Bubble Reduction in Casting

There are several things that can be done to reduce the number and size of bubbles in casting.

• Fire higher - to 830ºC instead of 815ºC - and soak for at least four hours. This allows more bubbles to rise to the top and burst. If there are still more bubbles than wanted, increase the soak time.

• Stack the glass in the centre of the mould, allowing a few centimetres from the mould walls. This allows the glass to spread and flow from the bottom and up the sides, reducing the likelihood of trapping air. If you have more than one stack, keep the same space between the stacks as the mould walls.

• Make sure that the way you stack the billets or casting plates so there is a smaller space at the bottom of any cavity than at the top. The reverse allows the glass to soften and seal in the air in the space.

• You can construct a mould to make billets of the general shape of the final object. This of course, is much more work, needing two moulds.

• A major thing to avoid is the use of frit, especially at the bottom or deep in the mould as bubbles will collect around each piece and lead to a multiplicity of bubbles throughout the casting.

Thinfire as a Separator on Moulds

"I was told that it was possible to put a piece of thinfire paper under a circular piece of glass that is to be draped over a mould....  Has anyone done this or heard of doing it?"

Yes this can be done. Some caveats are in order though.

It is important to put a separator on the mould -normally kiln wash - before using it. Once coated, it will not need to be re-coated unless the surface is damaged, scratched, etc., for a very long time. If the kiln wash seems to be rough you can smooth it in various ways as noted in this tip.  The advantage of kiln wash is that it does not cost much and lasts a long time. The thinfire or similar is a one-time-use product.

The binder in the thnifire burns away during the heat up and allows the now unbound separator to drift down to the mould before the glass begins to conform to the mould. In that way it is very similar to a fine dusting of kiln wash powder over the mould surface before firing.

I don't see the point of using Thinfire or similar during slumps (although I can see that there are occasions where this method would be useful). For drapes, placing a bit of thinfire over the mould - especially if it is metal – is an additional precaution.

My view is that you have to kiln wash the mould anyway. KIln wash is cheap and long lasting while thinfire needs to be replaced after each firing.

Clearing Small Core Drill Bits

Core drill bits are very useful, as only a thin wall of glass is removed, leaving the main part of the hole as a single piece. This means that it is quicker than a solid drill bit through not having to remove so much glass. However with small diameter core drills they often jam up with waste glass. This is evident when the drill ceases to drill through the glass, leaving a broad circular mark as it moves around on the glass.

Inevitably, at one time or another, the glass core will get stuck inside the drill bit. This needs to be cleared before any further drilling can be successfully done. It seems to be more common with the smaller diameter drills – from 5mm down.

The broken off core needs to be cleared from the back of the bit, not the diamond coated end. So you need to take the bit out of the drill and use a thin nail or piece of wire into the hole to push the glass out.

My collection of core drills.  At the front you can see copper wire and a thin punch for clearing blocked drills

Core drills need a more accurate means of steadying the bit than your hands. Imagine that you are trying to get that core out of the glass in tact. Any wobbling as you drill will break that core. When the core breaks, it is inclined to jam the cylinder. So a drill press is almost essential when using a core drill.

They also need water like any other drilling of glass. Normally this is supplied through the hollow core of the bit, which is beyond hand held drill motors.

For something as small as 3mm, a solid drill that removes all the glass rather than a core is more likely to be most successful. You can get diamonds bonded to piano wire that will give you holes from a fraction of a mm up to 2 or 3 mm and these can be used in dremmel-like tools that take small diameter bits.

It is absolutely NOT recommended to use a core drill at an acute angle to start the hole. The wall of a 3mm core drill is very thin and easily damaged. It is ok to start a solid drill that way, but not a core drill.

There are a number of related posts beginning with this one which give advice on the many ways to drill holes in glass.

Finger protection

Grinding lots of glass pieces often leads to a number of small cuts on the tips of your fingers. There are several things that can be done to reduce these cuts and the tenderness that comes from lots of grinding.

The first thing is to take the sharp edges off the glass. You can do a simple, light grind all the way around the piece. This removes the extra sharp edges that often remain after breaking the glass.

You can go a step further and do a light arris around the piece. This is just lightly holding the glass at about 45degrees to the grinding bit and going all the way around on all sides. This does not take off the shape of the piece, but gives a more rounded feel to the piece.

Illustration of the effect of holding the glass at an angle to the grinding head - not so much needs to be taken away as in the illustration to get the effect

It is not generally recommended that you wear gloves around rotating machinery. There is too much risk of injury, even on a small grinding machine. Some of the alternatives to gloves include plasters (band aids), masking tape, electrical tape.

Other purpose-made things you can buy include rubber finger protectors, finger tip pads, finger caps (as used in counting money).

Other tools are made to hold the glass such as the grinder cookie

and Nick's Grinder's Mate 

Reclaiming Solder

Re-using solder can range from simply soldering the ends of the solder sticks together (if you are using blowpipe solder). This will then form a useable stick with solder blobs on its length.

If you have a number of blobs and splashes, don't throw them out. Collect them together and when you have enough you can make them into another stick of solder.

You can tape two narrow pieces of glass onto a length of marble or heavy steel about 3-4 mm apart. Put the pieces in the channel formed by these two pieces of glass and melt the pieces with your soldering iron. This will form a useable stick.

Lifting the new solder stick from the wood

The same can be done by cutting out a 3mm wide and deep channel in a piece of timber and doing the same as above. The wood will smoke a bit and blacken, but not ignite.  And you can use a blowtorch to melt the solder if you do it this way.

Using the (slightly irregular) solder stick

Assembling Foiled Pieces

Keeping foiled pieces together while assembling them prior to soldering is sometimes a problem.

If the panel is rectangular - or at least one with multiple straight sides - you can use short battens of the length of the sides. Nail or screw them down to a board so that about one half of the panel is contained. So if it is a rectangle, two sides will be enough. If it is six-sided ,three pieces would do.

An assembled piece illustrating the two battens - although with a leaded panel

For ease of assembly, a copy of the cartoon should be fastened to the board first and then the battens fixed on top of the cartoon. The pieces can then be placed against battens and held there with pins or nails until the next pieces are ready.

Illustration of the cartoon fixed by the surrounding battens

Some prefer to tack solder the pieces together as they foil. This can be done in combination with the use of battens. However, leaving the soldering iron idling while foiling and using it only occasionally is very hard on the iron's tip and your electricity bill. I prefer to assemble the whole and then solder all at once.

Illustration of placing pins all the way around an oval panel

For circular or irregular shapes a slightly different approach is required. You can use multiple pins or nails along the perimeter to hold the foiled pieces together. An alternative is to cut a piece of scrap window glass to the shape of the external perimeter of the panel. Hold it is place with nails or pins and proceed as with a rectangular shape.

Placing nails all the way around an irregularly shaped piece

Rates of Advance

There is a lot of literature about annealing and cooling rates, as they are the most critical elements in producing a piece with minimum stresses within it. But there is not so much information on initial rates of advance.

It is possible to break the glass in heating it up by going too fast during the initial temperature rise. How fast you can increase the temperature is dependent on how even the heat is within your kiln. So any suggestions have to be tested within your own kiln and setup rather than relying exclusively on others' experience. Some of the considerations relating to the kiln are given in this blog about initial rates of advance.

So with those precautions, I put forward a suggestion based on my experience and information gleaned from the Bullseye site, education section and from Graham Stone's work. These lead me to suggest that the initial rate of advance can be twice the actual or planned first cooling segment. This rate of advance applies up to the softening point of the glass.

So this theory implies that a piece of glass 6mm thick - that might be annealed at 80ºC per hour during the first cooling segment - can be taken up at rate of 160ºC/hour to the softening point. And by extension:

• A 12mm thick piece could be taken up at 110ºC
• A 19mm piece could have an initial rate of advance of 50ºC/hr
• A 25mm thick piece of glass could be taken up at 30ºC/hour.

These all depend on a number of factors:

• how the glass is supported,
• the nature of the shelf,
• the composition of the mould, and
• the kiln characteristics as well as
• the colour combinations and
• whether the piece is tack fused or full fused.

If the glass is heated only from the top with no ventilation beneath the shelf, more caution is required.

Slower rates of advance are indicated if 

• the glass is supported only at a few points, 
• or if the kiln is side fired or has cool spots.
• If the piece is tack fused, you need to slow the rates of advance. 
•  Consider the rate of advance for the next thicker glass as your starting point as a minimum.

Remember that these numbers can only be used as a guide in conducting you own experiments.

Fusing with Painting

Combining Painting and Fusing

Painted oyster catchers with frit and stringer

It is possible to combine glass painting with fusing. Tracing paints are generally powdered iron rust and fused to the glass by the glass powder that carries this pigment. So it is possible to paint and fuse a project at the same time without loosing the intensity of the paint.

In general it is best to work down from the highest to the lowest temperature in your firings. This does require planning of the firing sequence in addition to the usual design considerations.

This sequence of firing depends on the glass stainers' paint you are using. The tracing paints (blacks, browns, some whites, some blue greens) that fire at 650ºC and above can be fired up to around 800ºC without losing much of their intensity. If you use Debitus paints, they can be fired to 850ºC without loosing their depth of colour.

Fused, painted and slumped piece painted both at fusing and slumping operations

If the paint is under glass pieces or under frit, the paint will appear to spread and the lines thicken. This is due to both the lens effect of the covering glass and the weight of the glass over the lines. If you require the lines to be of consistent thickness, you probably should paint after fusing.

You can, of course, use low firing ceramic glazes as they mature in the region of 700ºC to 850ºC. These can be painted on to the unfired glass and taken to full fuse without any fading. You do need to make sure the glaze has time for any volatile materials to burn off, so a slow rate of advance up to the slumping temperature of the glass is advisable.

Painted and fused, then painted and slumped.  Note the paint lines and coloured glass do not always match or need to.

If you are using glass stainers' enamels, you need to fuse and shape before firing. You can fire in the mould for the enamel firing as the temperature range is in the 520ºC to 580ºC range and will not add more mould marks to the glass. Keeping the glass in the mould protects against any tendency for the glass to alter shape.

Moulds for Bottles

One of the many styles of commercially prepared moulds

An alternative to buying moulds for slumping bottles is to use a sand bed. You can place the bottle into the sand and roll it a little from side to side to create a depression in the sand that then becomes the mould.

I use a fine sand (not builder's or garden) and coat it with alumina hydrate (slaked alumina). I use about 1 part alumina to 5 sand, but the mix is not critical, just enough alumina to coat the sand particles. It can go directly on your kiln floor if you have an easy way to pick it back up, as it is re-usable. Or you can put it in a stainless steel tray or any open topped box that will withstand the temperature.



Make your depressions and then sprinkle or sift a fine layer of alumina over the area - I use an old sock to hold some and dust it over the sand. Then lay the bottle in the depression.

The amount of sand impression you get is dependent on the temperature you use - the higher, the more sand texture you get. 





This way of slumping bottles eliminates the need for a mould and it is variable for different sized bottles.

Annealing a Stressed Piece

If a project has not been sufficiently annealed, it is possible to re-fire to do a good anneal.

You need to establish a slow rate of advance; one that is much slower than for the heat up of a fully annealed piece of the same thickness. This is because you can heat shock the glass much more easily than one which is adequately annealed. I suggest going at about half the rate of a normal firing for a piece of the same thickness.

You have to make sure all the glass is above the annealing temperature so it is advisable to go up to near the slumping or softening point of the glass to ensure all the stresses are removed before beginning the annealing process. If you want or need to retain the level of texture of the previous firing, you will need to go to about 20ºC above the annealing soak point of the glass and soak there for at least as long as you plan to soak at the anneal point.

Anneal at a rate (after the anneal soak) that is considerably slower than previously used. Look at the Bullseye annealing schedules for thick glass, or their project notes for annealing bowls to get some good guidance on the rates of cooling.

Thin Glass Uses

• Thin glass is often used in jewelery as it allows more layers of differing colours to be built up. It also is very useful when building a channel in the piece.
• Thin glass allows more layers to be built up before going over the 6-7mm when the glass begins to expand due to the height overcoming the surface tension of the glass.
• This also allows colours that are not present in the manufacturer's palette through combinations made from two or more colours.
• Thin glass versions of dense colours provide a lighter tone which can fit in well with other lighter colours.
• Thin glasses are also useful in tack fusing, as the height is not so great and so can be used over 6mm thicknesses without adding greatly to the volume.

Seams on the Edges of Jewellery Pieces

Sometimes the edges of pieces can show the places where the glass layers join.

This happens when there is not enough heat to completely fuse the pieces together, as in a tack fuse.

It can happen to only some pieces, due to differences in temperature within the kiln. If this occurs take note of the cool spots, or better yet, run a test for the cool areas of your kiln as in this Bullseye Tech Note.  Then avoid those areas for full fused items.

If you run your kiln as fast as it will go, you can be left with seams at the sides if you do not soak for long enough at the top temperature. I usually try to achieve my desired results with a 10 minute soak. Going slower to top temperature can give you fully formed edges at a lower temperature.

You can also get visible seams if you are using iridescent glass, mica, metal or other inclusions that come to or near the edge of the piece. The solutions are to move the inclusions further in from the edges. With iridescent glass, you should remove two or three millimetres of the iridescence to allow the glass to stick together.

If you want to conceal the layers of colour used to build up the piece, you can cut your top piece - assuming it is not clear – 3mm larger all around. This allows the top layer to fold over and enclose the whole piece, concealing the different colour layers.

Fire Polishing Jewellery

Sometimes jewellery pieces come out of the kiln with imperfections that need to be ground away.

You can do a quick shaping on the conventional grinder and then fire polish. However, you will not get a fire polish at a low enough temperature to avoid distorting the piece. The way to get a fire polished surface or edge is to make the edge less rough before putting the glass into the kiln.

After you shape the pieces on the grinder, make sure you scrub them well to remove any particles created by the grinding process. Dry them and then cover the rough areas with a white paint marker. This will indicate when you have removed the scratches caused by the previous rougher grinding.

Get out your wet and dry sandpapers. Start with the most coarse (about 200) and work the ground area until the paint has been removed. Keep the sandpaper and the area being worked damp. When all the white paint has been sanded away, wash, dry, paint and go to the the next finer grit. Repeat this with progressively finer grits – normally use the grit number twice the previous grit (larger numbers indicate finer grits).

Normally, going down to 400 or 600 grit will be enough to enable a fire polish at a low enough temperature to avoid distortion of your piece. This heat range will be at a low temperature tack fuse.

This smoothing process does not take very long and is much cheaper than buying several finer grinding bits - most grinder bits are 60 to 80 grit, although it is possible to get bits up to 600 grit.

Channels for Jewellery

There are many ways to attach jewellery to the wearer.

One of these is to create a channel in the glass to allow a chain or jump ring to be placed through the piece. This is done by stacking the glass around a narrow strip of thick fibre paper – 2mm or 3mm – thick enough to allow the chain or wire to pass through.

Channel formed with 2 layers and fibre paper

This can be built with layers of thin fusing glass. These are normally 1.5mm to 2mm thick. Volume control is important, as enough thickness of glass is required to stop it contracting. The piece might be constructed with a 3mm base, and several thin layers to make a total thickness of 6mmm to 9mm.

For the channel, cut a narrow piece of thin layer about 6mm to 8mm for the top. Then cut another short enough to leave a channel of the desired width. Fill that channel with fibre paper. Then continue to build the upper layers. These layers should be around 6mm to keep the glass around the channel from drawing in.

Pendant formed in the manner described above

Mini anniversary

I've just noted that it is five years since starting the Glass Tips blog.
And this is the 600th entry.

Installing Your New Kiln

You have your new kiln unwrapped. Now where to put it?

First read the manufacturer's recommendations. If you are still uncertain there are a number of things you should think about.

One of these is getting access around it. You need to be able to work around three sides ideally if it is a top loader, two sides for smaller kilns is usually enough. If it is a front loading kiln you only really need to consider the space in front of the kiln. You are going to be carrying pieces, even shelves to the kiln so you need enough space for you and the shelf in front of the kiln. At the sides you only really need enough space for yourself.

Getting access is part of the consideration of distances from other things. Often people are worried about the heat that the kiln will be giving off. Kilns are well insulated to reduce the costs of firing, so the heat release is slow. Still, you want at least 300mm space from anything inflammable.

You also will want to think about the support surfaces. Sometimes the kilns come with their own stands, but usually these are to raise the kiln to working height. You may want to protect against any (unlikely) meltdowns, so you should put the kiln on steel, ceramic or concrete surfaces. There are a number of table top models and in these cases a large ceramic tile or ceramic fibre insulation under the kiln is an entirely adequate safety precaution.

You will need a place to put things down just before loading the kiln, so placing the kiln near adequate flat surfaces is important.

You also should think about putting the kiln out of the main traffic areas of the studio to avoid disturbance to the kiln or the rest of the studio activity.

Now that you have the ideal location for the kiln you have only begun.

You need to make sure the kiln is as level as possible. The first stage of this is to make sure the casing is relatively level. Use of a spirit level on the top front and sides is probably enough. Put hard spacers under the legs to level things up. You can if you want, level the internal base of the kiln instead of the casing. Many find that more re-assuring. Then you need to put the kiln furniture to hold up the kiln shelf into the kiln and the shelf on top of that. This is the part that really needs to be level. Spend time on it. Place pieces of ceramic fibre under the shelf supports as required to get things really level. A circle or three-way level is good for this purpose. The shelf needs to remain level to get good, consistent results. Any time you move the kiln, the shelf, or the supports, you need to check the level of the shelf.

Once you have the shelf level you are ready to do a test fire. Normally you need to have a firing without anything in it to burn out binders used in the making of the kiln. There is no reason that you cannot have the furniture (shelf and supports) in the kiln for this first firing as they need to be test fired too. In addition you can run a test to discover where the cool spots are in your kiln (every kiln has them). Look up and follow this technical note on how to run a test for discovering how even the heat is within your kiln.

Once you have run your test firing, you will want to protect the kiln floor from any spills of hot glass and the glass from sticking to your furniture. If the manufacturer has given you some kiln wash with the kiln mix it up about 1 part powder to 5 parts water and lightly paint the floor of the kiln - not the walls. The kiln furniture needs this too as does the shelf. This note on applying kiln wash will give you information on how to do it. 

It is important that you have some protective gear to do the work with kilns. At first and for fusing temperatures, you need eye protection and gloves. You need to look frequently and briefly into the kiln to monitor the firings, especially at the start of your career. For this you need eye protection. Sun glasses will not do as you need protection against infrared rather than UV light. There are a number of things that will do from welders' goggles to special lenses as used by bead makers. Use them! Every time. You will need gloves, at the start leather gloves with sleeves going half way up your forearm (such as welders' gloves) will do. Later and for higher temperature work you will need better and much more expensive gloves, sleeves, and body protection.

These things will get you off to a good start.

Brown Ceramic Fibre Paper and Board

A frequent concern about fibre board and fibre paper that appears brown when peeking into the kiln is that something has gone wrong.

Ceramic fibre papers and boards have organic binders that have to burn out during the firing. What starts as white later turns brown as the binders are burning out – often the smell is like burning paper. As the binders burn away, the fibre paper or board returns to its original colour.

The amount of smell or smoke is dependent on the amount of ceramic fibre you have in the kiln. If you have a thick board, you should think of venting the room, as there will be a lot of smoke.

Also if you are firing a large piece of glass, you should think of firing the binders out of the ceramic fibre before placing the glass on top. If you do not, the binders may not fully burn out, leaving marks of the smoke fired into your glass.

Colour “theory”

You will need to decide which colours combine well, whether they are toning, harmonious or complimentary.  By getting to grips with the rules of colour, you can give your work a unity of concept.

Primary colours

Primary colours are three key colours - red, blue and yellow. They cannot be made from any other colour.

Secondary colours

If you mix equal amounts of the primary colours, you get the secondary colours - purple, green and orange.

red + yellow = orange
red + blue = purple
blue + yellow = green

Tertiary colours

If you mix a primary with a secondary colour, in a ratio of 2:1, you get a tertiary colour. red-orange, blue-green etc.

Colour wheel example

Cool versus hot

Look at the colour wheel and you will see the left hand side of the colours are 'warm' or 'hot' and the ones on the right are 'cool' or 'cold'. This is useful when you want to create a mood in a particular room or need to make your space cosier or lighter.

Neutrals

Neutrals are one of the easiest groups of colours, or non-colours to work with. They don't appear on the colour wheel and include Black, Grey, White and sometimes Brown and Beige. They all go together and can be layered and mixed and matched. No neutral colour will try to dominate over another.

Accent colours

An accent colour is a colour used in quite small quantities to lift or to add punch to a colour scheme. An accent colour should be in a complementary colour. It works best if it's a bright, vibrant colour. Accent colours are perfect if you're concerned about using strong colour - simply add a splash of an accent colour. Keep most of your piece in shades and variations of one single harmonious colour. Then pick out just a few objects in an accent colour.

Clashing colours

Using clashing colours is thought to be inappropriate in formal settings. But in other settings they can provide drama, if they are used carefully. If they are of equal tonal strength, you can mix them together. You don't have to stop at two, you can try three or four. But if one is paler or weaker than the rest it will get lost in the overall scheme.

Sticky Labels

There are a number of products to help get the label adhesive off the glass. However you need to make sure you get the product residues off the glass before firing.

A simple and usually effective method is pull label off, use white spirit or turpentine substitutes with a plastic kitchen scrubber or fine brass wire brush to remove the adhesive. Then clean up with alcohol or methylated spirits, followed by good cleaning with your normal cleaning method, as here.

Why do kiln shelves break?

Kiln shelves are made of clay – a very hard clay, sometimes called mullite. So when firing you need to remember that like other ceramic materials it can be heat shocked.

examples of broken shelves

The recommendation is that you put the shelf on supports to keep it above the base of the kiln and allow air to circulate around both the top and bottom of the shelf.

The question remains, why do the shelves break. There are at least two reasons: physical impact and thermal shock. It is possible to knock the shelf while moving it around the studio. This impact does not always cause a break, but sometimes creates a stress point that later can develop into a crack and break. You can sometimes see the start of the crack from the edge of the shelf. In this case, you can either continue to use the shelf with support under the crack or dispose of it immediately, because at some point during a firing it will separate.

The thermal shock that causes the break occurs because (usually) uneven cooling. It seems the shelves are pretty resistant to rapid heating, but less tolerant of rapid or uneven cooling. In general un-dammed fusing and using moulds elevated a little from the shelve do not create that uneven cooling.

However placing a large refractory mould directly on the shelf can promote cracking either immediately or on subsequent firings.

Broken shelf with casting moulds laded to one side 

The main culprit in any breakage seems to be large or heavy and damp refractory moulds directly on the shelf. The mould is giving off water vapour which cools the immediate area around the mould. So as the temperature rises, the covered part of the shelf stays cool, in addition to being shielded from the general heat of the kiln while the uncovered parts of the shelf rise in heat. At some point the temperature differences in the shelf are too great for its strength to resist. The solution is to remove the shelf from the kiln and place the mould, slightly raised, on the floor of the kiln. The bricks, being softer, do not react in the same way as shelves to uneven heating.

For thick fusing with dams all around, it seems best to do this on a shelf that almost fully covered with glass and dams. This promotes more even heating and cooling of the shelf than having a small part of the shelf covered. It does mean having different sized shelves, but then you may already have some of them due to the breaking of other shelves. Just cut the broken shelf to the size you want on a tile cutter.

Small Glass Balls

You can create tiny balls of glass in several ways. You can cut small squares of glass as small as 5mm, place them in the kiln and fire to at least a rounded tack fuse temperature.

You can do the same with frit. Make your own by smashing the glass within some container, which can be as simple as newspaper. Sieve out the finer and larger glass and put the chosen size into the kiln at the same temperature. This will give more irregular pieces than squares, so you may want to include these pieces of frit in a regular fuse firing in the spare spaces.

You can also make balls individually by putting the piece of glass on a graphite block and heating it up with a blow torch. This will round up even a rectangular piece of glass. You can put these into a fibre blanket, but it really is not necessary as the round form will contain a lot of stress that will later be removed by subsequent firing. It is possible to over heat some glass, especially opalescent, so be careful about the amount of heat you apply.

Cutting thin strips

Cutting thin strips of glass such as used in Mission Style patterns and precision fusing projects requires skill and assistance. For transparent and translucent glass you can arrange a right angle guide on a board and tape a piece of lined notepaper to the jig. Use a cutting square and move it right along the lines on the note paper making four or six scores at a time and then breaking on the last score first and then every other score, and then each one in half.

Another method is to use the edge of the bench as a guide. With a small adjustable carpenter’s square, you hammer in nails at the predetermined width (plus half the thickness of the cutter head). Align the glass to the edge of the bench between the nails. Place a straight edge against the nails and score. This gives strips of the same width every time, but works best with strips of 10mm (3/8”) or more. This is illustrated in the processes section.

The thinner the strips are to be cut, the more important it is to make the scores and then divide the sheet in half - the two halves in half each - the 4 quarters in to halves, etc, until you are down to the piece that only needs to be divided in two.

The thinnest strip that can be cut is a fraction wider than the thickness of the glass.  This is because the glass will always break toward the weakest area.  If the strip is thinner than the glass is thick, it will break within the strip.  The narrowest strip that can practically be cut is at least one or two millimetre wider than the glass is thick.  So, if you have 3mm glass, the narrowest you can cut is 4 or 5 mm.  Four millimetre wide strips can only be cut from really smooth consistent thickness of glass sheets.  It is much more practical with decorative glass to limit the width to twice the thickness of the glass you are cutting.

Slumping Etched Glass

In general slumping will not remove the evidence of etching. There will be very little effect on etching on the bottom even at fire polishing temperatures.

When the etching is on the top side exposed to the radiant heat of the elements, you need to be careful to use the lowest practical temperature for slumping. It is possible to achieve a satin finish to a sandblasted surface at 677ºC with a soak of two hours. It depends on the delicacy of the etching texture as to whether the slumping will affect it much.

The more the glass will need to move during slumping the more distortion will be apparent in the finished piece. This can be minimised by using a low heat for considerable time.

If the mould is very detailed, it would indicate that etching should be done before the slumping due to difficulty in attaching the resist to the shaped glass, unless you paint it on. But again, a significantly long soak will be required to achieve the detail of the mould.

If it is a simple and relatively shallow slump it may be easy to etch after shaping. It is a question of how easy it is to get the resist to conform to the curve.

Keeping Copper Inclusions from Oxidising

The colour change in the copper foil is due to oxidisation - if the copper foil is completely deprived of oxygen it stays shiny and copper coloured. If you leave copper exposed at all it will go metallic blue or even bottle green, mostly it turns a lovely burgundy red colour- an intermediate oxidisation stage.

Klyr fire or borax solutions may help the copper stay bright.

Through doing some experiments with art school students, I have found the speed of firing is critical in an electric kiln. In a gas kiln the speed is normally fast anyway and produces better results than an electric kiln. It also is a kiln with a reducing atmosphere rather than oxidising one of an electric kiln.

Summary:

The main elements in keeping copper inclusions (and by extension, other metals) bright is to keep the metal from oxidising. Two elements are important in this:

• Keep oxygen from the metal
• Reduce the time the metal is exposed to high temperatures

Various methods are used to keep the metal from exposure to oxygen. Some of these involve: 

• coating the metal with fluxes to reduce the amount of oxygen in contact with the metal. 
• using a reducing atmosphere, such as a gas kiln. 
• placing an oxygen hungry material in the kiln with the glass and metal. 
• coating the metal with glass powder before encasing it within the glass.

Reducing the heat exposure of the metal also indicates that firing fast would provide better results. This requires very even heating within the kiln to avoid heat shocking the glass.  This is where a gas kiln is most advantageous - it can be fired fast without breaking the glass and it has a reducing atmosphere within it.

In general, it is easier to make use of the effects of the oxidised metal rather than striving for bright metal inclusions.