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

(°C)

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.