this temperature impractical.

Slump not quite complete

Now observations need to become more frequent – possibly every 10 minutes or less. When you reach a temperature where the glass is visibly distorting, it is time to cease the temperature advance and begin the soak. Record this temperature and continue to observe, recording the time it takes at this temperature to fully slump. Continue to the anneal.

Inspect the piece when cool. If you have the result you want, you have the temperature and soak time needed for this thicknesses and size of glass on this mould. Record this information. If it is not fully slumped you can try either extending the time (if that is practical, it is the best option) or increasing the temperature on another piece. This increase should be by no more than 10ºC, so that you do not over fire the piece.

Glass conforms to the bottom of the mould

Of course, it is possible that the piece was slumped at too high a temperature as evidenced by stretch marks, mould marks, uprisings in the centre, distortions on the edges. Then you need to reduce the temperature on the next slumping of a piece of the same dimensions. Start with 10ºC less than your first piece, and programme the same amount of time. Observe, record and inspect as on the previous one.

This process shows why it is important to have a kiln with observation ports to be able to follow the progress of your work. In some ways, it is more important to have observation ports than whether the kiln is front or top loading, coffin or clam shell opening. But that is by the way.

Heat

The second important element in avoiding stretch marks is to enable the glass to be at the same temperature throughout its thickness. This involves the concept of heat work.  In general terms it means you can achieve the same result by putting the heat in fast and at a high temperature or slowly and at a low temperature. The “slow and low” approach allows more control and allows the glass to be the same temperature on top as on the bottom.

It is important to heat the glass slowly and steadily all the way up to the slumping temperature. The temptation to increase the temperature rapidly after the strain point needs to be resisted. Getting the top too hot can at the worst, cause a split on the bottom of the glass as the tension from slumping glass on the top splits the stiff glass at the bottom.

This means there is no need for a soak at the strain point, nor a speed up in the rate of advance up to the slumping temperature. Exactly the opposite is indicated. Choose a rate of advance for the glass according to its thickness – at 6mm a rate of 150ºC will be adequate. Maintain that rate of advance all the way up to the slump temperature. This also is required when you are making observations to determine what the slump temperature should be. The moderate rate of advance all the way to slumping temperature ensures the whole thickness of the glass is at the same temperature.

Heating the glass slowly to enable all of it to be at the same temperature, allows the glass to change shape at the lowest possible temperature and avoid picking up so much of the mould texture. The glass at the edge and upper sides is in contact with mould longer than central parts as it changes shape and slides along the surface of the mould at elevated temperatures. The lower the temperature used with a long soak, means that the glass is less likely to slide along the mould and so adds to the avoidance of stretch marks.

Annealing Range

Among the critical temperature ranges is the temperature around the annealing point. These are known as the strain points. The higher one is the highest temperature that annealing can begin and is the softening point. The lower one is the lowest point at which annealing can be done and is called the strain point. Soaking at any lower temperature will not anneal the glass at all. The ideal point to anneal is the annealing point, because annealing occurs most quickly at this temperature.  This temperature is defined by various characteristics both mathematical and observational. The manufacturer can give this temperature, and most do on their websites.

This annealing range is traditionally calculated as being 40ºC either side of the stated annealing temperature. Your aim should be to spend as little time in the temperatures above the annealing range to reduce the chances of devitrification.

Most glass kilns are not really accurate in recording the temperature within the glass. They are measuring the air temperature. The glass on the way down in temperature is hotter than the recorded temperature. If you do a soak at 515°C for example, the glass is actually hotter and is cooling to the 515° point during the soak. So, long soaks at the annealing temperature are required. Longer for thicker is required. The slow cool to at least 5C below the lower strain point does the annealing, and reduces the risk of inadequate annealing.

Recent research at Bullseye indicates that the use of the fact that temperature readings are above the actual temperature of the glass indicates lower annealing points for thick glass. This has been based on temperature probes at various points within the glass, comparing the glass temperature with the air temperature. The results of their research suggest an annealing soak at about 30C below the annealing point with a long soak, and slow anneal cool for the next 55C.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken in the continued cooling. 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.

Maintaining a Single Colour on the Edge

The piece in the middle distance shows the different colours of the two layers

Keeping the edge one colour on a two or multiple colour piece can be done by cutting the upper layer larger than the lower one(s). If you are making a 6mm thick piece, the upper piece needs to be 3mm larger all around. So if you have a 300mm diameter base piece, the top will need to be 306mm in diameter. This allows a coloured top to bend over a clear base, giving the appearance of a single colour throughout.

However if you are building thicker than 6mm and are willing to allow the glass to flow, you do not need to add the full thickness of the glass to the size of the capping piece. I find that at 9mm thick, I need only 4mm all around to cover the layers. This may be because the outer edges are nearer 6mm than 9mm thick.

Of course, if the final piece needs to be a pre-determined size the lower layers can be cut 3mm smaller than the top all around (for 6mm thick pieces).  The top is cut to the final size of the piece.

Glass Dust

This is from Greg Rawls' website.  He is a glass worker and a certified industrial hygienist. A huge amount of practical information on safety in glass working is available on his web site:

http://www.gregorieglass.com/Health_Saf ... mical.html

Ground Glass

OSHA classifies glass dust as a “Nuisance Dust”. Ground glass does not cause silicosis. You can wear a respirator if you are concerned about exposure.

Glass is made from sand, which contains silica - a naturally occurring mineral silicon dioxide (SiO2). Crystalline forms of silica, also known as “free” silica, can contribute to the development of silicosis under prolonged exposure conditions.

It is important to understand the difference between glass and crystalline silica because exposure outcomes are extremely different! Glass is a silicate containing various other ingredients which have been melted and upon cooling form an amorphous, or non-crystalline structure. While silica (SiO2) is a primary ingredient in the manufacturing of glass, when glass is formed under heat, the crystalline structure is changed to an amorphous structure and is no longer considered crystalline.

Ground glass is rarely respirable because the particle is too big. Always use wet methods when grinding glass! Water captures the dust. Sometime other chemicals are used to add colour to glass such as arsenic, lead, cadmium. These are usually present in low concentrations and are bound to the glass and not readily available but could present an exposure issue under some circumstances.

Observation Ports for Kilns

Observation Ports for Kilns

When choosing a kiln, an often overlooked element is the observation ports. These openings in the side or top of the kiln enable you to observe the progress of your work during a firing without opening the kiln lid or door. They have ceramic or fibre plugs to keep the heat in the kiln when you are not using them to observe what is happening. 

A kiln with a very large quartz observation panel

Some newer kilns are built with quartz observation panels in the kiln. These serve the same purpose as the ports, but without the (small) additional heat loss.

When doing any new work it is important to observe the progress of work, rather than just hope for the best and see what has happened after the whole process is finished. Observation can tell you when the piece has reached the desired stage and progress to the next part of the programme.

A port located too high to be of use for observation of the interior.  It is sealed with a ceramic fibre plug.

The location of the port is important. You need to be able to see the relevant part of the kiln or they are useless. 

This relatively large kiln has two ports, one at the center of the door, and one on top.  The top is mostly for ventilation.  The one in the door may be too high to observe work while firing unless the shelf is put up on tall kiln furniture

Although a small kiln, the observation port at the top is not so useful as one at the side.

A popular kiln with an appropriately placed observation port.  Often these have an additional one on the side opposite the controller. 

 Some kilns have multiple ports to make observation of various parts of the kiln easier.

There are a variety of shapes of these ports. The shape is not so important as the location and what can be viewed within the kiln through the ports.

A round port, but probably too low to be of much use

A rectangular port viewed from the inside showing the field of view that can be allowed

A kiln with multiple square ports

If your kiln has come without a port or one that is not placed where most suitable for your use, you can drill the casing and brick or fibre to provide another viewing port.  Make a ceramic plug or wad up ceramic fibre blanket to fill the hole when it is not in use.

Boiled Glass

This is a technique that will obtain a random, organic feel to glass that would otherwise be scrap (cullet) – remembering that you have to use compatible glass throughout. The principle is to take the temperature up high enough for the glass to begin to flow easily and bubbles to blow through and burst.

The results can be used as they come out, or they can be cut to provide points of interest in other work, or the glass pieces can be damed before firing to obtain thick pieces which can be cut into slices for other work. And I am sure, there are numerous other ways to use the resulting glass too.

The effects are rather like colourful molten rock with gases bubbling through. These bubbles mix the glass colours. So you need to be sure you do not use a wide variety of colours, or your result will be similar to the molten rock - muddy. Use a few contrasting colours, and ensure you include a significant proportion of white to maintain bright colours. Also remember that the hot colours – reds, yellows, oranges – opalise at high temperatures, so the transparents can be used as opals.

You can use whole sheets of glass or scraps. In either case, it is useful to start with a clear base to help avoid picking up kiln wash when the glass is moving about. The glass must be clean to reduce the incidence of devitrification. Stack you glass on top of the base glass in what ever order you like. Contrasting colours alternated give a strong result.

You can put shelf paper of 0.5 mm or thicker on the shelf or simply kiln wash the shelf with several layers of wash until the shelf surface is no longer visible through the wash. Use of thinfire is not recommended as the powder can be pulled into the glass.

If you do not dam the area to contain the glass calculate how far the glass will expand on the shelf, so that you do not put down too much glass and have it spill over the edge of the shelf.  

You can use bubble powder onto the base layer to promote the bubbling during the firing. However, if you are using cullet, you can just take the temperature up rapidly without a bubble squeeze, which will give you plenty of air pockets to burst through the layers of glass.

You can take the temperature up at about 300ºC per hour to 925ºC with no bubble squeeze and soak for 10 – 15 minutes. Then allow the kiln to drop the temperature as fast as possible to about 815 and soak there for around 30 minutes to allow the little bubbles to rise to the surface an burst too. Then reduce to the annealing temperature and soak for the thickness you calculated in preparation for the firing.

Precautions

You need to be careful in firing and annealing pieces using this glass. Any glass that has been fired to a high temperature tends to begin changing compatibility. So you need to be careful on your rates of advance, and on the annealing and cooling portions of the firing when using the glass in other projects. You may want to consider using a schedule for twice the thickness of the piece on subsequent firings.

There may be devitrification on the surface. You should sandblast or abrade away this devitrification in some way to be able to get a shining surface when you fire polish.

There may also be a number of pin hole sized bubbles at or just below the surface. These will close with a fire polish also.

Lead Corrosion in Acids

Lead forms a protective film, which if undisturbed preserves the metal below this layer.

The corrosion resistance of lead is based on its ability to readily form a tenacious coating of a reaction product. This then becomes a protective coating. Protective coatings on lead may form as the result of exposure to sulphates, oxides, carbonates, chromates, or chemical complexes.

Handbook of Corrosion Data, by Bruce D Craig, p26

Lead is resistant to corrosion especially “with solutions containing sulphate ions, such as sulphuric acid.”

However, the new or bright metal reacts quickly with a variety of alkalis and many organic (although not most inorganic) acids.  “...Lead is not stable in nitric and acetic acids, nor in alkalis. The metal does not resist nitric acid. Lead corrodes rapidly in acetic and formic acids.” (Handbook of Corrosion Data, by Bruce D Craig, p.29)

Lead has very limited resistance to acetic acid.... Dilute [acetic acid], even at room temperature attacks lead at rates exceeding 1.3mm/year. These rates increase rapidly with increasing aeration and velocity However … acetic acid … has little effect at strengths of 52% to 70%.

The corrosion rate in acid increases rapidly in the presence of oxygen and also in oxygen in combination with soft waters such as rain and distilled water. Corrosion increases at the rate approximately proportional to the oxygen content of the water.”

Handbook of Corrosion Data, by Bruce D Craig, p.26, 29

This another good reason to avoid vinegar as a cleaning agent for leaded windows.

Lead dissolves in organic acids (in the presence of oxygen). Lead also dissolves in quite concentrated alkalis (≥10%) because of the characteristic of the lead salts that can act as either an acid or an alkali. These salts are soluble in the presence of water and oxygen.

Alkali salts are soluble hydroxides of alkali metals and alkali earth metals, of which common examples are:

• Sodium hydroxide (often called "caustic soda")
• Potassium hydroxide (commonly called "caustic potash")
• lye (generic term, for either of the previous two, or even for a mixture)
• Calcium hydroxide (saturated solution known as "limewater")
• Magnesium hydroxide is an example of an atypical alkali since it has low solubility in water (although the dissolved portion is considered a strong base due to complete dissociation of its ions).

Although this has been a rather technical posting, these data show that lead is subject to rapid attack by both organic (and some inorganic) acids and alkalis in relatively low concentrations when in the presence of aerated water. However in normal environmental conditions the protective reaction layer avoids much of this vulnerability.

Tack Fusing Considerations

1 – 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. More information on this is given in this entry. 



2 – Annealing 

The tacked glass can be considered to be laminated rather than fully formed together. This means the glass sheets are still able, partially, to act  as separate entities. So excellent annealing is 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 addition, in tack fusing there are very uneven thicknesses meaning 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 on cool down.

On very difficult tack fuses it's not unusual to anneal for a thickness of four to six times greater than the actual maximum thickness of the glass. That extended cool helps ensure that the glass has time to shift and relax as it's becoming stiffer, and also helps keep the temperature more even throughout.

So 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 – including the tacked elements – 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, possibly up to 5 times the total thickness of the piece.

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. The anneal cool over the next 110ºC is to ensure this piece of different thicknesses will all react together. That means tack fusing takes a lot longer than regular fussing.

3 – Effects of thicknesses, shapes, degree of tack

The more rectangular or pointed the pieces there are in the piece, the greater the care in annealing is required. How you decide 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 simplistic estimation of the schedule required 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 section as well.

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

·         Tack fusing of a single additional layer on a six millimetre base

·         Rectangular pieces to be tack fused

·         Sharp, pointed pieces to be tack fused

·         Multiple layers to be tack fused

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

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 up a piece of the configuration you intend, but 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. So your annealing soak needs to be longer, if stress shows. You can speed things 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, you need to do a mock up in clear.

Pot Melts – Weight of Glass Required

Circular pieces
This table assumes that a 150 mm diameter pot is being used, and assumes that 125 grams of glass will be left in the pot. Larger diameter pots or even pot trays can be used, but more glass will remain in the container. The following table gives the desired diameter of the melt and the weight of glass needed to achieve an average 6 mm thick result. To achieve a uniform six millimetre thick disk will require long soaks at both melting and fusing temperatures to allow the glass to even out in thickness.

50 mm diameter disk requires 154 grams of glass
100 mm diameter disk requires 243 grams of glass
150 mm diameter disk requires 390 grams of glass
200 mm diameter disk requires 596 grams of glass
250 mm diameter disk requires 861 grams of glass
300 mm diameter disk requires 1185 grams of glass
350 mm diameter disk requires 1568 grams of glass
400 mm diameter disk requires 2015 grams of glass

Thicker melts
Of course if you want a thicker pot melt — one that is confined so that it cannot grow larger, only thicker — you can use the following method to estimate the amount of glass required.

Choose the diameter wanted from the above table, and subtract 125 from the weight of glass required. Then multiply by thickness wanted divided by 6 mm. Add back 125 gms — the amount that will be retained in the pot — and you have the required amount.

For example: a 200 mm disk of 6 mm requires 596 gms. You want a 12 mm thick disk of 200 mm.
First subtract 125 from 596 to get 471 gms. 417 gms times 12 equals 5652. Divide this by 6 mm and you have 942 gms required. Add 125 gms — the amount left in the pot — and you have a requirement of 1067 gms for a 12 mm thick disk of 200 mm.


Rectangular pieces
These are easier to calculate than discs, as the calculation is length times height times depth (all measurements in centimetres).  

If you are making a billet and using an empty margarine pot of 7 cm wide, 12 cm long and 7 cm high you will need enough glass to fill a volume of 588 cubic centimetres.  As the specific gravity of glass is 2.5, you multiply the cubic centimetres to give the weight required in grams — in this case, 1470 gms.

If you wanted a 6 mm tile of 100 mm square you would need 150 grams of glass.

To make a 1 cm slab of the same size you need 250 grams of glass.

To make a billet of 5 cm by 10 cm square you need 1250 grams of glass (this is pretty close the the maximum that can be loaded in a 12 cm diameter Pot).

To make a small sample billet of 2 cm thick by 4 cm by 8 cm you need 160 grams of glass.

A billet or pattern bar of 5 cm by 10 cm by 5 cm needs 625 grams of glass.