Storing Moulds

Requirements

• protection – some form of container is required to maintain the life of the mould. A container also prevents the kiln wash or other separator from being rubbed off.
• ease of access – It should be easy to get to the moulds, especially as some can be heavy.
• flat vs vertical – some like to store moulds in boxes vertically rather than horizontal with them stacked one above another.
• In either case you need shelves of the right depth and height to store and support the moulds.
• custom made vs bought in – Moulds you have purchased normally come in their own boxes which can be retained for storage for a number of years. You can also make light weight boxes either from old boxes or card cut to size and taped and glued together. More robust boxes can be made from thin plywood or thin pressed board.

Quarry Glazing

For some reason that I have never learned, the regularly shaped pieces of glass that are leaded into a window are called quarries.  The derivation of the word relating to excavation comes from its middle English variant of medieval Latin quareria, from Old French quarriere, based on Latin quadrum 'a square'.   How it came to be applied to pieces of glass is unknown apparently.  However, in stained glass, "quarries" can relate to rectangles, diamonds, circles, etc., not only squares

These kinds of glazing are weak as there are long straight lines created with these designs. The best way of counteracting this weakness is to “weave” the lead joints within the panel combined with more frequent than usual glazing bars.

It may be wise to use a glazing bar at every second set of joints rather than merely consider the distance between bars.

Soldering corroded metals

Some investigation has shown that zinc corrodes more easily than lead (lead being a medium anodic and zinc a high anodic metal). Bronze, Copper and Brass are among the least easily corroded materials other than precious metals. Lead comes immediately after copper in the ease of corrosion.

The implication is that more easily corroded materials need more aggressive/abrasive cleaning before soldering and that their longevity in the weather will be shorter than the less easily corroded metals.

Slumping Tack Fused Stringers

With a piece made up of tack fused stringers it is important to anneal carefully. There are a multiplicity of small contact points where the glass is little more than laminated. This means that each piece of glass will try to contract into itself rather than acting as a single unit. An annealing soak and cool should be chosen to be several times longer than its nominal thickness.

Slumping of this piece will also need to be done carefully. First, to ensure all the contact points can relax as soon as the thinner parts of the stringers. Second, the weight of the piece is not as great as one of the same size and so will not fall into the mould in the same way – it will take longer. Third, the movement of the glass will be within the fragile stringers so it is important to take the heat up slowly so they can bend rather than fracture . So, fourthly, it is important to establish a schedule that will have a slow rate of advance to the lowest possible temperature with a long soak for a slump of the required depth.

Rods in Glass

Inserting large rods into glass

Normally large diameter rods are incorporated into a glass piece for display supports. However, rods over about 2mm will break the glass when incorporated in the fusing. This means that rods to support the glass that will be of sufficient diameter for the strength required cannot be fused into the glass.

You can use a rod of the same - or slightly smaller - size if you wrap it with thin fibre paper to cushion the differentials in expansion and contraction. When the glass has cooled, pull the rod out and clean out remaining fibre paper with water and a pick. 

Alternatively, you can use fibre paper to create a void of similar size to the wire or other metal support that will be inserted.

If you find that you do not like the hump that the rod or fibre paper create, you can use a third layer between the top and bottom.  Cut the glass to leave a channel of the length you want.  This means that the intermediate glass will need to be in at least two vertical parts, with half the channel width taken out of each side.  You can cut the glass in three parts horizontally.  The main piece is cut to a length minus the depth of the channel.  Then two strips are added to the bottom which leave the desired width. Finally, put the top layer on.  

Alternatively, the middle layer can be the design, with the bottom and top being clear or any other colour desired.

Once fired, clean out the channel and glue the rods into the glass. Silicone is a good all purpose adhesive that will allow a bit of movement.

Slumping Single Layers

Slumping small single-layer pieces

A different set of conditions are formed when slumping small pieces. It takes more time or heat to slump small pieces, because there is less weight to pull the glass down into the mould. Because there is less weight in a single layer than in a two layer fused piece, more time is required for the glass to conform to the mould. The exact amount of time and heat will have to be observed for each kiln and kind of layup.

The alternative strategy is to increase the heat. This can work, but leaves more mould marks on the finished piece. The increased temperature causes the forming to occur much more rapidly and may lead – if not closely observed – to over stretching or “bubbles” at the bottom of the mould.

If you are not concerned about the exact nature of the shape of the piece you can fire pretty fast and high. This will produce a curve, but probably not good conformation to the mould. Even if there is good conformation, there will be significant marking of the back of the glass.

If you want a relatively unmarked and slumped piece, you need to change strategy. You can take the glass up in temperature rapidly to ca. 600C and then use about 40C/hour to 677C. Begin watching from about 635C. This slower advance in the slumping range gives better control of the slump. Slower rates of advance give less distortion and less movement within the mould. It also provides a less marked piece, because the slumping is done at a relatively low temperature.

When the piece achieves the shape you want, record the temperature and advance to the next – cooling - segment. The recording of the temperature will give you information for future slumps of this nature.

On any slump that is new to you, either in terms of the mould or the thickness and size of the glass, you need to make records to help determine the optimum combination of conditions. You should of course, record each firing anyway.

You will find more information on factors affecting slumping at these locations:

Weight/thickness

Mould sizes

Mould shape and detail

Uneven slumps

Firing speeds

Rounded edges on slumped pieces

Often people want to know how to get rounded edges during slumping of a single layer piece, especially when trying out techniques with their spare art glass.

Achieving a rounded edge on a slumped piece is a combination of temperature, thickness, larger top layer and cold working.
Rounding of the edges of a piece of glass occurs at tack fusing temperatures, which are beyond the slumping temperature. It is possible to take the glass to a tack fuse within the mould as long as you are prepared for some consequences.

More mould marks are evident on the bottom of the vessel.
Mould life is reduced.  You get a lot more mould marks on the bottom of the glass because the bottom of the glass is softer than in a standard slump. These marks will be directly related to the surface texture of your mould.

You need to re-coat the mould before the next slumping to avoid the kiln wash sticking to the glass.  Ceramic based moulds last a long time if fired below 680C. But numerous firings at tack fusing and higher temperatures increase the possibilities of glass sticking to the mould and occasionally, thermal shock. If you insist on tack fusing in your mould, you need to renew the separator each time, as the kiln wash breaks down at tack fusing temperatures leading to it sticking to the bottom of the next piece you fire in that mould.

You may get an uprising at the bottom, as the glass slowly sinks down the mould and pushes the glass up at the bottom in any mould other than a simple, shallow shape.  

These things indicate that it is best to tack fuse first and then slump at the lower temperature.

Of course the best result can come from using 6 mm of glass, with the top layer 6 mm larger than the bottom layer. This allows the upper layer to sink over the outer edge of the lower one, giving a rounded edge with no sign of any differences between the two layers.

Cold working solution 
If however, you want to work with a single layer, you need to realise that the edge will be the same when it comes out as when it went in. So you need to cold work the edge before slumping. You do not need sophisticated machinery to do this. A few diamond grit hand pads will do the job. Start with one at about 100 grit to shape the edge. Make sure you keep the pad and working surface damp. If you begin to get a white paste appearing, you need more water.

After shaping the edge satisfactorily, take a pad of about half the grit size (twice the number) and begin the smoothing of the scratches created by the shaping. When finished with one grit move on to the next. You can use a paint marker to help tell when one grit is finished. 

This process will give a sheen that will change to shiny during the slumping.
  

  

Mould preparation

The preparation of a mould is more than adding a separator to it, although that is essential.

The glass will always take up the mould texture if it touches. So start with smoothing the mould itself. You can do this by wrapping very fine wet and dry sandpaper round a flexible sanding block. You can use this wet or dry, although wet is more healthy. When you get the desired surface texture you can let the mould dry or immeidately apply kiln wash. When this is dry you can again sand the kiln wash but with dry paper only this time and very lightly. Alternatively you can ball up nylon stockings to provide a soft abrasive action. If you sand through to the mould surface, simply apply the kiln wash again.

You can minimise mould marks by slumping at the lowest temperature which can achieve the slump you want.

If you want to avoid mould marks altogether, do an aperture drop and cut off the rim. There is no mould contact with this technique, so the glass retains the surface it had in fusing.

Moretti Rods

Information on Moretti/Lauscha specifications:

Coefficient of Expansion (C.O.E): 104

Strain Point: 448C

Working Temperature: 926C

Annealing Range: 493 – 498C


Softening Point: 565C

Rods per pound: 15-17

Rod 
Length: Approx 1metre

Rod Diameter:5-6mm

from yglass.com

Mesh Melt Grids

You can construct your own grid rather than relying on the barbecue manufacturers to use high grade stainless steel.

You can buy stainless steel rod and cut it to the lengths you need for your kiln or project. Push them into soft fire brick at each end. Do the same with another pair of fire bricks and put one pair at right angles to the other.

In this way you can vary the intervals to suit yourself for different projects.

You could also put the rods on strips of mullite and weight the ends so the rod does not roll about while assembling them or when firing.

Alternatively, you can buy the grids available for various purposes. Not all are high grade stainless steel, but normally the spalling happens at a temperature that is low enough to avoid incorporation with the glass.