Wednesday, 16 April 2014

Making Billets

One of the uses of cullet (small pieces of glass) is in casting. However, simply placing the glass into a mould and firing, leaves many bubbles and often shows the edges of the original pieces of glass. Billets (ingots of glass) are more useful because they have fewer of the small bubbles and fewer edges than cullet.

It is possible to make your own billets. This can be done in a fashion similar to pot melts, although the temperature does not have to be so high. And the results are easy to store, if the dimensions are kept regular.

You need to have a mould for the melting glass to be contained within. These moulds can be made from plaster. A simple way is to use old margarine tubs placed upside down and fastened to the base within a dammed area. Pour the plaster of paris over the tubs to make the moulds. An alternative is to use strips of refractory material (fibre board or cut up kiln shelves) surrounded by heavy bricks to stop any movement due to the weight of the glass.

The glass to be formed is put into ceramic flower pots and can be directly onto the plaster of paris or dammed areas. You should put at least one piece of glass to cover the hole at the bottom of the pot. All this glass must be clean. Calculate the amount of glass required by determining the volume of the containment area (in cubic centimetres) and multiply by the specific gravity to give the number of grams required.

Don't get too ambitious about size, as these billets need to be fitted into the mould reservoir for filling the mould. A small margarine tub is approximately 12 cm wide, 7 cm deep and 7 cm high. This is as large as required, and smaller may be better. If you are making your own from dams, something like 4 cm by 8cm by 2cm may be better. This size is convenient for filling a reservoir, and has the advantage of being able to compare the intensity of colour the different thicknesses will give to the casting.

Remember that the thicker you make the billets, the longer you have to anneal. So the annealing time of the billet may be the factor that determines time. A 2 cm billet will take at least 9 hours of annealing time; one of 4 cm will take 28 hours of annealing.

When setting up the kiln for making the billets, remember that in general the higher the reservoir above the billet mould, the fewer bubbles you will get in the billet, although you are confined by the height of the kiln. Although there still will be some bubbles, these will further reduce by the second flow of the glass during the casting process.

To fire the set up, you can advance the temperature rapidly to 650/670ºC with a long soak there (possibly 3 hours). The final temperature can be below pot melt temperatures, so a casting temperature of 830ºC with a long soak (possibly 6 hours) will be sufficient. Take note of your final thickness – including any containment material – to determine the annealing soak and schedule.

Wednesday, 9 April 2014

Writing Your Own Schedules

Most introductory kilns are now being supplied with pre-set schedules. This can make moving on to the schedules you need for the new work you are doing appear to be difficult.

The first thing is to get the print-out of the pre-programmed schedules and determine what each stage of the programme is designed to achieve. If you compare the programme temperatures with a description of what is happening with the glass at that temperature, you will be going a significant distance to making your own schedule with an understanding of what you will be achieving with each stage of your purpose made schedule. A very good guide to what is happening to glass at various temperatures is this note from Bullseye. This also has the advantage of telling you what happens with different thicknesses of glass.

Next compare the pre-programmed schedules with those printed on the manufacturer's website, for example:

So, now you know what temperatures you are trying to achieve, how fast should you go to get to that temperature? I have developed a guideline that the initial rate of advance should be no more than twice the rate of your initial cooling rate for the final piece. This means that you start planning the schedule from the annealing portion of the full schedule. If you will have a final flat thickness of 6mm, the annealing rate will be around 80ºC, so the initial heat up rate could be about 160ºC. This is a conservative rate, and experience will guide you to how much quicker you can heat up the glass. This initial heating phase can be all the way up to the bubble squeeze/ slumping temperature, but must be to a temperature at least 40ºC above the annealing point.

There are at least three elements that will reduce this initial rate to less than this general guidance: Thicker pieces need more care. The more layers, the more difficult it is to get the heat to the bottom layer, so slower rates of advance are needed. The greater the unevenness in thickness, the slower the rate of advance.

There are, of course many other variables relating to the kiln, some of which are:
Side or top elements
Distance to the elements – side or top
Distance to the sides of the kiln
Placement in the kiln – e.g.,floor or shelf and how high
Nature of the firing surface – e.g., ceramic, fibre board, fibre paper
Placing in relation to the hot and cool spots in the kiln
How the glass is supported - especially on a slump or drape

At the initial stages of learning about fusing schedules, you need to make notes of all these things (and the results) on your firing records so that you can refer back to get guidance on what rates of advance are acceptable for any given firing.

Wednesday, 2 April 2014

Glue Placement

Many people use glue to hold their arrangements of glass together to get it to the kiln. There are many kinds of glue that can be used. It is best to avoid resin based adhesives, but most other kinds of glue can be used – including hair spray, lacquer, super glue, CMC and PVA in addition to the proprietary fusing glues. The cheapest with the fewest additives seem to get good results.

Remember the glue burns away long before the glass becomes sticky, so if the glass won't stay in place while you are assembling it, it won't in the kiln either. The glue is only to keep things together while being transported to the kiln.

But this note is about were to apply the glue you choose to use.

The glue should always be used in minimum amounts. If it is a strong water based glue, such as PVA, it can be diluted with water and still provide sufficient adhesion. The glue should be runny, not thick or a gel. Unless the adhesive is a spray, a small dot at the edge of the piece to be glued will be sufficient. Capillary action will draw enough glue under the piece to stick it to the base glass.

If you are spraying the adhesive, that should be done at the end of assembly, to avoid flooding the base glass with adhesive. It is often best when using these lacquer based adhesives to spray a small amount of liquid into a container and use tooth picks or other pointed implement to dot the lacquer at the edge of the pieces to be attached. This way you can glue as you assemble rather than waiting to the end.

Adhesive under the middle of a piece of glass is likely to give black marks and even large bubbles, as the combustion gasses cannot get out from under the glass. So always confine your glueing to the edges of the pieces. A dot at each end is all that is required.

Wednesday, 26 March 2014

Hangers for Sun Catchers

Unless you are using some manufactured system or a frame, the most frequent way to provide hanging points for copper foiled sun catchers is to create a loop from copper wire.

Hangers should originate in a solder bead that goes some way into the piece. The loop's tail should lie a significant distance into the solder line to ensure it does not pull the piece apart. If this is to remain invisible, some planning will be required to allow the small extra space between the foiled glass.

The loops for hanging a piece of any size should not be soldered to the perimeter foil without reference to the solder bead lines within the piece, as the adhesive and foil are insufficient to hold the weight without tearing.

Reinforcement of free hanging or projecting elements can be done by placing wire around the piece with a significant excess going along the perimeter in both directions. The supporting wire can go into the solder line, if it is a continuation of an edge of the free hanging piece.

An example of a piece that needs reinforcement around the wings to keep them firmly attached to the body

The strongest method of proving hangers is to wrap the wire around the whole perimeter of the piece. Choose easily bent copper wire. This will be pretty fine, but when soldered, will be strong enough support the whole piece.

The perimeter wire can also be concealed by edge cames

The hanger can be made by leaving a loop of wire free along the perimeter. This way you can hang from any convenient place on the perimeter. This loop can be made by a single 180 degree twist in the wire, or by bending a loop into the perimeter wire. In all cases you will need to tin the wire to blend it with the rest of the piece.

An example of wire running between the yellow and purple on the left and incorporated into the design

This perimeter wire can be simply butted at the start/finish of the wire. It could be overlapped, but this is unnecessary on any piece where this method is adequate for support. The start can be at the top or bottom, although I prefer the top, so the wire is continuous from loop to loop. The reason for continuing beyond the loops is to provide support to all the edges of the sun catcher.

Wednesday, 19 March 2014

Annealing - Effects of Chemistry

Affects of Chemistry on Annealing Point

The change in the transition temperature is affected by the rate of cooling; it is also affected by the chemistry - or composition - of the glass. The transition temperature in silicates (glass of various compositions) is related to the energy required to break and re-form covalent bonds in an amorphous (or random network) lattice of the tetrahedra form of the glass molecules.

A covalent bond is one that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is what covalent bonding refers to.

The transition temperature is influenced by the chemistry of the glass. For example, addition of elements such as Boron, Sodium, Potassium or Calcium to a silica glass helps in breaking up the network structure, thus reducing the transition temperature and the melting temperature. Alternatively, Phosphorus helps to reinforce an ordered lattice, and thus increases the transition temperature.

The modifiers commonly used in glass-making are: sodium oxide, potassium oxide, lithium oxide, calcium oxide, magnesium oxide, and Lead oxide. Although there are over 2,000 known additives to glass. The minerals used to colour the glass seem to have minor affects upon the glass composition as they generally are in a colloidal suspension without forming bonds to the silica atoms.

If an oxide, such as sodium oxide, is added to silica glass, a bond in the network is broken and the relatively mobile sodium ion becomes a part of the structure. With increase in the amount of modifier, the average number of oxygen-silicon bonds forming bridges between silicon atoms decreases. The principal effect of a modifier is to lower the melting and working temperature by decreasing the viscosity. An excess of modifier can make the structural units in the melt sufficiently simple and mobile that devitrification (crystallization) occurs in preference to the formation of a glass. The skills of the glass makers lie in the balance of factors relating to the transition and working temperatures, and the maintaining the resistance to devitrification.


Wednesday, 12 March 2014

Annealing - Physical Changes

Physical changes of Glass at the Annealing Point

What happens at the annealing point and what is its relevance to compatibility? There are two main changes that occur – physical and chemical. They both affect the temperature of the annealing point, but in different ways. These notes are an attempt to understand these changes and how they affect compatibility.

The first requirement is to understand what the annealing point is. First it is a range of temperature during which the glass transforms from a liquid to a solid. It has a definition:

The annealing point is the point at which the material reaches the glass transition temperature. It occurs in a temperature region at a point where stresses can be relieved in a very short time. It is defined mathematically by a specific viscosity. In simple terms, this is the temperature below which viscosity prevents any further configurational changes.

Any contraction beyond the transition temperature range is due only to the lower kinetic energy of the groupings of the tetrahedra molecules. Thus, the compatibility of the glasses is determined at the annealing range as a combination of expansion/contraction and viscosity at the annealing range of temperatures rather than at the lower CoE which is more suited to crystalline solids. The transition temperature of a given “glass composition” depends both on its constituents and upon the rate of cooling.

The physical changes of glass during the transition/transformation range of temperatures are various:

  • Viscosity has a very large increase with temperature reduction, but without any discontinuity. Viscosity has an enormous effect on the activity of molecules in glass. As the glass cools below its transition temperature it causes the progressive immobility of the molecules.
  • The expansion rate (CoE) shows a relatively sudden change around the annealing temperature. Below the annealing point, the glass expansion and contraction behaves much like the CoE at the lower, measured temperatures. This means viscosity may be the most important element in creating a stable fusing compatible glass.
  • The amount of heat required to increase the glass temperature rises quickly rather than the previous regular heating rate needed to achieve unit changes.
  • The shear modulus changes rapidly, making the glass much more brittle below the annealing point.
  • The rate of heating or cooling can affect the exact temperature at which the glass transition point occurs.

The annealing phase (glass transition) is a dynamic process where time and temperature are to some extent exchangeable. This allows annealing to occur at the lower part of the range of the transition phase, but the glass then needs a slower cool from there. From the (higher) annealing point temperature - as defined by viscosity - the cool can be a little more rapid than at the lower temperature range of the transition phase. The anneal at the lower part of the transition saves annealing and cooling time for thick slabs, but for thinner pieces (less than 9mm), soaking at the annealing point and cooling from there is the simpler process.

Slow cooling results in a lower transition range because the tetrahedra forms of the molecules have more time to rearrange (to the degree that this is possible). This slower cooling results in tighter packing of tetrahedra as the mass reaches its transition range. When the glass reaches room temperature, its volume will be smaller when cooled slowly than glass melt which has been cooled rapidly. Hence, slower cooling from the melt results in a denser glass.

Wednesday, 5 March 2014

Heat Up Events

This is based on Graham Stone’s work with float glass. The temperatures are applicable to float glass, and so need to be adjusted for any other glass, but illustrate the principle of how heating temperatures affect the glass. Temperatures in degrees Celsius.

10-250 Slow rate heating up. Risk of thermal shock. Venting often done in this phase.

250-500 Medium rate heating. Risk of thermal shock diminishing.

400 + Many glasses now tolerate fast heating up ramp rate.

550 Glass surface beginning to soften slightly

600 Safe from thermal shock above this temperature

610 Glass bending slightly, picking up texture.

680 Glass begins to stick to itself. Tin bloom becomes iridescent.

690 Fusing glasses reaching their softening points.

715 Glass beginning to stretch. Tack-fired pieces adhered by now.

720 Subtle devitrification and iridisation burn off becoming a factor with some glasses.

730 Softening point of float.

750 Edges no longer sharp. Tin bloom stretching becoming "frosty".

760 Tack fuse range for fusing glasses.

770 Float glass fused, but still "sitting up".

790 Trapped air can cause bubbles under sheet glass at this temperature.

800 Full fuse for most fusing glasses.

820 Fused float glass nearly flat.

825 Full fuse for float glass. Devitrification more pronounced.

850 Glass flowing.

950 Glass soft enough to "rake".

1000 Approximate liquidus temperature.

Based on Firing Schedules for Glass; the Kiln Companion, by Graham Stone, Melbourne, 2000, ISBN 0-646-39733-8, p24
Post revised 5th March 2014

Metal Framing Materials

Lead is a very weak metal. Therefore various other metals are often considered for the perimeter of the panel to strengthen the whole.

Zinc is a metal often used for strengthening the perimeter of panels. It is stronger than lead – by about 8 times. It is relatively easy to solder. However it is subject to more rapid corrosion than lead.

So an alternative is aluminium which is about about the same strength as zinc. However it does not accept soldering, so professional joining or cold fixing solutions are required to make the frame.

Copper is over 10 times the strength of lead and can be considered as an alternative to zinc. It accepts solder well, but as a came is extremely expensive. It does corrode to a verdigris unless protected and maintained. However, because of its strength, copper wire - as a single strand or several twisted - can be used inside other came such as lead or zinc to provide strong support.

Brass is about 19 times stronger than lead. It is available in came profile as well as “U” and “L” profiles. It accepts solder well and resists corrosion. It is more expensive than lead, but similar in price to zinc.

Mild steel strength varies but is at least 27 times stronger than lead. It does not accept solder easily, and does corrode without painted protection, but is a less expensive option than aluminium, zinc or copper. As an angle or “T” shape, mild steel and iron have been used for centuries to support leaded glass panels.

Stainless steel is at least 37 times stronger than lead. It is difficult to weld and does not accept solder at all. It is very resistant to corrosion.

When considering framing solutions for panels, the main factors to consider are relative strength, corrosion, and joining methods possible.

Brass, Copper, Lead and Zinc all can be joined by solder. Aluminium and stainless steel cannot be joined with solder. Although mild steel can be joined with solder, a good strong joint is difficult.

The stronger the metal, the thinner profile required, which can make metals that are more expensive by weight an economical solution, as metal prices are most often by weight rather than shape.

It also is possible to combine a stronger metal with a weaker metal, such as including copper wire or steel rods in the lead came.

It is not absolutely necessary to solder the panel to the framing material. A frame can be made and the panel fixed within it by other than hot soldering methods. In this case the frame takes the whole weight of the panel.

Wednesday, 26 February 2014

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
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.

Wednesday, 19 February 2014

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.

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.

Wednesday, 12 February 2014

Grinder maintenance

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

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.

Wednesday, 5 February 2014

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


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.

Wednesday, 29 January 2014

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.

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.

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.

Sunday, 26 January 2014

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.

Saturday, 25 January 2014

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.

Wednesday, 22 January 2014

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:

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.

Wednesday, 15 January 2014

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.