Dams should normally be lined with Thinfire and fibre paper to get the best release. If you are using fibre board that has not been hardened, you do not have to line, but you will get smoother edges if you do.
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| As described by Helios |
The lining papers should be about 3mm shorter than the expected final thickness of the finished panel. I find that 3mm paper against the dam provides the required standoff between the dam material and the glass. The lining of the fibre paper with Thinfire provides a smoother surface than just the fibre paper. Both of these liners should be the same height – 3mm less than the final height of the finished piece.
To calculate the expected final height you need to do a few calculations in the metric system. Weigh the glass in grams. Divide by specific gravity (2.5) to get the number of cubic centimeters. Divide the cc by the area enclosed by the dams in square centimeters. This will give the fraction or multiple of centimeters thick the glass is predicted to be.
Example:
The weight of glass = 500 gms
The specific gravity = 2.5
The area is 10cm by 10 cm = 100 square cm.
Divide 500gms (the weight) by 2.5 (the specific gravity) = 200 cubic centimeters. Divide 200 (the volume in cc) by 100 (the area) = 2 cm thick final piece for the amount of glass put into the pot.
This indicates the fibre paper should be 1.7cm high to allow enough space for the bullnose edge to form.
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.
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| 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.
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| 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.
