Sand Beds for Kilns

Sand beds can be used for shaping directly into the sand or as a bed for a large kiln to avoid having to place a number of shelves together and patch cracks. It also saves on the purchase of a large shelf, at the expense of some labour before each firing.

Mix (by weight)
The mix is largely sand with powders to coat the sand particles to reduce the amount of sticking to glass. The mix ranges from 75% sand 25% alumina hydrate, up to 67% sand, 33% alumina. The proportions can be adjusted by experience.

Some kiln formers include plaster or china clay (kaolin) in various proportions so the whole mix can range from 70% sand, 25% alumina, 5% plaster/kaolin up to 60% sand, 20% each alumina and plaster/kaolin. Some have found that the plaster started to scum up the glass after a number of firings, and this can happen with china clay too.

Use
The use of sand as the bed of the kiln requires screeding of the sand before each firing. It is possible to smooth the sand with a plaster’s float. This presses the sand down and allows a fine film of powder to coat the surface. A more certain way of avoiding the sand sticking to or texturing the glass is to lay fibre paper over the sand.

You should be aware that a sand bed requires a little different cooling than glass on a suspended shelf does. Effectively you are cooling the glass on the bed of the kiln. Thus, you should use annealing and cooling schedules for one or two thicknesses more than is actually being fired to be really safe.

Sand forming
If you are using the sand for mild shaping, sprinkle alumina hydrate over formed sand to reduce sand particle take up and textures on the glass.

Renew sand regularly if you are using powders on the sand. An annual renewal using half the old sand mix and the rest new will be sufficient in most cases. The effect of too much powder is to promote large bubbles as air cannot move through the sand with the same ease as when there is less powder. An easy way to tell if you have too much powder is that the mix flows ahead as you screed it.

Pot Melt Contamination

Pot melting occurs at temperatures above that for which kiln washes are designed. This means the kiln wash sticks to the back of the melt.

If you have a sandblaster, it is easy to take the kiln wash off leaving a matt surface. You can live with this for many purposes, but if you want a more polished surface you can take the melt up to fire polishing temperature. You will need to flip this over and fire again, if the original top surface is what you want to present.

If you are going to cut the pot melt up for other uses, there is no need to fire polish as the surface does not matter, only the cleanliness.


Another thing to do is to avoid the kiln wash contamination.

If you put only fiber paper – not Thinfire – at the bottom, the dripping glass will tear and move it about. The best solution appears to be to put a disk of glass on top of the fibre paper. It can be clear or any colour you wish. This seems to keep the fiber paper from tearing and being incorporated into the glass.

It also works very well when you are confining the melt to get a thicker disk. Make sure you have kiln washed the sides of the container or dam very well, or it will stick to the sides. You will need to grind the kiln wash off the edge of the disk, but this is much easier than grinding it off the bottom.

Pot Melts – Weight of Glass Required

This table assumes that a 150mm diameter pot is being used, and assumes that 125 grams of glass will be left in the pot. This gives the desired diameter of the melt and the weight of glass needed to achieve a 6mm thick result. To achieve a six millimetre thick disk will require considerable amounts of time at both melting and fusing temperatures to allow the glass to even out in thickness.

50mm diameter disk requires 154 grams of glass
100mm diameter disk requires 243 grams of glass
150mm diameter disk requires 390 grams of glass
200mm diameter disk requires 596 grams of glass
250mm diameter disk requires 861 grams of glass
300mm diameter disk requires 1185 grams of glass
350mm diameter disk requires 1568 grams of glass
400mm diameter disk requires 2015 grams of glass

Of course if you want a thicker pot melt - one that is confined so that it cannot grow larger, only thicker - you can use the following method to estimate the amount of glass required.

Choose the diameter wanted and subtract 125 from the weight of glass required. Then multiply by thickness wanted divided by 6 mm. Add 125gms - the amount that will be retained in the pot - and you have the required amount.

For example: a 200mm disk of 6mm requires 596 gms. You want a 12mm thick disk of 200mm.
First subtract 125 from 596 to get 471gms. 417gms times 12 equals 5652. Divide this by 6mm and you have 942 gms required. Add 125 gms - the amount left in the pot - and you have a requirement of 1067gms for a 12mm thick disk of 200mm.

Pattern Scissors Usage

The purpose of pattern shears/scissors is to cut out the space between pattern pieces equivalent to the came heart or the space needed for foil.

The scissors come in two thicknesses – one for leaded and the thinner for copper foil.

If you must use them:
Use in short cutting motions. Use only the first 50mm of the blades which are closest to the pivot point. Otherwise the paper jams in between the blades. It remains difficult to cut long straight lines without quickly having an “accordion” of paper blocking the cutting action.

Some suggestions to make things easier:
- Clean the blades regularly. If you are cutting anything with adhesives, clean the blades after each use with spirits.
- Often running a little soap along the blades helps to smooth the action of the blades.
- Use stiff high quality paper so you do not catch fibres in the scissors. Waxed paper or stencil card are good materials to use.

Organising pattern pieces.
- You have made a second and third copy of the cartoon haven’t you?
- Now that you have a lot of pieces what do you do with them?
- Mark any grain direction before you cut the pieces apart.

You need to code the pieces in some way.
- Numbering with reference to the main cartoon is most common.
- It is a good idea to colour code the pieces and if the surface will take it, a shading of the colour makes a quick visual reference.

Keeping the pieces together.
- Envelopes are easy to write on for colours, or areas such as borders, background, etc.
- Freezer bags that are transparent and have a band to write on are very good, as you can see the pieces without opening the bag.
- You need a labelled bag or container to keep all the envelopes together.


Alternatives to pattern scissors
- For copper foil, you can use normal scissors, by cutting to the inside of the pencil or inked line. - You can also use a scalpel or craft knife to cut to the insides of the marked lines.
- For leaded glass you can use a felt tip pen (a bullet point is almost exactly the right width when new). Cut with scissors or craft knife at the sides of the line.

Alternative to pattern pieces
Use the European or trace cutting method as described here.

Removing Stick-on Lead and Film

It is possible to do this. It is a labour intensive process. You do need to be careful to avoid scratching the glass.

Cut through a lead line and carefully rip away the lead tape, being careful to not pull so hard that you flex the centre of the glass and cause it to break. You will need considerable force. The bulk of the lead is probably positioned over the film, so bulk of the the glue residue from the lead tape will come off when the film is peeled away. With a spray bottle mist the glass with white spirit and scrub using a cloth. If the glue is especially resistant use a broad wallpaper scraper and cover it with the kerosene soaked cloth to scrape the glue off. Use vinyl or latex gloves.

However, the manufacturer comments that stained glass overlay is virtually impossible to remove. It is better to replace the glass. It will save time, expense and possible tears.

Plating in Copperfoil

Plating is used to modify the colour, or intensity of local areas in a window or panel. Plating for leaded glass is normally putting two pieces of glass in the same came, although there was a common practice at the turn of the 19th into the 20th century to have the plate cover several pieces of leaded glass. In principle, the plating of copper foil panels is the same as for leaded glass, except there is no came to fit the glass into. So there are some variations.

Build the flat, single thickness window first. This provides a solid panel to work on. It also enables you to see whether you really need the plating, and if so the exact areas where it will be applied.

You should solder the whole panel except where the plate is to be soldered. In this/these areas just lightly tin the back, although you will have already put a solder bead over the whole of the front.

Patina the back of the panel, except where the plate is to go. Allow this to dry and clean up any spills, especially in the neighborhood of the plating.

Foil the plate with a backing to match the colour of the patina. So use copper-backed foil where the panel is in copper patina, but black-backed where the patina is black.

Tin the foil on the plate with solder. If the piece is to cross a number of the base pieces, you need to patina the tinned face that will be placed toward the viewer with the same colour patina. You need to make sure this is absolutely dry before proceeding.

Clean the plate and the base glass where the plate is to cover very well. Make sure there are no oils or tarnish on the solder, and that everything is dry.

Solder the plate to every seam that it contacts with no flux and a small amount of solder. This is to insure there is no leakage of flux - by not using any - or solder between the two pieces of glass.

Put a small amount of clear silicone between the edge of the plate and the base glass where you were not able to solder. Just lightly fill the gaps to ensure a seal against moisture and insects.
When the silicone has cured, carefully patina the plate so no fluid seeps between the glasses.

Protect the uneven back when handling by placing a soft foam pad, or a polystyrene sheet with cutouts for the plating, on the back to protect the panel from the carrying board.

Commissioning

Commissioning a stained glass window, screen or lamp involves entering into a contract with the designer/maker. It is therefore important that both client and maker know exactly what is involved.
· The price of the work should be established. The materials used in the making of a window, especially the glass itself, can be expensive and the possibility of commissioning a well-designed leaded light should not be ignored.
· The maker will need to know the budget for the work and will provide an estimate, and may require a down payment before beginning work and perhaps payment by instalments, depending upon the cost of the materials involved.

The designer will prepare a preliminary design, according to the client's brief.
· The design should indicate the nature of the construction and the position of any ferramenta or physical support.
· This design should be as detailed as possible. It may be accompanied by samples of the proposed glasses.
· The client must be prepared to recompense an artist for design(s) prepared according to a brief, whether or not it proceeds to execution.
· The copyright in all cases remains the property of the artist.

The arrangements for the execution of the commission must also be satisfactorily established, including those for installation. If necessary, the advice of an architect should be sought; for church commissions, the architect responsible for the church should be involved from the outset. If the window is to be sited in an exposed position or in an area where vandalism is known to be a problem, protective measures should be considered.

Also look at Commission Agreements

Effect of Plaster-Water Ratio on Some Properties

Plaster-water ratio (by weight) 100/30
Setting time (min) 1.75
Compression strength (kg/sq.cm) 808
Dry Density (kg/cu metre) 1806

Plaster-water ratio (by weight) 100/40
Setting time (min) 3.25
Compression strength (kg/sq.cm)474
Dry Density (kg/cu metre) 1548

Plaster-water ratio (by weight) 100/50
Setting time (min) 5.25
Compression strength (kg/sq.cm)316
Dry Density (kg/cu metre) 1352

Plaster-water ratio (by weight) 100/60
Setting time (min) 7.24
Compression strength (kg/sq.cm)228
Dry Density (kg/cu metre) 1206

Plaster-water ratio (by weight) 100/70
Setting time (min) 8.25
Compression strength (kg/sq.cm)175
Dry Density (kg/cu metre) 1083

Plaster-water ratio (by weight) 100/80
Setting time (min) 10.50
Compression strength (kg/sq.cm)126
Dry Density (kg/cu metre) 990

Plaster-water ratio (by weight) 100/90
Setting time (min) 12.00
Compression strength (kg/sq.cm)98
Dry Density (kg/cu metre) 908

Plaster-water ratio (by weight) 100/100
Setting time (min) 13.75
Compression strength (kg/sq.cm) 70
Dry Density (kg/cu metre) 867


This table of relationships makes it clear that the less weight of water added to the plaster, the stronger the resulting mould will be. It also is clear that with less water, the setting time is reduced. So some compromise may be needed to be able to pour the mixture before it sets.