Damming for Exact Shapes

 Many times, exact dimensions of the final piece are not critical.  When they are and the piece is 9mm and thicker, or has irregular amounts of glass near the edge, damming is required.

 If the dimensions are rectangular, you can use straight edged refractory materials, usually sawn up broken kiln shelves, vermiculite, or fibre board strips.  

 These need to be kiln washed and lined with fibre paper.  The dams should be lined with 3mm fibre paper that is 3mm narrower than the final height of the piece.  This allows a bullnose shape at the edge to form.

 If the shape is a circular or irregular shape the dams can be made from thick fibre board or vermiculite.  The lining of the dams is the same as for rectangular shapes.  

 The use of 3mm fibre paper means that you have to make rectangular shapes 6mm bigger in each direction to achieve the exact final dimensions.  For circular or irregular shapes, the edge will need to be only 3mm larger.  This is because the edge goes around the whole shape, rather than only one side.

 

Vitrigraph Pots from Refractories

Many people are now buying or having made stainless steel square pots for doing stringer and murinni pulls.  This may be the best material for the purpose, but square pots can be made from other refractory materials.

Vermiculite

One material that can be used multiple times is 25mm vermuculite board.  These can be cut to a convenient size and fastened together with stainless steel or brass screws.

If you want to make a pot 100mm square and 100mm high, cut four boards 125mm square, and one 100mm square for the bottom.  This last one will need to have a hole of the desired size drilled at its centre.  A 19mm diameter hole is a medium sized hole.  Keep in mind that you cannot make it smaller, but you can make it bigger. 

Place the four larger squares around the base.  Drill pilot holes for the screws (if you don’t you will split the boards).  Two or three holes along each edge should be as much as needed for long term security. 

The screws at the left side of the box are omitted in the drawing, but are required

Alternatively, you can make the base to fit onto the bottom rather than inside.  In this case, cut the base to 150mm square and fix it to the sides with stainless steel screws from the bottom.

The screws at the left side of the box are omitted in the drawing, but are required

A disadvantage of the vermiculite is that glass sticks to it.  You can overcome this by lining the bottom and inside of the square with 2mm or 3mm fibre paper.  You cannot hammer out the residual glass without destroying the whole box. This lining will protect the surfaces, and the fibre can be removed after a firing, leaving clean sides.  This will not be as long lasting as the stainless steel pot is, but it is economical and adaptable to your specific requirements.

Refractory fibre

Another refractory material that can be used to construct square pots is 25mm refractory fibre board.  The sizes of the components are the same as for vermiculite.  This time rather than screws, you need stainless steel pins – 50mm long sewing pins are suitable.  You can also use high temperature wire, about 50mm long with a small right-angle hook at one end.

Assemble the four sides around the bottom as previously, and push the pins into the board to secure them.  You will need more pins than you did screws for the vermiculite.

As an additional securing measure, wrap the box horizontally with two bands of 0.5mm kanthal or high temperature wire and twist the ends together.  Then on two sides wrap more of the high temperature wire under the bottom and twist the ends together on the top of the two sides.  These wires do not have to be really tight.  They are there to prevent any failure of the pins.

This refractory fibre box is light weight although it will not last as long as the vermiculite one, but it is quick and easy to put together with a minimum of tools – knife, wire cutter, straight edge.  You can line this with fibre paper as for the vermiculite.

Safety note:

When working with vermiculite or refractory fibre, you should wear breathing protection and dust your clothing outside or change after the box is complete and put those clothes in the washing machine.

You can make a vitrigraph box from refractory materials rather than buying a stainless steel one.  Information on making one is given.

Materials for making dams

Rectangular or straight sided shapes

Broken shelves

Accidents happen to mullite shelves causing breaks or cracks.  Rather than throwing them out, you can cut them into rectangles or 50mm strips with a tile saw.  The resulting shapes need to be kiln washed to keep glass from sticking.  They can be used flat or stood on their edges with supports on the outside.

Thick ceramic tiles can be used in much the same way.  You do need to remove the glaze from the tile to make sure they don’t stick to the glass.  Or you could use the unglazed side toward the glass. Again, the tiles need to be kiln washed.

Stainless steel can be used as a dam.  It will need treatment with a separator such as boron nitride or kiln wash.  In addition, it needs to be lined with refractory fibre paper to cushion the force of the greater contraction of steel than glass.

These materials cannot easily be adjusted in length to fit the size of the glass piece being dammed.  Instead, arrange them in a swastika like formation. 

This photo also shows how shorter lengths can be incorporated to make the whole dam.

Vermiculite board is a refractory material that can be used to form dams by cutting with a wood working saw.  The saw you use to cut the vermiculite will be dulled and only be useful for cutting vermiculite in the future.  Do not use any expensive cutting equipment!  

Credit: Bullseye Glass Company

Refractory fibre board is available in many thicknesses.  It can be cut with craft knives even though it dulls the blades quickly.  The thicker boards can be used without rigidising.  This avoids the need to kiln wash and allows adjustments in length.  If you do rigidise fibre board, you must coat it with a separator such as kiln wash or boron nitride.

Weighted fibre paper can be used.  It is sometimes the quickest and easiest to use, as there normally is a stash of scraps around the studio.  It is easily cut with a craft knife.  You can build up the thickness of the dam by layering pieces on top of one another.  Sometimes people put metal wire or pins in the layers to ensure there is no movement between the layers. I’ve found that if weighted, the fibres interlock enough that the layers do not shift.  But you need to line the layered fibre paper dams with vertical strips of fibre paper, so the glass does not take up the layered dam profile on its edge.

Note that you need to use breathing protection when cutting all these materials.

Curved and circular pieces

Many times, the shape to be dammed is not formed of straight lines.  Different materials need to be used in these cases.

Formed stainless steel is a good durable and reusable material.  You need to line the shape with fibre paper if it completely contains the shape, because it contracts more than the glass and can crush the piece.  It is expensive to have made and so needs to have multiple uses to justify the cost.  A cheaper alternative is to make your own shape using stainless steel strapping as used for shipping crates and pallets. 

Fibre paper is an excellent material for damming irregular shapes.  It can be cut into complicated shapes, and it can be layered to attain the required height. You can weight it if you are taking things to a high temperature and fear that the glass will flow under the fibre paper.

You can also use the thicker fibre papers upright by backing up with multiple pieces of kiln furniture to maintain the shape you desire.

Vermiculite board is a good material for making shapes, although not as complicated ones as possible in fibre paper.  Vermiculite can be shaped with wood working materials, but cheap ones should be used as they are quickly dulled. You can rough out a shape with a jigsaw and refine it with various wood working tools, including coarse sandpaper.  Because it is a relatively rigid material, a lot of inventiveness can be used in forming the edges by altering angles from the vertical, incising designs into the edge, etc.  Be certain that you have adequately kiln washed or put other separator on the board, as it will stick to the glass if left bare.

Fibre board is a less rigid material than vermiculite, but is easier to work with simple craft tools.  It is simple to use for a unique one-off shape. It only needs smoothing and does not have to have a separator applied because it does not stick to the glass.  If you create a shape that you will want re-use, you can rigidise the board after shaping, but it will require separators then.

Note that when working with refractory materials, you need to wear respiratory protection and clean surfaces with a HEPA vacuum or by dampening dusty surfaces and wiping them clean.  Dispose of cleaning materials safely.

Effects of Dams on Scheduling

 I recently made a statement about the effects of various dam materials on the scheduling.  This was based on my understanding of the density of three common refractory materials used in kilnforming – ceramic shelves, vermiculite board and fibre board.  I decided to test these statements.  I found I was wrong.

I set up a test of the heat gain and loss of the three materials.  This was done without any glass involved to eliminate the influence of the glass on the behaviour of the dams.  The dam materials were laid on the kiln shelf with thermocouples between.  These were connected to a data logger to record the temperatures.

 

The schedule used was a slightly modified one for 6mm:

300°C/hr to 800°C for 10 minutes

Full to 482°C for 60 minutes

83°C to 427, no soak

150°C to 370°C, no soak

400°C to 100°C, end

 

The data retrieved from the data recording is shown by the following graphs.

 

Highlights:

·        The dam materials all perform similarly. 

·        This graph shows the dams have significant differences from the air temperature – up to 190°C – during the first ramp of 300°C/hr. (in this case). 

·        There is the curious fall in the dams’ temperatures during the anneal soak.  This was replicated in additional tests.  I do not currently know the reasons for this.

·        The dams remain cooler than the air temperature until midway during the second cool when (in this kiln) the natural cooling rate takes over.

·        From the second cool to the finish, the dams remain hotter than the air temperature.

 

Some more information is given by looking at the temperature differentials (ΔT) between the materials and the air.  This graph is to assist in investigating how significantly different the materials are. 

This graph is initially confusing as positive numbers indicate the temperature is cooler than the material being compared and hotter with negative numbers.

 

As an assistance to relating the ΔT to the air temperature some relevant data points are given.  The data points relate to the numbers running along the bottom of the graph.

Data Point   Event

    1                Start of anneal soak.

    30              Start of 1^st cool (482°C)

    45              Start of 2^nd cool (427°C)

    65              Start of final cool (370°C)

    89              1^st 55°C of final cool (315°C)

    306             100°C

 

At the data points:

·        At the start of anneal soak the ΔT between the dams is 16°C with the ceramic shelf temperature being 18°C hotter than the air.

·        At the end of the anneal soak of an hour, the air temperature is 20°C higher, although the ΔT between the dams has reduced to 12°C.

·        At the end of the 1^st cool the ΔT between the dams has reduced to 9°C and the ΔT with the air is 3°C.

·        At approximately 450°C the air temperature becomes less than the dams. 

·        At 370°C the hottest dams are approximately 17°C hotter than the air.  The ΔT between the dams is 10°C.

 

More generally:

·        The air temperature tends to be between 17°C hotter and 17°C cooler than the ceramic dams during the anneal soak and cool.  The difference gradually decreases to around 8°C at about 120°C.

·        Ceramic and fibre dams loose heat after annealing at similar rates – generally having a ΔT between 4°C and 1°C, with a peak difference of 9°C at the start of the second cool. This means the heat retention characteristics of ceramic strips and fibre board are very close.

·        Between the annealing soak and about 300°C the vermiculite is between 12°C and 9°C hotter than the same thickness of fibre.  Vermiculite both gains and loses heat more slowly than the ceramic or fibre dams do.  This means that vermiculite is the most heat retentive of the three materials.

Conclusions

·        Dams will have little effect during the heat up of open face dammed glass.  The slight difference will be at the interface of the glass and the dams where there will be a slight cooling effect on the glass.  Therefore, a slightly longer top soak or a slightly higher top temperature may be useful.

·        The continued fall in the dams’ temperature during the anneal soak indicates that this soak should be extended to ensure heat is not being drained from the glass by the dams to give unequal temperatures across the glass with the risk of inadequate annealing.  I suggest the soak should be extended to that for glass of 6mm thicker than actual to account for this.

·        The ability of ceramic and fibre dams to absorb and dissipate heat more quickly indicates that they are better materials for dams than vermiculite board.  The slightly better retention of heat at the annealing soak, indicates that ceramic is a good choice when annealing is critical.

Scheduling Effects 

Based on these observations, I have come to some conclusions about the effect of dams on scheduling.

·        There is no significant effect caused by dams during the heat up, so scheduling of the heat up can be as for the thickness of the glass.

·        The lag in temperature rise by the dams indicates a slightly longer soak at the top temperature (with a minor risk of devitrification), or a higher temperature of, say 10°C can be used.

·        The (strange) continued cooling of the dams during the annealing soak indicates that extending the soak time to that for a piece 6mm thicker than actual is advisable.

·        The cool rates can continue to be as for the actual thickness, as the dam temperatures follow the air temperature with little deviation below the end of the first cool. 

·        Ceramic dams perform the best of the three tested materials.

 

Pot Melt Saucers as Dams for Melts

Preparation

Many ceramic plant pot saucers can be used as circular moulds.  Most are unglazed and will accept kiln wash easily.  Some are unglazed, but polished to such an extent they are no longer porous.  These and glazed flower pot saucers need some preparation before applying kiln wash.

Plant pot with saucer

Polished and glazed saucers require roughing to provide a key for the kiln wash solution to settle into.  This can be done with normal wood working sand papers.  You may want to wear a dust mask during this process, but not a lot of dust is created.  You could also use wet and dry sandpaper or diamond handpads with some water to reduce the dust further.

If the sanding of the surface does not allow the kiln wash to adhere to the saucer, you can heat it.  Soak it at about 125C for 15 minutes before removing it from the kiln to get the heat distributed throughout the ceramic body.  One advantage to the ceramic is that it holds the heat, because of its mass, for longer than steel.  Apply kiln wash with a brush or spray it onto the warm saucer.  As it dries, apply another layer of kiln wash.  Two or three applications should be enough to completely cover the surface.  If not, then you probably will need to heat up again before repeating the process.

Alternatives to plant pot saucers

There are alternatives to the saucer approach to getting thick circles from a pot melt.

 

Fibre paper

You can cut a circle from fibre paper and melt into that.  The advantage of fibre paper is that it requires little preparation other than cutting and fixing.  You may have only 3mm fibre paper and want a 9mm thick disc.  Simply fix the required number of layers together with the circle cut from each square.  The fixing can be as simple as sewing pins, copper wire, or high temperature wire.  Then place some kiln furniture on top of the surrounding fibre paper to keep it in place on the shelf during the melt.  This furniture can often be the supports for the melt.

Fibre board

If you find cutting multiple circles of the same size a nuisance, you can use fibre board.  Simply cut the circle from the board with a craft knife.  You will probably want to line the circle with fibre paper, as the cut edge of fibre board can be rough.  Alternatively, you can lightly sand the edge.  Wear a dust mask and do this outside, if possible, to keep the irritating fibres away from the studio. If you want a thicker melt than one layer of board can give, just add another in the same way as for fibre paper.

In both these cases, you may wish to put down a layer of 1mm fibre paper to ensure the glass does not stick to the shelf and does not require sandblasting.  

The advantage of the fibre paper or board alternative to flower pot saucers is that you do not need to kiln wash anything unless you want to. If you do not harden the fibre paper or board, it will not stick to the glass.

Vermiculite board

Another alternative is vermiculite board.  The advantage of this is that it comes in 25 and 50 mm thicknesses, so you can make the melt as thick as you like without having to add layers.  You can cut the vermiculite board with wood working tools.  Knives will not be strong enough to cut through the vermiculite board. You will need to kiln wash or line the vermiculite with fibre paper, as the board will stick to the glass without a separator.

Damless circles

Of course, if you want a circle without concern over the thickness, you can do the melt without any dams. You need to ensure that the shelf is level.  Any supports for the pot will need to be both kiln washed and far away enough that the moving glass does not touch the supports and distort the circle.  In general, one kilogramme of glass will give a 300mm circle, so your supports need to be further apart than the calculated diameter of the circle.  An undammed circle will vary from 6mm at the edge to as much as 12mm at the centre, depending on temperatures and lengths of soaks.

Damming Ovals

There are various ways of damming oval shapes in kiln forming. Some of these are outlined here.

One set of methods depends on having a soft surface such as ceramic Fibre board or vermiculite.

Photo from Clearwater Studio

You can wrap your shape with fibre paper. For this you need to cut a strip or strips 3mm narrower than the height of the piece you are wrapping. You then stick sewing pins down through the fibre paper and into the shelf of fibre board or vermiculite. This will be easiest if you use 1 to 3mm thick fibre paper, as the pins must not contact the glass – the pins will stick to the glass if they do.

You can cut a form out of ceramic fibre board and use that as a dam. You can pin this to the base fibre board or allow it to merely rest on the board. It is possible to cut arcs from fibre board and place them around in sections. In this case they will need to be pinned together so they do not move apart. Staples can form the attachments. You can make your own – larger – ones from copper wire.

You can buy stainless steel banding which needs to be lined with any separator – batt wash or fibre paper.

Bonny Doon stainless steel dams

You also can layer fibre paper up to the height required – remember 3mm less than the thickness of the piece. You then need to fasten the layers together to avoid movement between the layers.


If you are firing on ceramic kiln shelves the same materials can be used but need to be supported a little differently.

If you are wrapping the piece on mullite shelves, use some pieces of kiln furniture to block the strips up against the glass. The thicker the glass, the more weight will be pushing out against the dams and the sturdier the dams will need to be. Make sure the strips contact the shelf evenly- if you have gaps, you'll have leaks.

The disadvantage to this method is that the glass can take up the irregularities of the kiln furniture.

You can use fibre board with a void cut out to the shape required and place it on the shelf.

You can also use layers of fiber paper around the shape and pin the layers to each other. This is the same method as used on ceramic fibre board.

Again stainless steel can be used to form the dam. Remember to line the steel with fibre paper that is 3mm narrower than the height of the piece.



In all these cases of dammed forms, the edges will be of varying degrees of roughness and some cold working will be required.

Formers

This post is not about the materials that go into the making of glass, but about ways of forming glass once melted or dripped into a space.

Formers are a bit different from moulds.  They are more like the formers used in concrete structures – they are there to resist the movement of the contained materials and give the form or shape desired rather than a natural flow.

These formers can be of anything that can resist the firing temperatures of the process.  Some of the materials are stainless steel, ceramics, fibre board and paper, vermiculite, kiln brick, and I am sure there are others.

Refractory Fibre

Most of these require a separator between themselves and the glass.  The ones which do not are untreated refractory fibre board and fibre paper. 

Most paper is not sufficiently strong to stand on its own. Instead it is used flat and the shape cut out of it.  It can be made in several layers and pinned together to achieve the height desired.  It should be lined in the interior with a thin fibre paper to avoid seeing the layers of the former in the edge of the glass.

For thicker work, fibre board can be used with the shape or form cut from it. Alternatively, it can be used on its side backed up by kiln brick or other material to resist movement. More information on methods and safety are here. 

If hardened, refractory board and paper will need separators between glass and former, just as most other materials will.

Sometimes the fibre board and fibre paper are not heavy enough to resist the flow of the glass.  You can use weights to help resist the movement.  At other times, the glass flows under the fibre and then you need something heavier.  Fortunately, there are a number of refractory materials that can be used.

Other common formers

Vermiculite board is another refractory material that can be cut and shaped much like fibre board.  The vermiculite needs to be covered with kiln wash where it might come into contact with glass or be lined with fibre paper or another separator.

Calcium silicate board can be used in much the same way.  It also needs a separator but does not stand up to such high temperatures as vermiculite.

Ceramics, especially in the form of cut up kiln shelves can be used as straight formers.  They have the advantage, over refractory fibre paper and boards, vermiculite and calcium silicate, of being heavy.  They can resist the movement of thick glass. They need to have a separator and usually a 3mm fibre paper, cut 3mm shorter than the final thickness of the piece, will provide the cushion in the movement that the glass needs.

Kiln brick is an often forgotten former.  The bricks can be cut and formed in many ways, even if not so freely as fibre board and paper.  The bricks do need fibre paper separators to keep the glass from getting into the pores of the brick.

Stainless steel is a common former too.  These are usually formed into an already determined shape and so are not so adaptable as many of the other formers.  Steel contracts much more than glass and needs a cushion of fibre paper, usually 3mm thick to avoid sticking to the glass.

More information on most of these formers can be found here.

Repairs to a Vermiculite Mould

Occasionally, during the demoulding of a form, the mould will break.  Not all is lost.  It can be repaired. 

In this example, the mould is not yet fully cured and is damp.  But this can be applied to fully cured and dried moulds too. Notes will be included where the practice varies for the dried mould.

The first stage is to make up a paste of the ciment fondue for the edge to edge repair.  This should be the consistency of pancake batter or slightly wetter.  The mixed cement is shown at the top of the picture in a small plastic tub.

Wet the edges of the mould pieces thoroughly.  This is to prevent the mould from sucking too much water from the cement, which would give a weak adhesion.  On dried moulds, you may have to do this several times to thoroughly wet the mould and the broken piece.

Then begin applying the wet cement thinly to all the edges.  Do not put it on thickly, as you want the pieces to fit back together smoothly. 

Place the pieces together with gentle pressure. 

Then begin to smooth the wet ciment fondue into the cracks between the broken pieces and the main body.  Be careful to smooth the ciment fondu immediately, as it is very difficult to change once cured.

Continue to work the ciment fondue into any cracks that appear as the mould is wetted.

Make sure the cement is smoothed into the cracks so there are no proud areas above or around the cracks.

This photo shows the smoothed ciment fondu on the interior.

Continue smoothing the cement into the cracks at the edges.

Fill the cracks from the outside also

When the application of the cement is completed, make up a mixture of 1:4 ciment fondue to vermiculite. 

The purpose of this is to strengthen the mould in the weak area.  It is not wise to rely entirely on the strength of the edge bonding of the ciment fondue.

You will need to estimate the total volume required, but it is better to mix too much rather than too little.  Make this mix a little wetter than for the original mould.  Water should not be standing in the mix, but you will be able to squeeze water from the ball of mix easily. 

This is especially important for moulds which have already been cured.  You should also put water on the surface that you are going to back up.

It is important to put a water proof material on the workbench to avoid the mould sticking to the bench, or water dripping over other things.

Having wetted the mould exterior again, begin applying the mix to the outside of the mould.

Continue building up the mixture in thin layers.  This allows the best adhesion of the material to the mould and to each layer.  It is easier to compact a small amount of material than a large amount all at one time.

In this photo, you see some of the water being forced out of the mixture by the compaction of the mix onto the mould.

Continue building around the broken area until you have applied sufficient material to the mould to strengthen it.

When you have finished, one area of the mould may be a little larger than the rest.  This is not a problem in its use, as it does not thermal shock, and it does not keep one part of the glass hotter than the glass touching the rest of the mould.

You can now loosely wrap the water proof material around the mould.  Do not seal it completely.  Place the mould in a plastic bag to cure for a day or more, just as for the original mould.

You can then unwrap the mould and fire it to cure it just as the original. The method for curing vermiculite moulds is given here.