Aperture Pours

The most commonly used aperture pours are Pot melts and wire melts. Pot melts use containers, and wire mesh for wire melts. In both cases they control the way the glass melts into a container or directly on the shelf below.

Emptied pot melt

The materials are stainless steel wire grids, and unglazed terracotta pots. The spacing of the steel grid will determine the number of trails of glass falling. So a finer grid will give more points of expansion in the resulting melt. But will mix the colours much more thoroughly than a coarser mesh will.

Finished screen melt

Doing a pot melt usually provides a simpler pattern of flow. A single round hole gives one circular point from which the glass expands. A single rectangular hole gives a single ribbon shape as the expansion point. You can, of course, have multiple holes in the bottom of the pot to provide a more complex interaction of the flowing glass. The wider the rim of the pot in relation to its depth, the more flexible it will be. You can put more glass in the pot and you can have it higher in the kiln.

The arrangement of glass in the pot will produce different results. There are two basic arrangements: colours layered one above each other as in a layer cake; and colours arranged on end around the sides of the pot. When loading the pot you need to remember that although the glass immediately above the hole will be the first to come out – and therefore be at the edge of the melt – the remainder of the glass comes out in a funnel-like order, with the glass at the bottom corner of the pot being the last to flow out – and become the centre of the melt.

There is a relationship between the hole size and distance to surface that affects the final appearance. The larger the hole the less likely the glass is to spiral as it falls, so you need a greater distance between the bottom of the pot and the shelf. The smaller the hole, the less distance you need. Only experience will tell you what distance and size you need or can use.

You can calculate the amount of glass for different sizes by using this table. If you have a rectangular space you are dropping into, you can calculate the volume of glass by multiplying the width, length and desired thickness – all in centimetres. This will give the volume in cubic centimetres and to convert that into weight, you multiply the volume by the specific gravity of glass - 2.5 is near enough – to get the number of grams of glass required. To convert into kilograms, divide by 1000.

By dropping directly onto kiln washed shelf, ring or circular container you will get some contamination.  There are some ways to avoid this given here.

You can also use this method to act as a crucible to pour glass into closed moulds.

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 piecesUse 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.

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.

Properties of typical gypsum plasters and cements

Number 1 Pottery Plaster
% of water to dry mix by weight - 70%
Set Time – 27 – 37 mins
Dry density – 1105 kg/cubic metre
Expansion on setting – 0.21%
Compressive strength - 126 kg./square centimeter

No. 1 Casting plaster% of water to dry mix by weight - 70%
Set Time – 27 – 37 mins
Dry density – 1058 kg/cubic metre
Expansion on setting – 0.2%
Compressive strength - 140 kg./square centimeter

Plaster of Paris% of water to dry mix by weight - 70%
Set Time – 27 – 37 mins
Dry density – 1105 kg/cubic metre
Expansion on setting – 0.2%
Compressive strength - 140 kg./square centimeter

Number 1 Casting Plaster% of water to dry mix by weight - 65%
Set Time – 27 – 37 mins
Dry density – 1162 kg/cubic metre
Expansion on setting – 0.22%
Compressive strength - 168 kg./square centimeter

Pottery Plaster
% of water to dry mix by weight - 74%
Set Time – 27 – 37 mins
Dry density – 1162 kg/cubic metre
Expansion on setting – 0.19%
Compressive strength - 126 kg./square centimeter

Hydrocal Cement
% of water to dry mix by weight - 45%
Set Time – 25 – 35 mins
Dry density – 1442 kg/cubic metre
Expansion on setting – 0.39%
Compressive strength – 35 kg./square centimeter

Hydroperm Cement% of water to dry mix by weight - 10%
Set Time – 12 -19 mins
Dry density – 
<641 br="" cubic="" kg="" metre="">Expansion on setting – 0.14%
Compressive strength – 35 kg./square centimeter

Hydro-Stone cement
% of water to dry mix by weight - 32%
Set Time – 17 -20 mins
Dry density – 1913 kg/cubic metre
Expansion on setting – 0.24%
Compressive strength – 703 kg./square centimeter

Ultracal cement
% of water to dry mix by weight - 38%
Set Time – 25 - 35 mins
Dry density – 1568 kg/cubic metre
Expansion on setting – 0.08%
Compressive strength – 421 kg./square centimeter

Break Down Temperatures of Common Mould Constituents

Binders are essential parts of mould materials. They hold the refractory parts of the mould together. Selection is dependent on the temperature you will be using. This also is important in choosing the refractory material to use.

Gypsum plaster - 704C – 816C
Hydrocal cement - 704C – 816C
Hydroperm cement – 760C – 927C

Colloidal silica – 1260C
Colloidal alumina – 1260C
Calcium alumina cement (cement fondu) – 1538C

There are of course, many other factors to take into account when choosing binders and refractory materials for moulds.

Temperature Characteristics of Various Glasses

Over the years I have collected temperature information for a number of glasses. They are of comparative interest and can assist with choosing a temperature or range of temperatures for the work you are doing. If the work is important, or critical, refer to the manufacturer for the latest information.

Bullseye
There has been a lot of information published about this glass. One interesting characteristic has been the different temperatures for the complete range of glass they produce. So there appears to be a difference between the transparent, opalescent and gold pink glasses.
Transparent:
Full Fusing 832C ; Tack Fusing 777C ; Softening 677C ; Annealing 532C ; Strain 493C
Opalescent:
Full Fusing 843C ; Tack Fusing 788C ; Softening 688C ; Annealing 502C ; Strain 463C
Gold Bearing:
Full Fusing 788C ; Tack Fusing 732C ; Softening 635C ; Annealing 472C ; Strain 438C

This also illustrates that not all the characteristics of a glass range are linear. The most apparent one is that the full fusing, tack fusing and softening points of the opalescent glass are higher than transparent, although the annealing point is lower.

Desag GNA
Full Fusing 857C ; Tack Fusing 802C ; Softening 718C ; Annealing 516C ; Strain 427C

Float Glass
Full Fusing 835C ; Tack Fusing ca. 760C ; Softening 720C ; Annealing ca. 530C ; Strain 454

Spectrum S96
Full Fusing 788C ; Tack Fusing 718C ; Softening 677C ; Annealing 510C ; Strain 371C

Uroboros
Full Fusing 788C ; Tack Fusing 732C ; Softening 663C ; Annealing 538C ; Strain 427C

Although the information above may be dated, the important element is that there is little correlation between glasses in the relationship of annealing point to other characteristics of the glass.

This listing also shows that the temperature characteristics are not linear between glasses. For example, Spectrum and Uroboros have the same full fuse temperatures, but different tack fusing, softening, annealing and strain temperatures. Sometimes one is higher than the other, and other times it is reversed.

Another example is shown by the Desag GNA and Float glasses. Desag GNA has higher full fuse and tack fuse temperatures than float, but lower softening, annealing and strain temperatures. This helps to make the point that you need to know the glass you are using as it will not have a proportional relationship at every point in the kiln working temperature range.

I emphasise that these temperatures have been collected over a period and may not be the current or absolutely correct information. They are used here to illustrate the differences within and between the glasses of various manufacturers.

Mesh Sizes

Mesh and grit sizes are most often refered to by a number. This relates to the number of wires per inch - and in a subsidary fashion also to the size of the wire used to form the grid through which the material falls and so is sorted into various sizes. The table below gives some of these figures most useful for mould making - mesh number, percentage of open area, the wire diameters in mm, and the mesh opening or material size in mm.

No.12 ; % open 51.8 ; dia. 0.5842 ; size 1.5240

No.14 ; % open 51.0 ;dia. 0.5080 ; size 1.2954

No.20 ; % open 46.2 ; dia. 0.4064 ; size 0.8636

No.30 ; % open 37.1 ; dia. 0.3048 ; size 0.5156

No.40 ; % open 36.0 ; dia. 0.2286 ; size 0.3810

No.50 ; % open 30.3 ; dia. 0.1905 ; size 0.2794

No.60 ; % open 30.5 ; dia. 0.1397 ; size 0.2337

No.80 ; % open 31.4 ; dia. 0.1143 ; size 0.1778

No.100;% open 30.3 ; dia. 0.0940 ; size 0.1397

No.120; % open 30.7 ; dia. 0.0940 ; size 0.1168

No.200; % open 33.6 ; dia. 0.0533 ; size 0.0737

No.325;% open 30.0 ; dia. 0.0356 ; size 0.0432

Properties of Some Basic Glass Types

Various types of glass have differing properties which make them suitable for a variety of applications. Some of the characteristics of three glasses are given here. The glasses are quartz, soda/lime, and lead crystal.

Quartz glass
Softening point (C) 1508
Annealing point (C) 1048
Strain point (C) 956
CoE at 10-7 metres/degree C: 3.1
Density (kg/m3) 1973
Refractive index 1.459

Soda/Lime glass
Softening point (C) 693 - 732
Annealing point (C) 516 - 549
Strain point (C) 471 - 493
CoE at 10-7 metres/degree C: 56 - 100
Density (kg/m3) 2203 - 2275
Refractive index 1.51 – 1.52

Lead glass
Softening point (C) 438 - 671
Annealing point (C) 366 - 527
Strain point (C) 343 - 449
CoE at 10-7 metres/degree C: 47 - 55
Density (kg/m3) 2505 - 4867
Refractive index 1.54 – 1.75