Glue in Kilnforming

 

There is some general guidance on using glue in kiln forming processes.

Avoid Glue Altogether

Do not use glue of any kind if at all possible. First look at other ways of stabilising the pieces. You can place clear frit or powder around or under the unstable pieces. Of course, if you are firing to less than a contour fuse, this will show. If the pieces are rolling, you can grind a flat spot to keep them stable.

Use Minimum Amounts

Use as little as possible if there is no other way to stabilise the pieces until you get them to the kiln. Use weak glues. Dilute the glues if water based. Place only a small dot of glue at one place.

Use Care in Placing

Place the glue at the edge of the glass pieces, not underneath. This allows the glue to burn out cleanly. Placed in the centre, the glue burnout is trapped under the middle of the glass. This leaves a black mark or a big bubble.

Avoid Glue with Additives

Use no glue containing additives. Many of these additives will remain after the adhesive part of the glue has burned off. These will promote devitrification.

Some Popular Glues

PVA also known as wood glue, white glue, carpenter's glue, school glue, Elmer's glue in the US, or PVA glue. This boils at 112°C/234°F.

         

Super glue and other cyanoacrylate glues have a boiling point 54-56°C (129-133°F).

Lacquer and hair spray have boiling points around 189°C/372°F)

Aloe vera gel has an ignition point of about 232°C/450°F. So, its boiling point is even lower than CMC.

CMC (carboxymethyl cellulose) includes wallpaper paste, Vitragel and most fusing glues. These have boiling points around 260°C –270°C (500°F –518°F).

Xanthan gum is a thickener sometimes used as a kilnforming glue. It boils at 311°C/592°F.

Proprietary kilnforming glues are generally without additives and diluted from the concentrate with demineralised water. They also boil off in the same range as CMC.

All commonly available glues evaporate well below the “sticky” range of glass. You cannot rely on them to hold the glass in place until the glass tacks together with the heat.

Quickly fired glue - wet or dry - boils. Sometimes with enough force to move the glass significant distances. So, slow down the initial ramp rate.

The general observation is that if the glass will not stay in place without glue, it will move during the firing.

Glue is only useful to stabilise pieces in moving the whole assembly to the kiln. Where possible, build the piece in the kiln without glue at all.

Best of all, use no glue.

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.

Glue boiling

People often find that their glued pieces shift in the firing.  Sometimes dramatically. This can be from placement or boiling of the glue.

Almost all commonly available glues evaporate well below the “sticky” range of glass. 

The boiling points of some of the popular glues:

PVA (also known as: wood glue, white glue, carpenter's glue, school glue, Elmer's glue in the US, or PVA glue)  

Boiling point: 112°C (234°F)

Super glue (and other cyanoacrylate glues) 

boiling point 54-56°C (129-133°F)

Lacquer (hair spray)

Boiling point: 185 to 189°C (365 to 372°F)

CMC  (carboxymethyl cellulose includes wallpaper paste, vitragel and most fusing glues)

boiling point: 260–270°C (500–518°F)  

melts at 274°C 

Aloe vera gel  

ignition point: ca. 232°C therefore, its boiling point is lower.

This shows the popular glues used in kilnforming are not effective above 300°C and many with much lower boiling points.  This means that glues only hold glass in place when cold. Glues may be useful in getting items with many pieces into the kiln, but they will not hold them in place until the glass becomes sticky.  The glass, in general, needs to be at or above the slumping temperature to begin to stick together.

If glue doesn’t work, what can I do?

Other means than glues are required to support the moveable items until the glass become sticky at around 620°C (about 700°C for float glass).

If placing frit supports is not possible without showing, such as in a tack fuse, you can use mechanical means. Two of these are grinding flat spots on rolling pieces; and placing supports under the balanced items. Other support and damming methods will depend on the nature of the project.  In general, if the pieces will not stay in place without glue before placing in the kiln, the pieces will move in the kiln after the glue evaporates.

This evaporation can be so fast as to be called boiling.  Glue boils off in the kiln whether dry before firing or not. Sometimes there is enough glue or rapid enough heating to cause displacement of the glass by the force of the boiling. Just as in boiling water, the evaporation can be explosive.  The force of the built-up pressure of glue trapped under glass can move small pieces relatively large distances.

How do I avoid boiling the glue?

• Use as little glue as possible.
• Use it at the edges of the pieces.
• Do not place it in the middle of large pieces.
• When you do use glue, advance slowly to at least 300°C allow the glue to evaporate, rather than boil.  I’d suggest a rate of 50°C per hour would be slow enough to avoid the boiling of sparingly applied glue.

Best of all, use no glue.

Quickly fired glue - wet or dry - boils.  Sometimes with enough force to move the glass significant distances.  Avoid gluing as much as possible and use sparingly when needed.

Glueing Glass Pieces

The best solution is to avoid the use of glue completely. If you cannot, use as little as possible and make sure it burns out cleanly.

The glues to which kiln workers have normal access, do not survive to tack fusing temperatures. Therefore they can only be considered as a means to get the glass assembly to the kiln. The glue will not hold the pieces in place until the glass begins to stick, so the pieces must have a stable placement. If not, the pieces will slip, roll and move once the glue has burned out.

The second requirement of glues is that they burn out without leaving a residue.

Glues that have been used with little or no residue include:

Powdered CMC that can be disolved in warm water

-CMC (carbylmethylcellulose). It is a cellulose based binder used in a wide variety of industries, including food. For our purposes, it is also used in the ceramics industry and is often called glaze binder. It is a main constituent of "glas tac" from Bullseye. This can be made up into a viscous solution to catch and hold frits and other sprinkled elements in place.

- PVA (Polyvinyl Acetate) is water-based glue. It is sometimes known as school glue. It can be diluted to about 10parts water to 1 part PVA. This is sufficient to hold the glass pieces together with only a drop for each piece of glass. It does not work so well for small sprinkled elements.

One of many brands of  Ethyl Cyanoacrylate
 glue

- Super glue burns off with no concerns about cyanide. It should be used sparingly and also works best for pieces of glass.

One of many hair laquers in pump spray bottles

- Hair lacquer is normally applied as drops at the edges of the assembled pieces and so can be used to hold pieces of glass as well as sprinkled elements.

In all uses of glue the principles to remember are:
- Use the minimum to hold pieces together while getting the work into the kiln.
- Put the glue at the edges of the glass or where its combustion gasses can escape easily.
- And in all cases, you need to test to see if a residue is left on the glass at full fuse when using a new glue.

An alternative to glue is frit as described here.

Super Glue - Kiln Forming Myths 29

The use of super glue in the kiln causes cyanide gas

This is not true.  But because it is such a persistent belief, a lot of detail is given below.  In short the precautions are: 

• use the minimum amount, 
• use an organic gas face mask, 
• do not wear natural fibres or gloves, 
• let the glue cure before placing it in the kiln, 
• have the solvents at hand while using the glue.

Super Glue Safety

Super glue is frequently used as a temporary fixative in assembly of kiln forming projects. There is some concern about safety, as it is known that super glue is made from cyanoacrylate, which it is feared will break down in the kiln into cyanide gas.

Greg Rawls, a certified industrial hygienist says

"I looked at the MSDSs for several forms of super glue. The main component is Ethyl 2-cyanoacrylate, which has a TLV of 0.2 ppm which is relatively toxic. [However,] the thermal decomposition products are carbon monoxide and carbon dioxide. I did not see a reference to cyanide gas. However, as I recall cyanide gas dissociates into elemental carbon and nitrogen at about 800 F. Since you use it in such small quantities, I would not worry about it. In my opinion the worst thing that could happen is you glue your fingers to the glass."

Safety issues

To treat the safety issues seriously and determine if you feel Greg Rawls' view is justified, you need to look at the issues of toxicity, reactions, adhesion of tissue, ventilation, first aid and decomposition products in the whole context.

Toxicity

The fumes from cyanoacrylate are a vaporized form of the cyanoacrylate monomer that irritate sensitive membranes in the eyes, nose, and throat. They are immediately polymerized by the moisture in the membranes and become inert. These risks can be minimized by using cyanoacrylate in well ventilated areas. About 5% of the population can become sensitized to cyanoacrylate fumes after repeated exposure, resulting in flu-like symptoms. It may also act as a skin irritant and may cause an allergic skin reaction. On rare occasions, inhalation may trigger asthma. There is no single measurement of toxicity for all cyanoacrylate adhesives as there is a wide variety of adhesives that contain various cyanoacrylate formulations.

The United States National Toxicology Program and the United Kingdom Health and Safety Executive have concluded that the use of ethyl cyanoacrylate is safe and that additional study is unnecessary. 2-octyl cyanoacrylate degrades much more slowly due to its longer organic backbone that slows the degradation of the adhesive enough to remain below the threshold of tissue toxicity, so the use of 2-octyl cyanoacrylate for sutures is preferred.

Reaction with cotton

Applying cyanoacrylate to some materials made of cotton or wool results in a powerful, rapid exothermic reaction. The heat released may cause serious burns, ignite the cotton product, or release irritating white smoke. Users should not to wear cotton or wool clothing, especially cotton gloves, when applying or handling cyanoacrylates.

Adhesion of the Skin

Various solvents and de-bonders can be used. These include:

·         Acetone, commonly found in nail polish remover, is a widely available solvent capable of softening cured cyanoacrylate

·         Nitromethane

·         Dimethyl sulfoxide

·         Methylene chloride

Commercial de-bonders are also available.

Warnings include:

·         It is a mild irritant to the skin.

·         It is an eye irritant.

·         It bonds skin in seconds.

·         Any skin or eye contact should be copiously flushed with water and medical attention be sought immediately.

·         Do not attempt to separate eye tissues – the bond will separate naturally within a few days.

Precautions

·         Use goggles.

·         Do not wear cotton or wool clothing while using super glue

·         Ventilate the area well. Since cyanoacrylate vapours are heavier than air, place exhaust intake below work area. Activated charcoal filters using an acidic charcoal have been found effective in removing vapours from effluent air so the bench top air filters are suitable for use while using super glue.

·         Avoid use of excess adhesive. Excess adhesive outside of bond area will increase level of vapours.

·         Assemble parts as quickly as possible. Long open times will increase level of vapours.

Evaporation Effects

·         The effects of heating cyanoacrylate are not completely known. The flash point is known to be greater than 85ºC. As a precaution do not remain in the area of the kiln after that temperature has been reached.

·         The decomposition products are carbon monoxide and carbon dioxide. There is no reference in the literature to cyanide gas. It is highly unlikely that heat will cause the release of cyanide gas at any time during the heating. To be certain, you should make sure the evaporation of the glue is complete before firing the kiln.

See this tip for the use of super glue in kiln forming. http://glasstips.blogspot.co.uk/2013/12/super-glue.html