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

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.

Stainless Steel Preparation

Preparing stainless steel rods and moulds for kiln work is done slightly differently from ceramic moulds.

Just to ensure that the steel is of the right grade, I fire it in the kiln to about 720C. This ensures that if the steel is not adequate for the high temperature work, you will find out that it spalls before the glass is put on top. It also has the advantage of removing any dirt and oils on the surface of the metal.

The separator that you need to put on the steel can be done cold if you use MR97 or other boron nitride coating. Its main advantage is that it can be put on cold and also that it has a very smooth surface. This should be put on thinly, or it will come off onto the glass.

You can also put standard kiln wash on the metal. The metal needs to be dry and clean. It could be sandblasted if desired for a bit of extra “tooth”, but is not normally necessary. Heat the metal to about 120C – 150C in the kiln. Remove it from the kiln with tongs or similar thing to grasp the hot metal. Spray or paint the kiln wash solution onto the hot metal. Return it to the kiln as necessary until you have a coating all over the metal. It does not have to be even all over, but noes need to have all of the metal covered.

If the kiln wash boils off the metal, it is too hot. So turn the kiln down a bit.

If the kiln wash runs off without sticking at all, the metal is not hot enough and needs to be returned to the kiln to heat up.

It is best to avoid applying the kiln wash to the metal in the kiln, as water and the hot elements do not mix well.

Metal Framing Materials

Lead is a very weak metal. Therefore various other metals are often considered for the perimeter of the panel to strengthen the whole.

Zinc is a metal often used for strengthening the perimeter of panels. It is stronger than lead – by about 8 times. It is relatively easy to solder. However it is subject to more rapid corrosion than lead.

So an alternative is aluminium which is about about the same strength as zinc. However it does not accept soldering, so professional joining or cold fixing solutions are required to make the frame.

Copper is over 10 times the strength of lead and can be considered as an alternative to zinc. It accepts solder well, but as a came is extremely expensive. It does corrode to a verdigris unless protected and maintained. However, because of its strength, copper wire - as a single strand or several twisted - can be used inside other came such as lead or zinc to provide strong support.

Brass is about 19 times stronger than lead. It is available in came profile as well as “U” and “L” profiles. It accepts solder well and resists corrosion. It is more expensive than lead, but similar in price to zinc.

Mild steel strength varies but is at least 27 times stronger than lead. It does not accept solder easily, and does corrode without painted protection, but is a less expensive option than aluminium, zinc or copper. As an angle or “T” shape, mild steel and iron have been used for centuries to support leaded glass panels.

Stainless steel is at least 37 times stronger than lead. It is difficult to weld and does not accept solder at all. It is very resistant to corrosion.

When considering framing solutions for panels, the main factors to consider are relative strength, corrosion, and joining methods possible.

Brass, Copper, Lead and Zinc all can be joined by solder. Aluminium and stainless steel cannot be joined with solder. Although mild steel can be joined with solder, a good strong joint is difficult.

The stronger the metal, the thinner profile required, which can make metals that are more expensive by weight an economical solution, as metal prices are most often by weight rather than shape.

It also is possible to combine a stronger metal with a weaker metal, such as including copper wire or steel rods in the lead came.

It is not absolutely necessary to solder the panel to the framing material. A frame can be made and the panel fixed within it by other than hot soldering methods. In this case the frame takes the whole weight of the panel.

Metal Strengths

Metal Strengths

The strength of metals is most often compared by their tensile strengths. These numbers are Newtons per square millimetre and represent the relative strength of each metal compared to another.  The range of numbers indicates the variations caused by various alloys.

Tin                      19

Lead                14 – 32

Solder 60/40        48
Zinc                120 – 246

Aluminium       120 – 246

Copper            220 – 270

Brass              340 – 540

Mild steel         500 – 750

Stainless steel  740 – 970

These figures may be of interest in considering what frame to place around a free hanging stained glass panel.

Keeping Copper Inclusions from Oxidising

The colour change in the copper foil is due to oxidisation - if the copper foil is completely deprived of oxygen it stays shiny and copper coloured. If you leave copper exposed at all it will go metallic blue or even bottle green, mostly it turns a lovely burgundy red colour- an intermediate oxidisation stage.

Klyr fire or borax solutions may help the copper stay bright.

Through doing some experiments with art school students, I have found the speed of firing is critical in an electric kiln. In a gas kiln the speed is normally fast anyway and produces better results than an electric kiln. It also is a kiln with a reducing atmosphere rather than oxidising one of an electric kiln.

Summary:

The main elements in keeping copper inclusions (and by extension, other metals) bright is to keep the metal from oxidising. Two elements are important in this:

• Keep oxygen from the metal
• Reduce the time the metal is exposed to high temperatures

Various methods are used to keep the metal from exposure to oxygen. Some of these involve: 

• coating the metal with fluxes to reduce the amount of oxygen in contact with the metal. 
• using a reducing atmosphere, such as a gas kiln. 
• placing an oxygen hungry material in the kiln with the glass and metal. 
• coating the metal with glass powder before encasing it within the glass.

Reducing the heat exposure of the metal also indicates that firing fast would provide better results. This requires very even heating within the kiln to avoid heat shocking the glass.  This is where a gas kiln is most advantageous - it can be fired fast without breaking the glass and it has a reducing atmosphere within it.

In general, it is easier to make use of the effects of the oxidised metal rather than striving for bright metal inclusions.

Rods in Glass

Inserting large rods into glass

Normally large diameter rods are incorporated into a glass piece for display supports. However, rods over about 2mm will break the glass when incorporated in the fusing. This means that rods to support the glass that will be of sufficient diameter for the strength required cannot be fused into the glass.

You can use a rod of the same - or slightly smaller - size if you wrap it with thin fibre paper to cushion the differentials in expansion and contraction. When the glass has cooled, pull the rod out and clean out remaining fibre paper with water and a pick. 

Alternatively, you can use fibre paper to create a void of similar size to the wire or other metal support that will be inserted.

If you find that you do not like the hump that the rod or fibre paper create, you can use a third layer between the top and bottom.  Cut the glass to leave a channel of the length you want.  This means that the intermediate glass will need to be in at least two vertical parts, with half the channel width taken out of each side.  You can cut the glass in three parts horizontally.  The main piece is cut to a length minus the depth of the channel.  Then two strips are added to the bottom which leave the desired width. Finally, put the top layer on.  

Alternatively, the middle layer can be the design, with the bottom and top being clear or any other colour desired.

Once fired, clean out the channel and glue the rods into the glass. Silicone is a good all purpose adhesive that will allow a bit of movement.

Pickling

Pickling Silver

This term relates to the removal of firescale from silver by use of chemicals, often slightly warmed.

When heated, silver blackens on the surface. It is common in silversmithing to pickle the object, bringing the shine back.

There are several methods.

Hydrochloric acid is the most common chemical used. It normally is used in concentrations of 10% or less and often is slightly warmed in a soup warmer or other similar temperature controlled container

Hydrogen peroxide (sparex) solutions can be used, but are a bit slower. This also is used in a soup warmer.

Acetic acid, available from most chemists and home-brew suppliers, can be used but is so much slower that significantly long soaks are required.

The best solution for this is a 5% solution of citric acid or similar concentration of tri-sodium citrate.  This latter is best for glass, as it chelates the corrosion or stuck kiln wash, but does not etch the glass even after 48 hours soaking.

Revised 6.1.2022