Capping

This term most often refers to placing a single piece of glass over the whole of the project. The decisions relate to whether to do it at all, in what circumstances and in what order. Whatever you place on top of the project is what the eye will first see. A tinted top layer will give that tint to all the pieces making up the object. So most often the top is a piece of clear glass.

Many times the purpose of capping is to give the volume of glass required to keep the piece contracting as a result of the surface tension of the glass trying to pull itself up to 6mm thickness.

When using opalescent glass as the main component in the work, you should consider capping with clear. Opalescent glass is slightly more prone to devitrification than transparent glasses, so any work to be fired a number of times might be best fired with a clear cap. It also protects against any bubble formed between the other glass and the cap showing as a clear spot within the opalescent as it pushes the colour aside and reveals the clear below.

There are some times when you should consider placing the clear on the bottom. If your design layer is made up of lots of pieces where air might be trapped, but is uneven enough to be the likely cause of bubbles, then the clear should go on the bottom to ensure there is sufficient volume. An alternative is to do a high tack or full fuse of the whole upside down on fibre paper, then clean up and fire right side up with the capping glass.

Annealing High Temperature Items

Every time you go above the annealing temperature, you must anneal again. You cannot skip or skimp on the annealing. You cannot rely on the annealing in the final firing to make your piece durable. Each time you fire a piece you are putting a lot heat stress into the piece.  If it has not been adequately annealed in the previous firing, it is much more likely to break on the heat up phase of the firing than if you annealed well on the previous firing.

The annealing at each stage in multiple firings is just as important as the previous one. In addition, pot melts and other high temperature items are inherently more delicate than those fired at their designed temperatures, so more careful annealing (including the annealing cool) is advisable. This is because the compatibility of glass alters a little at high temperatures. For example, you will observe that hot transparent colours opalise in the 900C range. This opalisation in itself will have altered the compatibility a little, because the opalescence alters the viscosity from what it was as a transparent. Other factors are at play too, such as some minor burning off of the colouring metals. So, careful annealing is required to ensure the maximum amount of stress is relieved. You also need to have a slower than usual initial rate of advance for any fire polish or slump firing after any high temperature process.

Even when firing at fusing temperatures, but beyond the tested number of firings, more careful annealing is required. In the case of Bullseye they have tested for three firings, although people get many more firings than that without difficulties. When taking glass beyond the design limits, more care is required in all phases of the firing to get durable results.

Writing Your Own Schedules, Part 2

Time Versus Rate

Schedules can be expressed as a rate per hour, or a time to get to the target temperature. What you feel most comfortable with relates largely to your background and teaching. Most ceramics based people use the time to get from one temperature to another. Most kiln formers without a background in ceramics tend to use rates per hour when writing schedules.

The rate of 100/hour to 100 degrees is the same as 1 hour to 100. 2.5 hours to 200 is the same as 80/hour to 200. So the conversion to a time to get to a target temperature is a simple one of dividing the temperature by the rate per hour to give the number of hours to achieve the target temperature. Some controllers will allow hours and minutes to be programmed; others allow only minutes – in which case multiply by 60 to give 150 minutes.

This is the same thing you do to find out how long a firing will take. If you see a schedule expressed as time e.g.,

3 hours to 677 for 0.5 hour,

1.25 hour to 800,

asap to 482 for 1 hour,

2.5 hours to 370

you already know approximately how long this firing will take – a bit more than 8.25 hours (3+0.5+1.25+1+2.5) plus cool down.

It can also be expressed as

225/hr to 677 for 30 mins,

102/hr (800-677=123/1.25) to 800,

afap to 482 for 30 mins,

45/hr (482-370=112/2.5) to 370.

The time to target temperature method of writing a schedule comes into its own when dealing with thick castings that require very slow cool downs. For example, a 60mm thick casting calls for an initial annealing cool of 2.4 degrees per hour over the range 482 to 428. I don't know of a programmer than can deal with decimals. So the alternative is to programme in time to target. In this case it would be a time of 22.5 hours.

The reason for avoiding the choice of 2 or 3 degrees per hour is accuracy. If you had put in 2 degrees per hour you would have spent 27 hours, possibly excessively long. If you had put in 3/hour it would have taken 18 hours, possibly not enough time for the glass to adequately anneal. So, for very slow rates of advance, time to target is much the most accurate method of writing the schedule.

Part 1

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.

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:

System 96

Uroboros

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.

Part 2

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.

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.

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.

Reference: http://glassproperties.com

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.

Reference: http://glassproperties.com

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.

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.