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.

Bubbles in texture moulds

People often assert that moisture is a cause of large bubbles on texture moulds.

 Let's think about this.

• Water evaporates by 100°C/212°F.
• Glass is not sticky until around 540°C/1000°F.
• Glass does not begin to slump until about 610°C/1130°F. 

Therefore, with a reasonable schedule the mould, shelf, etc., is dry before air can be sealed in.

 

Damp moulds are not the cause of bubbles in texture mould work.

This observation means you must seek other causes. Three related to glass are:

•        Air pockets: Texture moulds have lots of possible air pockets. Excellent, slow bubble squeezes are required to avoid creating bubbles. These bubble squeezes start about 50°C/90°F below slumping temperatures. The ramp rate for these can be as low as 25°C/45°F per hour, but more commonly are 50°C/90°F per hour. This rate applies to the slump temperature with a soak of 30 or more minutes. The deeper the texture, the longer the soak needs to be.

•        High temperatures: To resist bubble formation low temperatures are required.  The higher the temperature, the lower the viscosity. Low viscosity is less able to resist air pressure from below than glass at a lower temperature with greater viscosity. My testing and research show that 740°C/1365°F is hot enough to form the glass to the mould. Of course, low temperatures require long soaks. The soaks might be as much as 2.5 hours for a single sheet, or as little as 1.5 hours for three layers.

•        Thin glass: there is little weight or mass in a single sheet to resist bubble formation. Using low temperatures becomes even more important. The stiffer (higher viscosity) the glass is the more resistant it is to the pressure of trapped air. Using even lower temperatures will help resist bubbles, but much longer soaks must be used.

 

The moulds can be a cause of course. There may be boundary walls on the mould. There may be vertical sides to the relief. There may recesses that are deeper than the rest of the mould.

In extreme cases, pin sized holes may have to be drilled where a lot of air is trapped. Only experience will tell you where these low spots are holding sufficient air to create bubbles.

Most bubbles created in firing texture moulds come from the scheduling, the thickness, and the nature of the mould.

 

More information is available in the e-book Low Temperature Kilnforming, an Evidence-Based Approachto Scheduling.

Quoting for Fused Glass Commissions

When quoting on a fused glass commission, what are all the factors to consider?

Commission for Glasgow University

Quote the same way as for leaded or copper foil.  But if you don’t work in those forms, that statement will not be much help.

The elements to consider are:

·        Design time and value (making sure you retain the copyright of the design).

·        Amount of time to assemble. You need to think clearly about how long it really takes.  You need to be charging a reasonable amount for your time. Think about skilled trades people’s charges and that you have additional artistic skills.

·        Amount of glass to be purchased (rather than used) to make the piece, even if much is from stock – you must replace it after all.

·        Number and cost of kiln firings.  Be clear about how many firings might be required, if something does not work out first time.  Be clear about how much each firing costs including depreciation on the kiln.

·        Incidental supplies.  All the little things that are necessary to supply your practice, such as art materials, kiln supplies, etc.

·        Overheads. This is the cost to run your practice.  If the studio is part of the home premises, add a proportion of the running expenses of the house to the cost.  The cost of business - advertising, promotion, printing, etc., all need to be included.

·        Profit. You do need to make a profit to stay in business. Decide what that is and add that percentage to the cost.

·        Allowance for contingencies (20% of the price already determined is usual).

·        Delivery/installation costs (normally in addition to the cost of design and making).

It is advisable to find out what the client’s budget for the commission is before starting any designing.  If it is too small for their specification, decline the commission.  Otherwise, you can design to the budget.  A large budget allows expansive or highly detailed works.  A small budget restricts the size or detail possible.

Some people charge more for a commission. Some, like me charge less, as I am getting most of the money up front, rather than maybe sometime in the future.  Cash is important.

Some artists take 1/3 to make the design, 1/3 on approval of design, and final 1/3 on completion. This is widely used in the interior design field. You may want to consider requiring a non-refundable deposit of one third to make a start and the remaining two thirds on completion as an alternative. 

A contract of some sort is essential.  It needs to cover the expectations of both parties.  Cost, of course.  When is it to be completed? Requests for colours, shapes, location, style, etc.  If the client wants approval at various stages, you need to either state what these stages are, or more sensibly, decline the commission. 

The contract does not have to be legalistic.  It can be a letter stating the terms of the commission that is sent by you to the client and acknowledged by them.

Determining the price for a commission requires consideration of the costs of time and materials, and the values of what you do.  A contract of some sort is required. It can be a simple letter with a statement of the agreed conditions.

Cleaning masses of pieces

Are there any easy tips on how to clean off the cutting oil without having to wipe each of 168 pieces individually?

There are a variety of approaches. Some put multiples into a basin of water as they are cut.  Some with soap added, some with window cleaner or vinegar.  When all are cut, the pieces are swirled around in the water/additives solution and laid out on kitchen towels to drain while each is polished with clean towels.  Some put the glass in a bag into the dish washer.  This leaves the glass with the residue of a number of corrosive chemicals on the surface.

If you must put additives into the soaking water, I suggest you use a combination of 1 part water, 1 part isopropyl alcohol, and 1 part 5% citric acid.  Avoid the use of vinegar. There is a significant risk of etching the glass, leaving a dull surface. Citric acid will not affect the glass, nor leave residues after rinsing.  The alcohol will speed the drying.  But see this post on another better chemical than Citric acid. You can leave glass soaking in tri-sodium citrate for up to 48 hours without etching.

Essentially these practices are to soak the pieces until all are cut to have a mass cleaning and drying session.

But I don’t use oil in my cutter and so I can follow this procedure:

clean the glass sheet first,

score with no oil in the cutter,

break,

set aside to assemble.

Prior to assembly I clean each piece with isopropyl alcohol and a polish with paper towel.

Cleaning glass for fusing is much simpler if you do not use oil in the cutter.  There is no absolute necessity to do so.  The glass will score and break very well without oil.

Strain Points

A critical range is the temperature around the annealing point. The upper and lower limits of this range are known as the softening and strain points. The higher one is the point at which glass begins to bend.  It is also the highest temperature at which annealing can begin. The lower one is the lowest point at which annealing can be done. Soaking at any lower temperature will not anneal the glass at all. This temperature range is a little arbitrary, but it is generally considered to be 55C above and below the annealing point. The ideal point to anneal is thought to be at the annealing temperature, as annealing occurs most rapidly at this temperature.

Annealing Range
However, glass kiln pyrometers are not accurate in recording the temperature within the glass, only the air temperature within the kiln. The glass on the way down in temperature is hotter than the recorded kiln atmosphere temperature. A soak within the annealing range is required to ensure the glass temperature is equalised. If you do a soak at 515°C for example, the glass is actually hotter, and is cooling and equalising throughout to 515°C during the soak. The slow cool to below the lower strain point constitutes the annealing, the soak at the annealing point is to ensure that the glass is at the same temperature throughout, before  the annealing cool begins.

Strain Point and Below
No further annealing will take place below the strain point. If you do not anneal properly, the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken.  The cool below the anneal soak needs to be at a slow controlled rate that is related to the length of the required anneal soak. Too great a differential in contraction rates within the glass can cause what are most often referred to as thermal shock.  The control of the cooling rate reduces the chance of these breaks.

Softening Point

The glass is brittle below the softening point temperature, although it is less and less likely to be subject to thermal shock as it nears the softening point.  It is after the softening point on the increase in temperature that you can advance the temperature rapidly without breaking the glass.  So, if you have a glass that gives its annealing temperature as 515C, you can safely advance the temperature quickly after 570C (being 55C above the annealing point).

Playing in the Sandbox

This process provides flowing, abstract images that can be used as autonomous pieces or formed into other objects, such as free drops, bowls, cut for jewellery or into pattern bars.  The appearance provided is unique to this combination of using frit and pressing.

In principle, this process is the same as creating sand pictures.  The process is in three stages: making the box, adding frit, and pressing.

The Sandbox

Determine the size of the box.  It should not be more than two-thirds the size of your kiln shelf depending on thickness.  Thicker glass pressed to 6mm will spread more than thinner.  As a guide, 12mm should have an allowance to spread to about 1.3 times the original size; 19mm should have an allowance to spread about 1.5 times the original dimensions.

Cut two sheets of the same size from clear fusing glass. One will be the front. The other will be the back.

Determine whether the image you are creating will be portrait, landscape, or square.  Orient the sheets in the appropriate way to have the top away from you.  Choose the top piece of the pair and cut two 6mm strips from the designated top.  This gives you a lip to be able to pour the frit into the box easily.

 

Box formed with bottom and sides glued to back and front.  The filling lip shows on the right.

From another piece of clear glass cut two 6mm strips for the sides.  If you cut them the same length as the side of the glass, they will stick above the back about 3mm. You can cut this off, but it really is not a worry for the construction of the box.  These strips form the spacers to allow the frit to be poured into the box.  Their thickness will determine the amount of frit needed to fill the box.

Get out the back sheet and clean and prepare it for attaching the strips. My preferred method is to glue the bottom 6mm strip on its edge with super glue.  It is advisable to wear plastic gloves when gluing the strips, to avoid sticking your fingers to the glass.  Super glue cures quickly and does not delay the construction of the box.  It burns out cleanly without any health and safety concerns.  Place a thin film of super glue on one edge of the strip.  Attach it to the bottom by placing it carefully at the edge of the sheet.  Do the same for the sides.

  

When the strips are stuck down to the back, place  a thin line of super glue on the top edge of the strips in preparation for attaching the top sheet.  Using a strip of wood placed at the bottom of the backing glass will help in placing the sheet accurately. Lower the sheet from contact with the bottom to the strips forming the sides of the box.

When the glue is cured, inspect the sides of the box for gaps. If there are gaps, use clear Sellotape to seal the gaps in the sides. It will burn off cleanly in the kiln.

Adding the Frit.

Place the box on an easel or other support so it is slightly tipped backwards.  This helps ensure the box does not fall toward you while working on it.  It also allows the frit to slide toward the bottom rather than bouncing off the other frit.

 

The early stages of filling with the box on a stand

The size of frit you choose to use will affect the final appearance.

·        Generally, powder will appear greyer and more opaque than frit. This is due to the multiplicity of tiny bubbles between the grains of powder.

·        Fine and medium frit provide more clarity than powder.

·        Coarse frit provides the most clarity, but with fizzy bubbles between pieces of frit.

When preparing to place the frit in the box, it is a good idea to take small amounts out of jars and place it into small cups to avoid contamination of the main source of the frit.

Pouring the frit into the sandbox

You can use a jeweller’s scoop or a teaspoon to move the frit from the cup to the box.  Tip the frit into the box above where you want the colour to be placed.  

Moving the frit with a skewer

If the frit does not land just where you want it, you can move it with stiff wire that is long enough to reach the bottom of the box.  Gently sweep the frit with the end of the wire toward the place you want the coloured frit to be.

Using a jewellers scoop to add the frit.

Continue adding colours to create the profile and shapes you wish.

You can make additional alterations to the way the frit is placed.  You can poke the frit from one layer into lower layers with a stiff wire by pushing the wire directly downward.  You cannot do this more than 2 or 3 centimetres deep, as the frits and powders become compacted.

A thick copper wire being used to poke down from an upper layer to the lower ones.

When filled to the top or to your desired level, use the fourth strip to close the box.  If full, glue the strip to the top.  If not full, cut strip to the length needed to drop into the opening of the box.  Place a couple of drops of super glue on the top of the already placed strip to keep it in place while moving to the kiln.

The Pressing

Prepare the shelves

You will need two shelves for each pressing. One is the base to hold the glass and the spacers.  The other is to provide the weight to press the glass thinner.

Clean off old kiln wash from the shelves. Experience shows that adding new kiln wash over old for this process promotes the sticking of the kiln wash to the glass.  Add new kiln wash that performs well at extended times at upper temperatures.  I find Bullseye shelf primer works very well.

Once partially dried, with the pink beginning to pale, you can smooth the surface brush marks.  Some use balled up material such as tights to rub over the surface.  I find very good results from rubbing lightly over the kiln washed surface with a sheet of paper between the palm of my hand and the shelf.  The advantage of doing this smoothing while slightly damp is that no dust is created that needs to be cleaned away.  The disadvantage is that too much pressure will pull bits of kiln wash from the shelf.

Do not use fibre papers as the separator.  The glass will be moving within the space between the shelves.  It will pick up and incorporate parts of the fibre paper, if used.

If you have shelves of different thicknesses, reserve the thickest shelf for the upper, pressing one.  If all your shelves are the same size, put a second on top for adequate weight, or add heavy bricks or a steel weight to the top shelf.  (Note: if you use bricks for weights, they need to be dried first.  A two-hour to three-hour soak at 95C should be sufficient.)

Placing

Place the sandbox centrally on the shelf.  If you are doing more than one, ensure there is plenty of space between the pieces and from the edge, so they don’t contact each other, or drip over the edge of the shelf.  The allowances given for the size of the sandbox are a guide.

Two sandboxes placed on separate shelves

Place spacers of the desired thickness around the four corners of the shelf to restrict the extent of thinning.  This also regulates the evenness of the glass across the whole surface.  Usually, 6mm is a desirable height for the pressing.  Other thicknesses can be chosen for different purposes.  The spacers can be steel washers, although they will spall in the cooling stages of the firing.  If you have pieces of ceramic of the desired height, they can be used.  Fibre paper stacked up to the appropriate height are surprisingly robust spacers.  They also provide a cleaner set of spacers than steel.

A corner of the shelf with the 6mm fibre spacer

Place the upper shelf gently down onto the glass piece. The glass at this stage is taking the whole of the weight of the pressing shelf.  The shelf must be placed both gently and evenly down onto the glass to avoid breakage.

Check that everything is in place. This may require additional, directional light such as from your mobile phone or a torch.  It is now ready to fire.

The Firing

This assembly of materials has a lot of mass.  It is 2 to 3 times the normal mass for a standard firing.  

Pressing shelf placed on top of the glass sandbox

This promotes variations in practice:

• ·        Even with this additional mass, you can fire quickly.  This is because the glass is in small pieces and that the mass of the shelves gains heat slowly. 
• ·        The greater mass does require longer soaks than a normal fuse firing. 
• ·        The upper temperature for a full fuse is required to get the glass to a sufficiently low viscosity to allow the glass to move.
• ·        The long soak at the top temperature does not promote devitrification as in normal fusing.  My speculation is that the glass is not exposed to the air, so the devitrification cannot form. 
• ·        A further difference in a pressing firing is that the annealing can be at the rate for the final thickness of the glass.  The mass of the shelf and weights above the glass means the glass is cooling evenly from both sides, unlike normal fusing.  The glass may be cooling more slowly than programmed, but the programmed rates limit any possibility of too rapid a cooling.

A schedule for a 12mm thick Bullseye piece with a 19mm upper shelf might look like this:

300°C/hr to    670C       for   180 minutes

300°C/hr to    816C       for   180 minutes

AFAP      to    482C       for  120 minutes

55°C/hr   to    427C       for   0 minutes

99°C/hr   to    370C       for   0 minutes

200°C/hr to    50C         for   0 minutes

Off

A piece of 19mm should be slower:

150°C/hr to    670  for   240 minutes

150°C/hr to    816  for   240 minutes

AFAP      to    482  for   120 minutes

45°C/hr   to    427  for   0 minutes

90°C/hr   to    370  for   0 minutes

180°C/hr to    50    for   0 minutes

Off  

Both these schedules assume the final thickness of the glass will be 6mm.  The schedule for glasses other than Bullseye only needs to have the top and annealing temperatures altered to the ones appropriate to the glass.

Results

The pressed glass will have the texture of the shelves on both sides.  Normally, no kiln wash will be stuck to the glass.  If there is kiln wash to be removed, you can do this by abrasive means – sandblasting, diamond pads, wet and dry sandpapers or Dremel style tools.  It is important to keep the glass damp during this process.

Untreated result of pressing

If the surface of the glass is without sticking kiln wash or other marks, you can use it with the matte surface without further kiln work.  You can also fire polish the piece to get a glassy surface, once you have thoroughly cleaned it.

Alternatives

Tape box together

After super gluing the bottom and side strips, you can bind the box together with clear Sellotape.  Pull off at least three strips of tape and set them where you can reach them easily.  Place the upper sheet on the prepared base. Move the box to the edge of the work surface so a little of the box hangs over.  The first stage is to place a strip of tape at right angles to the side to bind the top to the bottom.  Do this for each of the three sides.  When the top is securely attached to the base and sides tape along the length of each of the three sides. 

This shows on the lower left a loosened piece of sellotape on the edge of the sandbox.

This process avoids any difficulty in attaching the top.   Attempting to use only Sellotape to bind the box together is very difficult and requires at least three hands.

Spacers for the frit

Spacers do not always need to be strips on edge.  The spacers can be one or two wider strips placed on their sides to provide the needed height.  They can be coloured, forming a border; but remember the border will become curved. The strips will need to be glued to the back.  The top can be attached with super glue, or taped to the sides and back.

Pressing without a box

It is possible to use the pressing technique without a box or frit.  You can arrange clear and coloured cullet on the shelf.  The arrangement needs to be such that there are no gaps between the pieces.  This means that the glass will probably be 3 to 4 layers thick.  Be careful to avoid creating thick layers of dark colour by interfiling with clear. Place the spacers at the corners of the shelf for the thickness desired and fire.  The slower rate of firing (as for 19mm) should be used.

This sandbox process is a combination of arranging frits and pressing.

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

Overlaying Fibre Paper with Thinfire

Some people use a 1mm or 2 mm fibre paper with Thinfire or Papyros laid on top to get a smoother surface. It also allows assurance that the air can migrate from under the glass through the fibre paper. 

However, if the Thinfire or Papyros is laid down too many times, it fills the spaces between the fibres of the paper, and resists the passage of air.  This results in the large bubbles your are trying to avoid.  Experience will show how many times you can layer Thinfire without blocking the passage of air through the fibre paper.

This effect can also happen on sand beds.  The separating powders can build up and fill all the spaces between the sand particles. This resists the movement of air through the sand.  It leads to large bubbles just as compacted Thinfire on fibre paper does.

Is this practice of repeated layering of Thinfire and Papyros sensible?  The tipping point between achieving a smooth surface and creating bubbles cannot be known with certainty.  This means there is always a risk of bubble formation.

It is possible to use a vacuum sweeper with variable suction control  to remove the Thinfire or Papyros residue.  Using the lowest power and holding the end of the hose above the paper can lift the Thinfire or Papyros without affecting the underlying fibre paper.  Then a new sheet of Thinfire can be placed over the fibre paper to maintain the smooth surface.

Repeated layering of thin fibre papers can lead to a compaction that no longer allows the passage of air from under the glass during fusing. 

Single Layer Circle with Decorative Rim

A question arose:

If you fuse a single 20cm diameter sheet of 3mm glass to full fuse, [with a decorative rim] what happens? … Would the lack of two layers in the centre be a problem for the 6mm rule?

This layup risks trapped air and a large central bubble.  The explanation involves the combination of volume control and weight.

Volume control

The volume control relates to the single 3mm layer in the centre.  The glass will thin in the centre and thicken at the perimeter.  This leads to the risk of thinning to the degree that bubbles are created in the centre.  The edges will also draw in as the viscosity - surface tension - of the glass pulls the glass toward a 6mm thickness.

Weight

The explanation is also about weight.  The decorative rim adds weight to the outside of the piece.  This weight will “seal” the rim of the glass to the shelf, reducing the possibility of air escaping from under the central portion of the piece.  This weight effect on the rim increases the risk of a large central bubble.

Profile

Another influence on the result of the fuse is the degree of fuse.  At full fuse the viscosity of the glass is less and so resists the force of expanding air much less than when cooler. Even at rounded tack fuse, the glass will be unable to resist the formation of bubbles. As the glass thins and viscosity decreases, any air at all will cause a bubble.

Changes for the future

Avoidance of bubbles in this piece relate to design, scheduling and technique.

Design

It is possible to design a piece of this nature to avoid the volume control issue.  The base piece could have a smaller circle or rectangle centralised on top inside the proposed perimeter.  The rim can then have the decorative elements placed.  If they are spaced widely, frit can be used to fill significant gaps.  The piece can then be placed in the kiln for a full fuse.

Scheduling

You can also fire the piece as originally described very slowly to a low temperature.  This uses the concept of heat work. By applying the heat over a long period, you can achieve the same effect as would be achieved by a faster rate of advance to a higher temperature. 

There are at least two ways to increase the heat work.  You can use a very slow rate of advance to a point slightly above the softening point of the glass.  This will be the lower end of the slumping temperature range of your glass.  The soak may be for hours.  You will need to observe when the effect you want is achieved.

You also can choose the same lower slumping temperature and reach it in your standard fashion.  This will require an even longer soak time to achieve the same result.

In both these low firing approaches, you will need to observe to determine when the piece is finished.

Technique

The “flip and fire” technique may also work on the single layer with an added rim.  To do this you build the piece upside down on the shelf.  It helps to draw an outline of design on Thinfire, or Papyros.  Place the decorative elements and cap them with the clear.  Take the whole to a rounded tack fuse.  When cool, clean well and fire to a tack fuse again.  This will give something less than a full fuse, but it will be more than a tack, as the heat work is cumulative.

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

Summary

A single layer piece with a decorative rim is most likely to produce bubbles in the centre.  There are some ways to overcome this: design, scheduling, and technique. Design is the most likely to be successful.

Bubble formation

Question:

I had 2 kiln loads where every piece had huge bubbles. The 3rd time I did a test fire and put 2 pieces on Thinfire and one on the kiln shelf. The one without thin fire got a bubble. Theories?

Response:

Your experiment showed that the Thinfire prevented the bubbling.  The question you are asking is why.

Fibre paper is a porous material allowing air to move through it and from under the glass.  If the shelf has only slight depressions, the Thinfire or Papyros will allow air out from under the glass avoiding bubbles.

Thicker fibre paper can provide a different and more level surface if it is thick enough to span any depressions in the shelf, while allowing air out too.

First, it is apparent that your shelf is not absolutely smooth and level.  This has been shown by your experiment where Thinfire prevented bubbles where previously there had been bubbles.  It showed that without the additional cushion that the depressions, although slight are enough to cause bubbles without additional fibre paper separators.

Second, although you do not give your schedule, the firing is too hot.  There is dog boning of your thin glass.  The bubbles on the thinner glass have burst and thinned greatly.

Firing hot causes the glass to become much less viscous than needed to perform a full fuse and allows the trapped air to push bubbles into and through the glass.  Lower temperatures with longer soaks/holds enable the glass to better resist the formation of large bubbles.  Also firing more slowly enables air to escape and allows the use of lower temperatures while still being able to achieve the fuse you want.

More information is available in Low Temperature Kilnforming, an evidenced based guide to scheduling.