Saturday, 12 November 2016

Heavy Metals in Glass

Some concern has been expressed about the metals used in colouring glass.  This centres around the temperatures used in fusing and whether kiln workers may be of risk from these heavy metals vaporising.

First of all, let’s get some sense of perspective. This is from Greg Rawles, an acknowledged expert on the hazards of working with glass.

Understanding Exposure:

In reality, unless you are doing:
High-volume production work that exposes you to a health hazard all day long
You are exposing yourself to high levels of a health hazard for a brief time
You are working with a very toxic material
You are not working responsibly

You are not really at risk for an unacceptable exposure when working in a glass studio  
http://www.gregorieglass.com/chemicals.html

Now, let’s think about how likely it is to have heavy metals vaporise at kiln forming temperatures. How stable would glass be if the metals that colour it vaporised when we fired it? the colour would vary with the heat and number of times we fired it.

Now, let’s think about how likely it is to have heavy metals vaporise at kiln forming temperatures. How stable would glass be if the metals that colour it vaporised when we fired it? the colour would vary with the heat and number of times we fired it.

Even if the metal were to evaporate, how much is in the glass. Apparently, Bullseye uses less than 3 pounds of cadmium for a pot of glass. We can tell from the sheet numbers that a pot of glass gives at least 2000 sheets of glass, so there is ca. 0.0015 lbs or .07 grams or less of metal in a sheet of 3mm glass. There is very little there to "vaporise", so even it were able to evaporate, it is in such small quantities as to be negligible, and the exposure so low as to be of extremely low risk. There is however, no risk in protecting yourself with dust masks. Just remember that the risks from vaporised heavy metals is much less than most of the other studio practices involving glass. If you need breathing protection for metals (and you may feel it is not worth the risk) then you need to be wearing a mask all the while you are doing glass work. It is about relative risk.

For complete information, the melting and boiling points of various metals relevant to glass colouring are given below.  The vaporisation will be somewhere above the melting point and toward the boiling point.  You will be able to see the relevant temperatures and take any precautions you feel are necessary.  Remember that the metals are not used in their pure forms, but as oxides.  These may have different melting and boiling temperatures.  In general, the oxides used in colouring glass have higher melting and boiling points than the pure metal.


Antimony -for whites
Melting point: 630C
Boiling point:  1635C

Antimony Oxide
Melting point:  380-930C
Boiling point:  1425C

Cadmium 
Melting point: 321C
Boiling point:  767C

Cadmium sulphide - yellow
Melting point: 1650-1830C
Boiling point:  2838C

Chromium 
Melting point: 1907C
Boiling point:  2671C

Chromic Oxide – for emerald green
Melting point: 4415C
Boiling point:  7230C

Cobalt 
Melting point: 1495C
Boiling point:  2927C

Cobalt Oxide- blue to violet
Melting point: 1900C

Copper 
Melting point: 1084C
Boiling point:  2562

Copper Oxides - for blue, green, red
Melting point: 1232-1326C
Boiling point:  1800-2000C

Gold
Melting point: 1337C
Boiling point:  2970C

Gold Chloride - red
Melting point: 170-254C
Boiling point:  298C

Iron
Melting point: 1538C
Boiling point:  2862C

Iron Oxide – for greens and brown
Melting point: 1377-1539C
Boiling point:  3414C

Lead – for yellows
Melting point: 327C
Boiling point:  1749C

Manganese 
Melting point: 1246C
Boiling point:  2061C

Manganese Dioxide – purple and a clarifying agent
Melting point: 535-888C

Neodymium
Melting point: 1024C
Boiling point:  3074C

Nickel 
Melting point: 1455C
Boiling point:  2730C

Nickel Oxide – for violet
Melting point (II - for green): 1955C
Melting point (III - for black): 600C

Selenium
Melting point: 221C
Boiling point:  685C

Selenium Oxide – for reds
Melting point: 118-340C
Boiling point:  350C

Silver 
Melting point: 961C
Boiling point:  2162C

Sodium
Melting point:  370C
Boiling point:   882C

Sodium Nitrate – a clarifying agent
Melting point: 308C
Boiling point:  380C

Sulphur
Melting point: 115C
Boiling point:  444C

Sulphur oxide - for yellow to amber
Melting point:  17C
Boiling point:   45C

Tin 
Melting point: 231C
Boiling point:  2602C

Tin Oxides – for whites
Melting point:  1080-1630C
Boiling point:  1800-1900C

Uranium
Melting point: 1132C
Boiling point:  4131C

Uranium oxide – for fluorescent yellow, green
Melting point:  1150-2765C
Boiling point:  1300C