Mould Elevation

 

The expansion characteristics of glass and ceramics components

Many people advocate the elevation of moulds.  Mainly for air flow to equalise temperatures above and below the mould.  But also, to prolong the life of the mould.  My observation on these reasons for elevating the mould are that they are not harmful, but not necessary, except for investment moulds.

My experiments have showed insignificant differences in temperatures above and below whether elevated or not.  Since the air temperature under the mould is much the same whether elevated or not, it indicates that elevation of the mould has no significant effect.  But, of course, elevation of the mould does no harm either. 

More important than elevation of the mould, is consideration of the nature of the ceramic mould.  Ceramics have two expansion/contraction temperatures called inversions.  The first is at 226˚C/439˚F, and the second around 570˚C/1058˚F.  The ceramic expands rapidly at these temperatures.  There is a 2.5% increase in volume at 226˚C and a slightly more gradual 1% increase around 570˚C.

This a main reason to use slow ramp rates up to at least 570˚C/1058˚F.  Slower rates ease the ceramic expansion speed and reduces the risk of breaking.  So, slower rates will lengthen the life of ceramic moulds. The cool down for annealing and cooling is slow enough that it presents no risk for the ceramic.

There are occasions when the mould must be elevated, though.  These are when the mould is large, heavy, or damp.  This is to protect the shelf rather than the mould or glass.

 

Stainless Steel Stringer Pots

Credit: Paul Gardner httpswww.facebook.com

 It is a consideration in stringer and murrini work that the pot be re-usable. This has led to the development of stainless steel square pots.  The thorough cleaning of these is difficult even with a lot of banging. Containers with removal bases have been developed as a result.

 The importance of a container with an integrated bottom is to ensure the glass is contained within the pot. To be reusable, the pot can be lined with fibre on sides and bottom. However, fibres can be drawn from the lining into the stream of glass.

Credit: Paul Gardner

 If you have a stainless-steel square with a removable bottom, the pot can be cleaned more easily and does not need the fibre lining. It also allows easy switching of bases with different hole sizes and shapes.

 However, some people have had the difficulty of the glass flowing out between the sides and bottom of the pot and onto the floor of the kiln. Glass is heavy and can float the much lighter stainless steel off the base, allowing the glass to flow sideways as well as through the hole in the base.

 This indicates that the stainless steel square should be weighted down. Placing kiln furniture on top of the pot can avoid it being floated off the base piece. These can be dams made from kiln shelves, dense fire brick, a small shelf, ceramic tiles, or other kiln furniture. Putting the furniture on two opposing corners will be enough to counteract the floating of the pot and still allow radiant heat to reach the glass.

Pots can be made from refractory materials too, such as vermiculite.

Liners for pots

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.

Vitrigraph Pot Liners

Stainless steel vitrigraph pots are durable replacements for ceramic pots that do not last many -  if more than one - firings. But cleaning is not straight forward. Most recommendations seem to concentrate on cleaning by banging the metal to break the glass away from the sides and bottom.  This seems more brutal and noisier than necessary. It will eventually dent the metal and possibly become unusable.

An unlined ceramic pot

An alternative is to line the sides with 1mm or 2mm fibre paper.  Paper of this thickness has enough fibres that the paper will stick together and not contaminate the pull.  It will still protect the metal from glass sticking At the conclusion of the firing and after the cool down, you can remove the fibre paper and have clean sides.

Instead of placing the glass in a bare pot, you can  line the pot with fibre paper 

 

It is possible to put a piece of fibre paper on bottom with a hole in it to match the pot’s hole.  There is a slight risk of drawing fibres into the pull, although I have not experienced it yet.

This method also works with ceramic pots.