Wednesday, 16 March 2016

Metal Inclusions in Glass

There are a number of reasons to include metals in glass, not least colour.  However there are some things of which you should be aware.

Coefficient of Linear Expansion of some metals and glass is very different.  This listing gives some of the characteristics:

(All numbers given as 10-7)
Aluminium          230
Glass              ca. 85
Brass                 180
Bronze               190
Copper              170
Borosilicate glass   33
Gold                  140
Iron                  116
Lead                  280
Nickel                130
Platinum              90
Quartz               7.7 to 14
Silver                195
Stainless steel     100 to 170
Mica                    30
Porcelain              65
Clay tile               59
Stainless steel     (in general) 100 to 170
Stainless steel     (418 series)          99
Stainless steel     (310 series)        144
Stainless steel     (316 series)        160
Tin                    234
Zinc                  297
Titanium              86

From this you can see there is little that is similar in expansion coefficient to glass.  Those that are, are expensive.  The implications of this difference in expansion are that the metals upon cooling contract more than the glass and so these are the effects you need to watch for:

  • ·         Metals create strain when fused within the glass. 
  • ·         Thin section is required to reduce the strength of the metals. 
  • ·         The tensile strength of the metal may be more important than the CoLE
  • ·         The amount of the metal should not be great or concentrated in one spot
  • ·         Where thick sections of metal are required, a space should be created for later insertion of the metal.

In addition to expansion characteristics, the strength of the metal should be considered. Numbers are MPa (approximately equivalent to one atmosphere pressure)

Aluminium          40-50
Glass (float)        55-138
Brass                 250
Bronze               172
Copper              210
Gold                  120
Iron                  350
Lead                    12
Nickel                140-195
Platinum            125-240
Quartz               48.3 (and borosilicate glass)
Silver                170
Mica                  250-300
Porcelain            110-160
Stainless steel     (in general) 860
Tin                    15-200
Zinc                  110-200
Titanium            200

The greater the strength of the metal, the thinner the pieces should be to avoid excessive stress.

Melting temperatures are also a factor in including metals in glass

Aluminium          660  
Brass                 930-1000
Bronze               913
Copper              1084
Gold                  1064
Iron                  1149
Lead                  328
Nickel                1453
Platinum            1770
Quartz               1670
Silver                961
Stainless steel     1510
Mica                  600-900
Tin                    232
Zinc                  420
Titanium            1670

This shows that aluminium, lead, tin and zinc are not good inclusions as their melting temperatures are below the fusing temperatures of glass. This means they will not retain their structure when fired.  It can of course provide a “frozen” liquid appearance.

Finally, the oxidisation characteristics should be considered.  The following metals tend toward the description after the arrow “>”

Aluminium    > brown
Brass   > some browning
Bronze  > sometimes a red cast
Copper > from red oxidising to green in the presence of soda or chloride
Iron  > black
Nickel  > retains its colour well
Platinum > > retains its colour well
Silver > reacts with sulphur to form a yellow
Stainless steel > blackens
Mica  > retains its natural colour, although some is low temperature coloured and so blackens, others have high temperature colours
Titanium  >  oxidises to white
Gold  > generally retains its colour except in leaf form when it becomes silver in colour

These are not exhaustive descriptions of oxidisation characteristics of metals in glass. They are a good starting point though.