Solarisation

Old glass can show changes in colour as evidenced by the different colour of the glass under the lead came where the light cannot reach the glass.

Drew Anderson has provided the explanation.

This change in color of some glass is known as solarisation.

The main ingredient of most glasses is silica, which is usually introduced as a raw material in the form of sand. Silica itself is colorless in glass form but most sands contain iron as an impurity, and this gives a greenish tint to glass. By adding certain other ingredients to a molten glass, it is possible to change the greenish color and produce colorless glass.

These ingredients are known as decolorizers, and one of the most common is manganese dioxide (MnO2). In chemical terms, the manganese acts as an oxidizing agent and converts the iron from its reduced state - which is a strong greenish blue colorant - to an oxidized state which has a yellowish, but much less intense, color. In the course of the chemical reaction, the manganese goes into a chemically reduced state, which is virtually colorless.

When pieces of decolorized glass containing reduced manganese are exposed to ultraviolet radiation for long periods of time, the manganese may become photo-oxidized. This converts it back into an oxidized form. Even in low concentrations this imparts a pink or purplish color to glass. The ultraviolet rays of the sun can promote this process over a matter of a few years or decades.

Selenium and cerium have also occasionally been used as a decoloriser and can produce solarisation colors, just as manganese does. The colors developed by these two elements are said to range from yellow to amber.

Mesh Sizes from a Typical Manufacturer

Mesh sizes have traditionally been measured by the number of wires per square inch used to sieve the material. This table gives a grit size measurement for the mesh/grit numbers in common use.

Mesh = Mesh opening (mm)
12 =   1.5240
14 =   1.2954
20 =   0.8636
30 =   0.5156
40 =   0.3810
50 =   0.2794
60 =   0.2337
80 =   0.1778
100 = 0.1397
120 = 0.1168
200 = 0.0737
325 = 0.0432
400 = 0.037
625 = 0.020
1200=0.012
2500=0.005

Break-Down Temperature of Common Mould Binders

The temperatures that various binders used in mould making is important to consider, as once they reach the break down point, they lose their strength and therefore ability to hold the mould together. The following table gives some indication of the characteristics of various binders.

Binders and Break-down Temperatures (degrees C)
Gypsum plaster - 704 - 816
Hydrocal cement - 704 - 816
Hydroperm cement - 760 - 927
Colloidal silica - 1260
Colloidal alumina - 1260
Calcium alumina (ciment fondu) - 1538

These of course, are not the only considerations in mould making but do show why combinations of materials is important. The common plasters and cement break down before the casting temperature of glass, typically 850C.

Polishing 3D Glass on a Wet Belt Sander

Polishing three dimensional objects depends on the shape of the glass you are sanding down to the polished surface.

Convex shapes can be done on the wet belt sander with ease.

You can polish slightly concave items on a belt sander if you have an unsupported section of the belt. On machines with a flat platen, you can remove the platen to use the ability of the belt to form into a slightly convex curve.