Removing Bubbles

Inclusions and Bubbles

The inclusion of material between two or more sheets of glass has the risk of creating bubbles.  The size of these often relate to the size of the inclusion.  The inclusion can be glass (powders, frits, cut pieces), mica, metals, foils, etc.

The important element in eliminating bubbles is to have a long slow bubble squeeze from the bottom of the forming temperature to the top slumping temperature.  If this is combined with supports at the edges or a fine film of clear powder, it will help reduce the interior bubbles to a minimum.  The supports at the edges may be as small as fine frit (and some use powder over the whole surface).

But, once you have bubbles in the piece, what can you do?

You can drill a hole in the bubbles, or break the bubbles and fuse again, but there will be distortions visible in the resulting piece.

Another method to reduce the effect of bubbles, is to flip the piece and fire upside down to drive the bubbles to the bottom of the piece.  Be careful to use low fusing temperatures to avoid enlarging the bubble.  At the finish, the bubble will still be in the glass but will not be protruding above the top surface.

It may also be possible to combine the two processes.  Drill a small hole in the bubbles and fire upside down.  If you do this you need to place the glass on porous fibre paper, not just Thinfire or Papyrus, to allow the air to be compressed out of the bubbles.  You also need to allow a significant amount of time around the slumping temperature for this to happen.

Once you have fired upside down, you will need to fire polish the surface again. Do not despair at multiple firings.  A lot of people fire their pieces many times to achieve the effects desired.

Mica - Kiln Forming Myths 23

Mica will not stick to glass unless it's capped with clear.

Almost by definition, any material that needs to be encased, does not stick to glass.

However, mica does stick to glass.  But it is only the surface that is in contact with the glass that sticks.  Mica shears into very fine sheets and particles (almost microscopic), meaning that there many layers of mica even with a thin layer.  So only a minor portion of the mica you sprinkle, sift or paint onto the glass can stick. 

It is possible to add a flux such as borax to the mica solution to soften the surface of the glass, allowing more mica to sink into and stick to the glass.

Of course you can encase much more mica than will stick to the surface.  However, you have to be very careful about avoiding bubbles.  There is so much air (relative to the volume of the mica) that bubbles in encased mica is a constant problem.  Very good bubble squeezes and supporting the edges on shards of glass to keep the glass open while beginning to slump are required.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Organic and Mineral Inclusions

Encasing organic materials adds a new level of complexity to inclusions.  In addition to bubble formation, you need to consider how to eliminate the combustion gases from the vegetable matter.  On the other hand you don’t need to worry about expansion differences.

Some examples of metal and glass inclusions

The requirement is to burn out all of the vegetable matter to avoid creating big bubbles from the burn off of the material.  There are two elements to this burnout.  One is the amount of moisture contained in the object and the second is the volume of dry material that has to burn out.

Drying

Unless you have dried the material before including it, you will need to leave enough soak time before the glass begins to move to ensure all the water is removed.  It is also advisable to place small shards of glass at the corners of the piece, to allow easy ventilation both for the moisture to evaporate and the vegetation to burn easily.  You can estimate the time required and then put a witness piece of glass or better mirror above the vent or peep hole to see if there is any fogging on the glass from escaping moisture.  You need to continue soaking until there is no fogging.

Burnout

The second element is to give the vegetable matter enough time to burn out.  The burn out should occur at about 400°C.  This is high enough to ignite carbon based materials, but not so high that an extended soak will allow the glass to sufficiently deform to seal the un-burnt material inside.  If you have a really good sense of smell you can tell when the carbon has burned away by the absence of the smell.  For the rest of us, we need to open the peep hole and use a strong light to tell how much is left to burn away.  The burning is much more like a smouldering with very little light coming from it.

An incompletely burned out leaf in a large trivet with felt feet at the corners. 150mm square

The length of time you need to soak below the softening point of the glass is directly related to both the water content and the amount of combustible material you have included.  The burning will not begin until everything is dry.  If the material is not dry, the time for this needs to be added to the burnout time.  The length of soak for burnout is much more difficult to determine and needs periodic observation beyond the time when the smoke stops coming out of the kiln ports.

Bubble squeeze
Once the drying and burnout are completed, you need to advance to the bubble squeeze.  This will need to be longer or slower than usual to ensure all the combustion gasses are out for organic materials.  Minerals will normally be thicker than the organic materials and so need long bubble squeezes. These can be at or just below slumping temperature, or a slow rate of rise, taking an hour or more, from about 50C below the slumping point.

It is possible to include other minerals such as bone, or ash, or other inert particles that will not stick to the glass. Materials that contain silica are not suitable, as they stick to the glass and cause breakages.  So most stone, which contain silica, however thinly sliced will not be suitable as an inclusion.

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.

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.

Bones as Inclusions in Glass

The major components of bones are calcium and organic materials making up the marrow. If the bones are not old and weathered a very bad smell will be produced. The organic material will cause bubbles. Finally, it takes a long time to burn out the marrow, so it is best to use bones that have weathered for a number of years.

Calcium “erodes” during firing, so fine and thin bones will leave a shadow of ash (or a big bubble if there is not a long bubble squeeze. The bone has to be encased or trapped by the glass as it will not stick permanently to the glass on its own.

It can make dramatic shapes if the bones are arranged in novel ways to represent other things. The whole of the bone does not need to be encased, as the thicker parts will be strong enough to support themselves.

Wire for Hanging

The most common wires used for inclusion in fused objects are copper, brass, nickel/chrome, stainless steel and sterling silver.

The strength of the wires – strongest to weakest - seem to be in the order of stainless steel, nickel/chrome alloy, brass, silver, copper. The metal you choose will be related to the weight of the piece, the available thickness of wire, and aesthetics.

All of these are subject to fire scale or fire stain, a blackened surface on the wire. This can be removed by abrasive cleaning of the exposed metal. The metal within the glass most often takes up the fire scale too. This can be reduced by thorough cleaning of the metal before enclosing it in the glass. Coating the metal with a flux such as borax often reduces the incidence of the fire scale too.

The techniques of cleaning the fire scale from the metal range from scrubbing and polishing to tumbling. The tumbling has the advantage of hardening the softer metals such as copper, and silver.

Copper looses much of its strength in the firing, and often needs gentle working to stiffen it. This is where tumbling is so useful.

Pure silver normally leaves a yellow stain on the glass. Sterling silver - an alloy of copper and silver – is less inclined to do this. However the exposed wire will stain the shelf and any subsequent glass unless well supported by 1 mm or more of fibre paper.

It is common in silversmithing to pickle silver to remove the fire scale after any heat work.

Metal supports

It is often desirable to have supports that terminate inside the glass rather than clasping or otherwise holding the glass. However, metal that would survive the firing and be strong enough to support a substantial piece would be of such a size that it would break the glass due to the differing expansion and contraction of the two materials.

The wire or rod to support the fused or cast piece does not have to be incorporated at the point of kiln forming. There is the risk of the glass breaking due to the large differences in expansion and contraction of the metal and the glass, especially with the harder metals.

Instead you need to plan for these supports and keep the glass open at the support points during the kiln forming. It is relatively easy to wrap short pieces of appropriately sized stainless steel rod with fibre paper, or coat with kin wash and build the glass around these, keeping one of each of the ends free of the glass.

You can of course, use other metals, although most – except brass – are likely to spall quite a bit, so wrapping them with fibre paper is best.

When the kiln work is finished, the short rods are pulled out.

You need to clean the cavity formed for the rods. If the glass is transparent or translucent, I like to have the cavity as clear as possible, so I prefer wrapping the rod with fibre paper what ever metal is being used. This provides a clearer surface when clean. Kiln wash leaves a white deposit that is difficult or impossible to clean away completely. Of course if the glass is opalescent, it does not matter whether there is a white deposit.

The cavity does need to be clean to enable the glue to stick to the glass. The support rods need to be secured with a silicone or other flexible glue to avoid any expansion problems in the future.

Information on inclusions of metals:
copper
Wire
Silver foil

Copper Backings

“Is it possible to to fuse copper to the back of glass?

The easy answer is - no.

But it can be done. There are a number of conditions that will help.

The copper needs to be thin and flat. It works best if you clean the copper of any oxidisation, 
and then coat it with borax or other devitrification spray that can act as a glass flux.

The fusing has to be done with a long soak to ensure the bottom of the glass is as soft as the top to assist the attachment of the copper. The devitrification solution will help soften the glass next to the copper sheet. You also have to protect your shelf from contamination by the copper sheet. This can be done by using 3mm fibre paper under the copper.

Not all attempts will be successful, showing that this process is on the edge of acceptability.

It is easier simply to glue the copper to the back.

“Does it matter whether it has been fused already?”

The glass does not have to be fused prior to attempting to attach the copper to the back. If it has been fused, you need to run a slower schedule than when fusing glass for the first time. A schedule for slumping, but with a higher target temperature – at least fire polishing – will be required.

Keeping Wire Hangers in Place

Even though it is normal to place the wire between two layers of glass, it often moves from its original placing.

Using glue only keeps the wire in place while moving the piece(s) to the kiln. The glue will burn off at just under 500C, which is before the sticky point of glass, so it cannot hold the wire in place at the critical temperature – from about 700C. In fact, if you fire quickly the glue can “boil” and cause the wire to move.

There are a variety of methods to help keep the wire where you placed it. Some of them follow.

You can try weighting the wire down with small scraps of glass to keep the wire in place until the glass sticks to the wire. The scrap will often form a small bead that can be used in other projects. Sometimes though, the scrap sticks to the wire.


Another method is to place a small piece of 3mm fibre paper under the wire to support it during the firing. This will be enough to keep the wire from moving, and the scraps of fibre paper can be reused many times.


You could also bend the wire loop so that the end touches the shelf. The part in between the glass needs to be flat with the bend starting after the wire emerges from the glass. You can bend the wire straight after firing.


Flattening the wire by tapping the wire – placed on an anvil – with a hammer will reduce the possibilities of movement, and certainly any rolling possibilities. It will also have a greater area of contact with the glass.


You can also make a shallow groove in the glass where the wire is to go. This can be done with a Dremel type tool with a diamond bit, or on the small diameter bit on the top of a glass grinder.
Lay the glass in the groove and cap with the top piece.