Metal inclusions

Two difficulties with metal inclusions in glass are common: stress and bubbles.

Stress

Metal inclusions always create stress in the glass. Different metals have different expansions and different strengths.  They also have different melting points - some so low that they liquify during the fusing process.

The trick in using metals as inclusions is to minimise the amount of stress. Small amounts of stress can be contained within the glass. The thicker or more mass inside the glass, the greater risk of stress breaks. The stronger or more rigid the metal is, the more stress will be generated.

Minimising stress is most easily achieved by using small amounts of the metal.  Thinning the metal as much as possible also reduces stress.  Flattening wire also helps reduce the amount of stress as well as keeping it in the place you want it without rolling away from its placement.

Bubbles

Bubbles often form around inclusions, especially of metals.  Metals that do not melt at fusing temperatures are stiffer than the surrounding glass.  You can see from the table noted above those metals which melt at higher temperatures than fusing.  These metals will create bubbles around their perimeter and elsewhere over the metal wherever there are wrinkles or undulations as the metal holds air in those places.

Thin metals

One possibility to reduce the bubbles is to thin the metal by hammering flat or use foil thicknesses of the metal.  Many specialist metal suppliers have very thin metals, often called shims.  They are increasingly available in online shops.

Weight

Another is to use enough glass on top to flatten the metal.  You should flatten the metal in the cold state as much as you can.  Then the weight of the glass presses down on the metal both in the cold and heated states. With a good long bubble squeeze, you can force more air out to the sides than with less covering glass.

Placing

A third possibility is placement. The further the metal inclusion is from the edges the more air is likely to be trapped to form bubbles.  If the air has less distance to travel, more is likely to escape.

Pressing

Supporting the edges or corners allows the centre to drop before the edges are sealed.  The weight of glass helps to press the air out to the sides.  Thicker glass (6mm/0.25") on top of the metal inclusion can help push the air away from the metal. You can also provide - within the design - paths for the air to escape. This can be elements such as powder, stringers and other glass accessories that can hold the glass up during the bubble squeeze process, but become invisible at fusing temperatures.

Fire in stages

A fifth possibility is to fire differently.  You can place the metal on a kiln shelf which is covered with fibre paper and put the glass on top of the metal and fire to a rounded tack fuse at the minimum.  To avoid dog-boning, you should cut the capping piece several centimetres larger than the final piece, so you can cut off the distorted edges. Clean the bottom and dry very well after firing and put the base under the top piece that has the metal attached.  Fire the combined piece slowly with a good bubble squeeze.  This can be applied to included vegetable matter too. 

Further information is available in the ebook Low Temperature Kiln Forming.

Inclusions often produce stress and bubbles.  There are some things that can reduce both when encasing metals or vegetation.

Zinc Health and Safety

So much is said about the toxicity of zinc, I thought to look up some facts.

As there is significant concern about health issues, it is useful to look in detail at the health and safety issues around the use of zinc at elevated temperatures.  Zinc is absorbed into the body by inhalation of fumes and consumption of zinc containing materials.

Toxicity

Although zinc is an essential requirement for good health, excess zinc can be harmful. Excessive absorption of zinc suppresses copper and iron absorption … [which results in the symptoms of zinc intoxication].  Stomach acid contains hydrochloric acid, in which metallic zinc dissolves readily to give corrosive zinc chloride. … The U.S. Food and Drug Administration states that zinc damages nerve receptors in the nose, causing [loss of smell].

Evidence shows that people taking 100–300mg of zinc daily may suffer induced copper deficiency. … Levels of 100–300mg may interfere with the utilization of copper and iron or adversely affect cholesterol. … A condition called the zinc shakes or "zinc chills" can be induced by inhalation of zinc fumes while brazing or welding galvanized materials. 

https://en.wikipedia.org/wiki/Zinc

Poisoning

Consumption of zinc can result in death, but requires large amounts (over 1 kg in one case).  Smaller amounts result in lethargy and gross lack of coordination of muscle movements or apparent intoxication. https://en.wikipedia.org/wiki/Zinc

Research and W.H.O. Information

The Essential Toxin: Impact of Zinc on Human Health, by Laura M. Plum, Lothar Rink, and Hajo Haase^*

Compared to several other metal ions with similar chemical properties, zinc is relatively harmless. Only exposure to high doses has toxic effects, making acute zinc intoxication a rare event. In addition to acute intoxication, long-term, high-dose zinc supplementation interferes with the uptake of copper. Hence, many of its toxic effects are in fact due to copper deficiency. While systemic [balance] and efficient regulatory mechanisms on the cellular level generally prevent the uptake of [cell destructive] doses of [environmental] zinc, … zinc [within the body] plays a significant role in cytotoxic [death of individual cells] events in single cells. … One organ where zinc is prominently involved in cell death is the brain, and cytotoxicity in consequence of [inadequate blood supply] or trauma involves the accumulation of free zinc.

Rather than being a toxic metal ion, zinc is an essential trace element. Whereas intoxication by excessive exposure is rare, zinc deficiency is widespread and has a detrimental impact on growth, neuronal development, and immunity, and in severe cases its consequences are lethal. Zinc deficiency caused by malnutrition and foods with low bioavailability, aging, certain diseases, or deregulated homeostasis [equilibrium] is a far more common risk to human health than intoxication.

Conclusions

Zinc is an essential trace element, and the human body has efficient mechanisms, both on systemic and cellular levels, to maintain [balance] over a broad exposure range. Consequently, zinc has a rather low toxicity, and a severe impact on human health by intoxication with zinc is a relatively rare event.

Nevertheless, on the cellular level zinc impacts survival and may be a crucial regulator of [the death of cells occurring as a normal and controlled part of an organism's growth or development]  as well as neuronal death following brain injury. Although these effects seem to be unresponsive to nutritional supplementation with zinc, future research may allow influencing these processes via substances that alter zinc [balance] instead of directly giving zinc.

Whereas there are only anecdotal reports of severe zinc intoxication, zinc deficiency is a condition with broad occurrence and potentially profound impact. Here, the application of “negative zinc”, i.e., substances or conditions that deplete the body of zinc, constitute a major health risk. The impact ranges from mild zinc deficiency, which can aggravate infections by impairing the immune defence, up to severe cases, in which the symptoms are obvious and cause reduced life expectancy.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2872358/

Zinc came
Credit: leadandlight.co.uk

World Health Organisation Document

10.2.2 Occupational exposure

Occupational exposure to dusts and fumes of zinc and zinc compounds can occur in a variety of settings in which zinc is produced, or in which zinc and zinc-containing materials are used. Typical airborne exposures observed include 0.19–0.29 mg/m3 during the smelting of zinc-containing iron scrap, 0.90–6.2 mg/m3 at non-ferrous foundries and 0.076–0.101 mg/m3 in hot-dip galvanizing facilities. Far higher exposures are possible during particular job activities, such as welding of zinc-coated steels in the absence of appropriate respiratory protection and/or fume extraction engineering controls.

Occupational exposure to high levels of zinc oxide and/or nonferrous metals is associated with metal-fume fever. [a condition in which the sufferer has influenza type symptoms - a raised temperature, chills, aches and pains, nausea and dizziness. It is caused by exposure to the fume of certain metals - commonly zinc].  This is usually a short-term, self-limiting syndrome…. Induction of metal-fume fever is most common with ultra-fine particles capable of deep lung penetration under conditions of exposure. Studies on volunteers conducted under short-term exposure conditions (77–153 mg/m3 for 15–30 min) have detected pulmonary inflammation responses (including [inflammation] induction) which are consistent with manifestations of metal-fume fever and support an immunological [cause] for this acute reversible syndrome.

Evaluation

Based on the available information, it is not possible to define a no-effect level for pulmonary inflammation from exposure to zinc oxide fume.

10.2.4 Risks of zinc excess

Toxic effects in humans are most obvious from accidental or occupational inhalation exposure to high concentrations of zinc compounds, such as from smoke bombs, or metal-fume fever. Modern occupational health and safety measures can significantly reduce potential exposure. Intentional or accidental ingestion of large amounts of zinc leads to gastrointestinal effects, such as abdominal pain, vomiting and diarrhoea.

In the case of long-term intakes of large amounts of zinc at pharmacological doses (150–2000 mg/day), the effects (sideroblastic anaemia [inability to make haemoglobin], leukopenia [low white cell quantities] and hypochromic microcytic anaemia [iron deficiency]) are reversible upon discontinuation of zinc therapy and/or repletion of copper status, and are largely attributed to zinc-induced copper deficiency.

High levels of zinc may disrupt the [balance] of other essential elements. For example, in adults, subtle effects of zinc on copper utilization may occur at doses of zinc near the recommended level of intake of 15 mg/day and up to about 50 mg/day. Copper requirements may be increased, and copper utilization may be impaired with changes in clinical chemistry parameters, but these effects are not consistent and depend largely upon the dietary intake of copper. Distortion of lipoprotein metabolism and concentrations associated with large doses of zinc are inferred to be a result of impaired copper utilization. In groups with adequate copper intake, no adverse effects, with the exception of reduced copper retention, have been seen at daily zinc intakes of [less than] 50 mg/day. There is no convincing evidence that excess zinc plays a [casual] role in human carcinogenesis. The weight of evidence supports the conclusion that zinc is not genotoxic [damaging of genetic information in cells] or teratogenic [affecting the development of embryos]. At high concentrations zinc can be cytotoxic [toxic to cells].   https://www.who.int/ipcs/publications/ehc/221_Zinc_Part_3.pdf?ua=1

zinc sheet 
Credit: Belmont Metals

Use and Risks of Zinc in Kilnforming

Zinc melts at 420°C and boils at 907°C, so any fumes will be emitted only around and above the full fusing temperature of glass.

The main problem in kilnforming is that the metal melts at such a low temperature that it is not useful for containing the glass.

There is anecdotal evidence to indicate that firing zinc contaminates the kiln, leading to subsequent devitrification issues.  This can be cleared by firing bentonite at high temperature in the kiln to absorb the zinc.

It is not a high-risk metal, even if it were to vaporise (above 900°C).

Research papers show zinc poisoning to be extremely rare. It is usually associated with taking too large daily doses of zinc as a dietary supplement, or swallowing USA pennies - made largely of zinc - which dissolves in stomach acid and creates large problems for the digestive system.  Where zinc intoxication occurs, it is largely reversible.

Conclusion

The idea that zinc will poison you in kilnforming conditions is simply not correct.

Strong Frames for Stained Glass Panels

Metals

Zinc is a popular material for framing copper foiled or leaded glass panels.  It is stronger than lead – up to eight times.  It gives a feeling solidity to the edges of the panel. 

However, it does have some disadvantages.  It is difficult to patina evenly and obtain the same colour as patinaed solder.  It resistance to progressive corrosion is weaker than lead. It requires special tools to fit around curves, making it best for rectangular panels.  It does need a saw to cut evenly, but so do a lot of the stronger metals.  A look at other options is worthwhile.

The strongest option is stainless steel.  This is difficult to cut and has special welding requirements, so is only useful in large and high corrosion installations.

Mild steel is widely available and cheap.  In certain circumstances – mainly small, thin profiles – it can be soldered.  The most secure joining is done with welding.  This requires equipment that stained-glass workers do not usually have.  However, there are a large number of metal workers that can to the work for you.

Brass is more expensive than mild steel.  It is an alloy of copper and tin and so can be soldered with the tools we normally use.  It is about half the strength of stainless steel, but three times the strength of zinc.  The tin content leads to a better patina result than zinc.

Copper is up to twice the strength of zinc, but the price fluctuates more than zinc.  It can be soldered. It requires different patina solutions than used for solder.

Aluminium is the same strength as zinc, but requires different joining methods as aluminium welding is a specialist activity.  Still, it will work on rectangular items with screws at overlapping joints.

More information on the relative strengths of various metals is given in a post on metal strengths.

Strengthening lead came

Lead is weaker than lead but can be bent to conform to curves and indentations for irregular perimeters.  If copper wire is incorporated and attached to the foiled glass, the soldering of the lead came to the joints at the intersections of the solder lines and the coper/came combination will provide greater strength than the zinc alone. 

When wanting to strengthen the perimeter of rectangular or shaped perimeter leaded panels, you can use 10mm “H” lead came soldered as usual to the whole piece as an alternative to soldering the wire to the panel.  Run the copper wire in the open edge of the “H”.  Pull the wire tight at the bottom and sweat solder at each corner.  Run the wire to the top on each side, where you can make a loop for attaching hanging wires and sweat solder the wires there too.  Then close the two leaves of the lead with a fid until they come together forming a single straight line.  If you want, a “U” or “C” edging came can be soldred to the outer edge of the "H" came to cover the line created by folding the leaves.

This post gives more detail about the process of incorporating copper into the perimeter of a leaded panel.

Firing Practices that Affect Kiln Elements

The way that you fire glass and other materials in your kiln affect the longevity of the kiln elements.  Some things you can do and avoid are given here.

Venting

Even if you have the best aluminium oxide coating, the fumes that emit from glazes, paints, organics, inclusions and devitrification solutions can still attack the element through cracks in the coating. Downdraft vents are your best defence against potentially harmful fumes. Downdraft vents pull the fumes from the kiln chamber before they have a chance to damage the elements.

If you do not have a downdraft vent your next best option is to prop the lid a couple of inches until the kiln reaches 540°C to allow the fumes out of the chamber. You should also consider leaving at least one peephole out during the entire firing for the fumes that escape above 540°C.

This presents a dilemma, as the recommendation is to keep the kiln closed from 540°C upwards to protect the glass from cold air drafts.   Those who rarely fire above 800°C do not have the same problem as those who regularly fire at 850°C and above for casting, combing, and melts.  The higher the temperature, the greater the effect of fumes on the elements.  At fusing and below temperatures the effect on the elements is not as great.  Thus, low temperature firings can follow the standard practice of closing the kiln above 540°C.  Those going higher, should consider venting the kiln all the way to the top temperature to reduce the wear on the elements.

Maintain an Oxidising Atmosphere

Elements need an oxidising atmosphere to provide a long dependable service.  Subjecting elements to reducing atmospheres will age the elements quickly.  This is be done by introducing organics or oils into the kiln without venting.  Among the things that will attack the aluminium oxide coating of the elements are

• ·        Carbon - this includes materials made from carbon and plant-based inclusions.   
• ·        wax burnout – it is best to steam wax out of moulds to eliminate most of the wax before any burnout, as the fumes are largely carbon.
• ·        halogens (such as chlorine or fluorine) 
• ·        molten metals (such as zinc, aluminium).  This is a more important reason for avoiding the use of zinc and aluminium in kilnforming than the possibility of health problems.
• ·        lead bearing paints and glazes – lead is a common component of paints, enamels and glazes.
• ·        alkaline metals – the main one we come across in kilnforming is magnesium which produces an amethyst colour of varying intensities.  This has a melting point of 650°C and boils at 1090°C, so some fumes can develop during firings and affect the elements.
• ·        borax compounds – used in enamel glazes and some devitrification sprays. 

If you use these materials in the kiln, you need to ensure that the kiln is well vented while these are in the kiln.

When you do have to use these elements - even when you vent - it is good practice to follow this firing by one without materials corrosive to the coating.  This allows the coating to re-form around the element surfaces after a corrosive firing.

Trying to do reduction firings in your kiln will greatly limit their useful life and is definitely not recommended.

Avoid Contaminants

Contaminants such as silica which is contained in kiln wash and some glazes attack the aluminium oxide coating of the wire.

Powders, paints and kiln wash accidentally touching the elements cause rapid corrosion of the elements if not cleaned off before firing.

Placing

Firing close to the elements allows any fumes from materials being used to affect the elements more than allowing some space between the glass and the elements.  This provides another reason to keep the glass away from the edges of the kiln in addition to the possible uneven heating of the glass.

High Temperature Firings

High temperatures with very long soak times will accelerate an increase in element resistance through the differential expansion of the inner wire and the coating. The higher the temperature, the longer the soak, the sooner the element will decrease in life. Usually short soaks work much better for the longevity of the element.  This is not such a big factor for glass kilns as it is for ceramic kilns.

The next part in this series deals with the maintenance of the elements.


Earlier relevant posts
Element Description

Aging

Copper inclusions

Inclusions of metals can be achieved with care.  Copper is a very good metal, as it is soft, even though its expansion characteristics are very different from glass.  This note provides some things you might consider when planning to include copper in your fused pieces.

The copper sheet should be stiff, but not thick. If the metal can be incised with a scribe and maintain that through gentle burnishing, it is suitably thick. The usual problem is that the copper is too thick rather than too thin.  Copper leaf can be very faint if a single layer is used.  Placing several layers of leaf improves the colour, but often provides wrinkles.  In summary, the requirement is to get a thickness of copper that will retain its structure, but not be so thick and stiff as to hold the glass up during the fusing process.  

Do not use the copper foil as used for stained glass applications. The adhesive backing produces a black colour from the adhesive and many bubbles -  sometimes a single large one.

Copper can provide several colours.

Copper sheet normally turns burgundy colour when oxidised.  This means that there is enough air reaching the copper to oxidise it to deep copper red.  This most normally happens, because a lot of air can contact the metal during the extensive bubble squeeze usually given to inclusions.

To keep the copper colour, clean the metal well metal well with steel wool or a pot scrubber. If you use steel wool, wash and polish dry the metal before fusing.  Reduction of air contact with the metal helps to retain the copper colour.  There are two methods I have used.  Addition of a glass flux like borax or other devitrification spray will help prevent the air getting to the surface.  Another method of avoiding oxidisation, is to cover the copper with clear powdered frit, as well as the surrounding glass.

In certain circumstances you can get the blue green verdigris typical of copper in the environment.  This is an extent of oxidisation that is between the clean copper coloured metal and the burgundy colour of extensive oxidisation.  The key seems to be be a combination of restricted air supply, shorter bubble squeezes and lower temperatures.  Experimentation is required to achieve this consistently.


The spaces under and over the copper give the opportunity for bubbles to form. 

This means that the copper needs to be as flat as possible for one thing.  Burnishing the copper can have a good effect on reducing the undulations in the copper.  Thinner copper is easier to make flat than thicker.  If you can stamp a shape from the copper with a stamper designed for card making, it is a good indication that it will burnish flat.  Thicker copper sheet holds the glass up long enough in the temperature rise during the bubble squeeze to retain air around the metal.  This remains the case even after burnishing to be as flat as possible.

The second element that can help to reduce bubbles around the copper is to sprinkle clear powder over the copper sheet once in place on the glass.  The spread of the powder over the glass assists in giving places for the air between layers to escape.

These two things combined with a long slow squeeze can reduce the amount of bubbles you get.  It cannot totally eliminate them.

Of course, a longer bubble squeeze allows air to be in contact with the copper and promotes the change to a blue green or burgundy colour.

Firing wire inclusions

Wire and other metal inclusions often cause bubbles to occur around them.  The standard solutions are to add frit to the corners, or powder or fine frit around the inclusions.   You can also flatten the wire or metal to reduce it height. These most often work well.  Sometimes though they don’t eliminate big bubbles around the metals.

In this case think about firing upside down. This is not the whole piece; it is only the inclusion and the bottom layer of glass.  Place the wire or other inclusion on the prepared shelf. It will be most successful if placed on 1mm or thicker fibre paper to allow any trapped air to escape through the fibre.  Place the base glass on top and take to a tack fuse with a bubble squeeze included.  You might even want to consider cutting the base larger than the final piece to be able to cut off the thickened edges and make a more successful piece at the end.

After tack fusing upside down, the inclusion will be imbedded in the glass with an almost flat surface and little in the way of air pockets at the edges.  Clean very well, especially any spalling from the metal and of course, clean the glass thoroughly.  Cap and fuse with a bubble squeeze again.  The bubbles around the inclusion should be minimal if not eliminated.

This method will allow the glass to sink around the glass making a much flatter piece for the capped full fuse. It should also make for a flatter finished piece with many fewer bubbles.

Colourising Metals for Glass

There are many minerals and chemicals that are used to give glass its variety of colour.  This note attempts to give information on the most common elements and combinations used to impart the colours to the glass.

Antimony oxides produce white glass as do tin oxides.  Together with lead, antimony results in yellow.

Cadmium together with sulphur forms cadmium sulphide and results in deep yellow colour.  Together with selenium and sulphur it yields shades of bright red and orange. 

 

Chromium is a very powerful colourizing agent, yielding dark green or in higher concentrations even black colour. Together with tin oxide and arsenic it gives emerald green glass. Chromium aventurine, in which aventurescence was achieved by growth of large parallel chromium(III) oxide plates during cooling, was also made from glass with added chromium oxide in amount above its solubility limit in glass.

The material can be introduced into glass either in the form of chromic oxide or potassium dichromate, the latter being a more convenient form.  Potassium chromate is yellow and this colour can be imparted to certain glasses. To produce emerald green glass in which a yellowish cast must be avoided, the addition of tin oxide and arsenic is necessary.

Chromium is associated mainly with the production of green glass, but other colours from yellow through bluish-red, red to dark green or even black can be achieved in combination with other oxides.

Cobalt is the most powerful blue colorant used in glassmaking producing rich blues when used in potash containing mixes, but it can also give shades of green when used with iodides.  The deepest of blues are produced when used in glass containing potash.

Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. The best results are achieved when using glass containing potash. Very small amounts can be used for decolourizing. Addition of 2% to 3% of copper oxide produces a turquoise colour.

Copper is a very powerful and versatile colouring agent when used in colouring glass.  Copper greens and blues are not difficult to produce, although the behaviour of copper in a silicate melt can be complicated.  Copper was used most profusely to produce green glass.  

Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold in the production of ruby-coloured glass. The art of using copper for ruby glass goes far back to ancient times but even so using copper oxide to make ruby glass can be very difficult. Today we find copper being used to produce turquoise blue tones.

Didymium gives green colour (used in UV filters) or lilac red.

Metallic gold, in very small concentrations (around 0.001%, or 10 ppm), produces a rich ruby coloured glass (gold ruby), while lower concentrations produces a less intense red, often marketed as cranberry. The gold is used as gold chloride. The colour is caused by the size and dispersion of gold particles.

Iron is a very useful and powerful colouring agent even though it can be an undesirable impurity in making glass. Iron in its metallic forms cannot remain in equilibrium with glass and can be disregarded. But its ferrous and ferric forms are of a great help in producing coloured glass.  Iron(II) oxide may be added to glass resulting in bluish-green glass.  

Together with chromium it gives a richer green colour.  In a reduced condition, it can be combined with chromium to produce a deep green glass. 

Used with the combination of carbon or other reducing agents, sulphur and iron sulphides give a dark amber colour.

Lead compounds produce a range of yellows.

Manganese can be added in small amounts to remove the green tint given by iron, or in higher concentrations to give glass an amethyst colour.

Manganese dioxide, which is black, is used to remove the green colour from the glass. This results in a very slow chemical process where it is converted to sodium permanganate, a dark purple compound. Windows made with manganese dioxide solarise to change to a colour which is lightly tinted violet because of this chemical change.

Manganese in its low state of oxidation is colourless, but it is a powerful oxidising agent and can be used for decolourising purposes to oxidise the iron content.  Manganese is mainly used in the production of purple glass resembling the colour of potassium permanganate crystals. The purple colour is achieved by the trivalent manganese however in its divalent state it only imparts a weak yellow or brown colour.

Nickel, depending on the concentration, produces blue, or violet, or even black glass. It is used in the production of smoky coloured glass

Lead crystal with added nickel acquires purplish colour. Nickel together with a small amount of cobalt can be used for decolourizing of lead glass.  When it is introduced into lead crystal it gives a purplish colour, which compensates for a yellow tint produced by other constituents.

Selenium, like manganese, can be used in small concentrations to decolourize glass, or in higher concentrations to impart a reddish colour, caused by selenium nanoparticles dispersed in glass. It is a very important agent to make pink and red glass. When used together with cadmium sulphide, it yields a brilliant red.

Silver compounds such as silver nitrate and silver halides can produce a range of colours from orange-red to yellow. The way the glass is heated and cooled can significantly affect the colours produced by these compounds.

Sulphur, together with carbon and iron salts, is used to form iron polysulphides and produce amber glass ranging from yellowish to almost black.  With calcium it yields a deep yellow colour.  

In borosilicate glasses rich in boron, sulphur imparts a blue colour. Cadmium sulphides, which have a deep yellow colour, are often used in the production of glazes and enamels.

Tin oxide with antimony and arsenic oxides produce an opaque white glass (milk glass), first used in Venice to produce an imitation porcelain.

Adding titanium produces yellowish-brown glass. Titanium, rarely used on its own, is more often employed to intensify and brighten other colourizing additives.

Uranium (0.1% to 2%) can be added to give glass a fluorescent yellow or green colour.  Uranium glass is typically not radioactive enough to be dangerous.  It is often referred to as Vaseline glass by USA collectors.  When used with lead glass with very high proportion of lead, produces a deep red colour.

Chart

A good visual chart of minerals and resulting colours is here.

Influence of the glass-making process on colour.

It is not only the minerals that give the glass the colour; it is combined with the way in which the materials are treated.  The physical conditions under which the glass is made also have an influence on the colour.  The main ones are:

1.   The temperature of the melt/batch

2.   Temperature of reheat during the working of the glass

3.   The temperature of the 'Lehr' (Annealing Oven)

4.   Duration of the melt/batch

5.   Time and temperature relationship at different stages in production

6.   The type of colorant being used

7.   Concentration of the colorant

8.   Atmosphere of the furnace

9.   The composition of the colorant within the glass composition, as is the case when iron is added to glass. The type of iron oxide formed decides if the glass will be blue or yellow

10.                The number of times the same glass is melted. Repeated melting of the cullet will usually give a darker tone to the finished piece