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.

Equalising Effects on Both Sides of the Glass in the Same Firing

The desire is to have the same degree of fusing on both sides of the glass.  An example is where a person wants to have their colourline paints equally matured on both sides of the glass in one firing.  This is difficult and requires a different strategy than normal fusing.

Background

A bit of background first. Glass is a very good insulator. This means that heat travels slowly through the glass. Its practical effect is that we have wavy lines on the top and very crisp lines on the bottom.  This results from the temperature differential between the two surfaces.  This can be many degrees different during the plastic phase of the glass.  It is dependent on how fast the temperature rise is.  The faster the rise in temperature, the greater the difference as the glass transmits the heat from top to bottom so slowly.  The problem is how to keep the temperature differential as small as possible.

Heat Work

The concept of heat work relates to the way heat is put into the glass.  It can be done quickly to a high temperature, or slowly to a low temperature and still get the same effect.  This shows glass reacts to the combination of temperature and time. Putting heat into the glass slowly allows lower temperatures to be used to achieve the desired effect, than fast rises in temperature.

The insulating properties of glass means that the heat work needs to be applied slowly to achieve similar temperatures on both sides of the glass.  The thicker the glass the longer it will take to temperature equalisation.

The mass of materials also needs to be considered.  The glass will normally be on a ceramic shelf of 15mm to 19mm.  This mass also needs to heat up to the temperature of the top of the glass.  Until it does, it will draw heat from the glass.  This also points to the need for slow heat input.

The question that prompted this note was how to get glass strainers paints to have the same degree of maturation on both sides at the same time.  The maturation temperature of Reusche tracing paints is around 650°C.  If you use a normal rate of advance – say, 200°C – the bottom of the glass will be considerably cooler than the top.  This is both because of the insulating properties of the glass and the mass of the shelf.

Methods to achieve the effect.

Some methods are worthy of consideration separately or in combination.

Use refractory fibre board as shelf.  This dramatically reduces the mass of the shelf to be heated up.  This kind of shelf requires more care to avoid damage than a ceramic shelf.  It would be possible to place smaller fibre shelves on top of the standard ceramic shelf rather than having one large fibre board shelf.  This will not be so efficient an insulating mass as fibre board on its own.  Also, it will not be sufficient on its own to obtain equal temperatures on both sides of the glass.

Use 3-6mm refractory fibre paper between shelf and glass.  This again reduces the heat sink effect of the ceramic shelf, but not as much as a fibre shelf on its own.  Again, the fibre paper on its own is not enough. The scheduling is important.

Use very slow rates of advance.  A slow rate of advance in temperature is important to achieving equal temperatures throughout the glass.  Even using 3mm glass, the rate of advance might need to be as slow as 50°C per hour.  The corollary of this is that you will not need to use as high a temperature to achieve the effect.  Heat work means that it is not an absolute temperature that will achieve the effect.  The slower you put the heat into the glass the lower temperature required.  The understanding of this relationship will require experimentation to establish the relationship to the rate of advance and the top temperature required.  For example, a satin polish of a sandblasted surface can occur at 650°C, if held there for 90 minutes.

In this case, a 50°C rate of advance will probably not require more than 600°C – and probably less - to achieve the shiny surface normally achieved at 660°C with a 200°C rate of advance.  At 50°C per hour, it will take 12 hours to reach 600°C, although a little more than 3.25 hours at an advance of 200°C to reach 660°C.  The input of heat acts upon the glass throughout the process, making lower working temperatures possible.  The reduction in temperature required is not directly related to the reduction in the rate of advance.  You will have to observe during the experimental phase of this kind of process.

If it was desired to fire enamels that mature at 520°C to 550°C you could put the sheets in vertical racks to allow the heat to get to both sides equally as Jeff Zimmer does.  But this will only work for very low temperatures and for quick firings, otherwise the glass will begin to bend.

There are limits to this strategy of getting upper and lower surfaces to the same temperature, both in terms of physics and practicality.  There are temperatures below which no amount of slow heat input will have a practical effect, for example,  due to the brittle nature of the glass.  Even where it is possible, it can take too long to be practical.  For example, I can bend float glass at 590°C in 20 minutes into a 1/3 cylinder.  I could also bend it at 550°C (just 10°C above the annealing point), but it would take more than 10 hours – not practical.

Fibre Dams

Fibre dams are a good and relatively inexpensive refractory material to form dams around regular and especially irregular shapes.  You need only cut the shape you want from the fibre board, if it is not a shape with straight lines.  

You can fire without any kiln wash or hardening if it is a one-off use.  For shapes you want to keep, you can harden the fibre board. 

Once hardened with colloidal silica, you need to paint the board with a separator – kiln wash, boron nitride or similar.

There are some precautions in the use of fibre paper and board.  The main physical one is that refractory fibre is lighter than glass and so will float on top of “molten” glass – that is fusing compatible glass higher than about 800°C.

Fibre board dams can be weighted with kiln furniture on the surface of the board.  If the board is flat this can be on the surface.  If the board is vertical, weights can be placed at the corners.

In the absence of fibre board, you can use layers of fibre paper.  If you have 6mm fibre paper, you need only one layer for two-layer glass, but remember that to get a bullnosed edge to the glass without needling, the fibre paper should be 3mm less than the final height of the fired piece. Thicker glass will require more than one layer of fibre paper.  Place as many layers of fibre paper as required to be at least equal in height to the finished piece on top of one another.  Push “U” shaped pins into the layers of paper to fasten the layers together.  Then cut the required shape out of all the layers all at one time. 

When finished cutting the shape out, you may want to line the edge with 1mm fibre paper to keep any of the layers of fibre paper showing through.  This dam will not need any kiln wash to prevent the glass sticking to it, unless you want multiple uses and so need to rigidise it with colloidal silica.

You can weight this fibre paper dam down by placing kiln furniture near the edge, all around the shape just as for the fibre board.

Safety in use of refractory fibre is described in Gregorie Glass.

Scroll down to Dusts/Particulates for safety recommendations.

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.

Pot Melt Formers

There are several suppliers of stainless steel and ceramic formers for pot melts.  They are not always necessary.

If you only want a circle, you do not need a former at all.  The shelf must be kiln washed and level.  The glass will pool in a circular manner ranging in thickness – thickest at the centre and 6-7mm at the edge. The variation in thickness depends on the time the glass is kept at the working temperature after the pot has emptied.

If you are wanting a thicker melt, you do need a dam of some sort.  You can purchase what you want, or you can make some from the materials you have at hand.

You can make a rectangle or square melt from existing straight dams.  You need to make sure the dams are kiln washed and lined with 3mm fibre paper.  You do not need to cut the dams to a predetermined length.  Instead, you can arrange them so that one end of the dam starts at the edge of your rectangle.  The next dam is butted at right angles to the first at the length wanted.  The other pieces are fitted similarly, until the last one passes the end of the first, so that they are butted together.  Then line with the fibre paper.  If you feel the dams are too light, you can back them up with bricks to prevent movement.

Using fibre paper, fibre board, or vermiculite board you can make any shape of melt that you can cut out of these materials.  If you don’t have refractory board, you can make your former out of layers of 3mm fibre paper.  It is possible to make a template for cutting of the multiple layers.  Cut your shape from the required number of layers of fibre to be as thick as your pot melt will become, according to your calculations.  Pin these layers together with stainless steel pins to be sure they do not move or float with the glass.  If you like, you can weight the layers of fibre paper with kiln furniture.

If you have refractory board – fibre or vermiculite – you can cut the required shape from them.  If you do not harden the fibre board, you do not need any further separator.  But you can line the shape with a thin fibre paper to ease the release and refine the edge.  Vermiculite always needs a separator, as it sticks to glass.  You should line the vermiculite board to get an easy release from the glass.

Using refractory materials releases you from the restrictions of commercially available forms and allows your imagination to take over.  It may not be cheaper than the bought ones, but will have the greater feeling of achievement.  In addition, you can develop all sorts of forms and depths not thought of by the commercial suppliers.

Fibre board moulds

A publication on moulds from fibre boards is available from Stained Glass Supplies. This gives much more detail than this note can.  However, the basics are outlined here.

Commonly available refractory boards are:

·         Calcium silicate

·         Standard fibre boards

·         Armstrong ceiling tiles

They can be used bare (except Armstrong ceiling tiles) or hardened.

A question that will arise is whether to harden or not.  This depends on the durability you require.  A board that is not hardened does not require kiln wash when fired.  However, as it is soft it is easy to break.  A hardened fibre board mould always requires kiln wash or another separator.  It does become durable and almost rings when tapped once it is hardened and cured.  If the shape needs to be preserved for further use, hardening is advisable.

Working methods

Usually hand tools are all that are required to get the results required.

Safety

Do any work on refractory boards outdoors if possible, and with a respirator.  If you must do it indoors, have good ventilation, wear a respirator, and clean up with damp sponges or other absorbent material to avoid putting the dust back into the air.

Do you need to pre-fire fibre moulds?

Moulds that are small or thin do not erequire firing before using.  Thick and large fibre moulds do need to have the binders burned out before use to avoid carbon marks on the glass.

Carved Fibre Moulds

The question of whether you can use carved moulds more than once will arise.

This refers to moulds made from refractory boards or materials.  Once fired, refractory boards and materials become more fragile as they have lost their binders.  If the carving is simple with lots of support, and the mould is kept supported in a container of some sort, rigidising is not essential.  The life of the mould may be short though.

To make a longer lasting mould, you can rigidise the refractory material using this method.  This can apply to board as well as blanket.  The process will make a much longer lasting mould that is light weight, and is not affected by rapid changes in temperature.

Do the fibre moulds need kiln wash?

This depends on both the nature of the material and whether hardened or not.  Refractory fibre boards – often called ceramic fibre – do not need kiln wash to separate the glass from the mould.  However, putting powdered kiln wash and smoothing it with a piece of glass or plaster’s float can give a less grainy finish.  If applied wet, the dried kiln wash can be gently sanded to give a very smooth surface.

Other refractory boards such as calcium silicate or vermiculite do need kiln wash to separate the glass from the mould.

Any refractory mould which has been hardened with colloidal silica will need to be coated with kiln wash to keep the glass from sticking.  The kiln wash needs to be re-applied each time the mould is used above tack fusing temperatures.  Otherwise it does not need renewal until or unless the kiln wash is chipped, scratched or in other ways damaged. 

Another popular separator is boron nitride.  It is sold under various brand names.  This must be applied each time the mould is used.

Fibre Board Moulds

There are a lot of moulds available in a variety of shapes and sizes through various suppliers.  But sometimes you want a simple or special shape that is not available to buy.  Fibre boards provide you with the material to make your own special moulds without great expense.

The ceramic fibre boards tend to come in a variety of thicknesses, mostly about 1 metre square.  Boards in 10, 15 and 25 mm thicknesses are commonly available. 

You should work in a well-ventilated area using a dust mask to avoid inhaling the dust and fibres.  See Gregorie's Glass for information on safety.

You can cut a wide variety of shapes into fibre board with just a craft knife.  You can smooth the shapes with just sandpaper. The shape can be smoothed with sandpapers in both natural and hardened states.  If a lot of material is taken off hardened shapes, it is advisable to coat that area with hardener and cure it again to ensure maximum durability.

A question that will arise is whether to harden or not.  This depends on the durability you require.  A board that is not hardened does not require kiln wash when fired.  However, as it is soft it is easy to break.  A hardened fibre board mould always requires kiln wash or other separator.  It does become durable and almost rings when tapped once it is hardened and cured.  If the shape needs to be preserved for further use, hardening is advisable.

It is also possible to stack the boards to make deeper forms.  The boards should be stacked and pined together with copper of high temperature wire such as kanthal to keep them from moving both while shaping and in use.

Fibre boards are relatively inexpensive, in comparison with commercial moulds.  They are not as durable as some, but provide a means of obtaining special shapes unique to your work.

Texture Moulds

Texture moulds are popular but expensive and with limited designs. You can make your own unique ones with only a few items of equipment. 

An example made by someone else

Clay

Various forms of clay can be used. Roll out a flat piece using boards of 8-10mm thick to support the rolling pin and give consistent thickness to the clay. The board underneath should be covered in paper or cloth to make an easy release. I have found that grease-proof paper as used in baking works very well.  It releases easily from the clay.  

Paper clay provides light weight moulds that do not hold a lot of heat, but any standard clay will do the job. There are two approaches to developing the pattern. You can stamp the pattern into the wet clay with any prepared design on a stamp or other textured material. The other is to dry the clay to leather hardness. You can then incise the pattern you desire directly into the clay. Fire to bisque temperatures, and sand to remove any rough areas or undercuts. Kiln washing the mould before use is essential.

Using a patterned roller to impress the design on the clay

Fibre board

Various fibre boards can be used. Ceramic fibre board, Kaiser Lee board, Vermiculite board, insulating ceiling tiles such as Armstrong, and calcium silicate boards can be incised and marked as desired. The advantage to the ceramic fibre, insulating ceiling tiles, and Kaiser Lee boards is that they allow air to pass through the material. Kaiser Lee board of these three provides the easiest surface for incising. Calcium silicate has no fibres, but requires a separator. Ceramic fibre and Armstrong ceiling tiles have fibres, requiring a bit more work to get a smooth surface. Armstrong tiles require a separator, but ceramic fibre boards do not unless you harden them for durability.

Fibre paper

Three millimetre fibre paper gives a easy material for cutting with craft knives or scissors to the design wanted. You can draw through an existing stencil or copy the design with carbon paper. It is not easy to produce designs with lots of detail.  It is quick, does not require separators, but is probably a single use material, unless you use mould hardener and then kiln wash, although it still will be delicate. For large projects, the paper should be fired first to ensure the combustion of the binders do not produce gasses to cause bubbles or fogging.

Loose Material

Sand, whiting, and kiln wash provide easy materials for one-off designs. You can quickly draw the design you want into the flat loose material with your fingers, or tools. You can also use found items to press into the loose material. Place the glass gently on top of the material and fire. If you use sand, you should dust it with kiln wash or aluminium hydrate to ensure the sand does not stick to the glass.

Unique Designs

All of these methods will provide unique designs which will distinguish your work from others.