Sticking Fiber Paper

People are reporting different behaviours of their thicker fibre papers such as small fibres sticking to the glass after a fuse, and a different smell from the burning binders.  These are most likely to be from a body soluble refractory fibre paper.

It seems more suppliers are selling the body soluble versions of fibre paper. It sticks to glass and it gives off a smell of volatile chemicals. I don't like it, but I may have to use it due to the unavailability of the more health risky refractory fibre that worked very well without so much sticking.

There are several ways to minimise the fibres sticking to the glass.  They all relate to adding a separate coating of separator to the fibre paper before firing.  Among the coatings that can be used are 

• shelf paper on top, 
• a kiln wash solution brushed on, 
• kiln wash powder dusted over, 
• sprinkled alumina hydrate, and 
• boron nitride (Zyp is one brand name).  

Others have found that simply soaking the fired glass in water overnight allows the fibres to be brushed off with stiff brushes.

It seems body soluble refractory fibre papers tend to stick to the glass at anything over low temperature tack fuses.  This requires an additional layer of separator to be applied over the paper.  It is each person’s choice, of course, but I will continue to attempt to get the older version of fibre paper.

Refractory fibres

 

There is a lot of imprecise terminology for refractory fibre paper and blanket.  My interpretation:

Shelf paper is a very thin - like cartridge paper - material held together with organic binders, and often containing fibreglass particles.  Thinfire and Papyros are two brand names.

Thinfire. photo credit Warm Glass

Fibre paper is rougher than shelf papers. The fibres are longer and not compressed so much. They seem to be available in 2-6mm thicknesses and are held together with organic or chemical binders.

Refractory fibre paper.  Photo credit Laurel Refractory

Fibre blanket tends to be uncompressed fibre from 12-75mm thick.  It relies on the interconnected fibres rather than binders to keep its thickness.

Refractory fibre blanket. photo credit Amazon

More information in this blog post. 

Multiple Layers of Kiln Wash and Fibre

Recently, there have been confirmations of multiple of layers of kiln wash on the shelf under pot melts, frit stretches, and flows.  Ten, and even sixteen layers are mentioned. Also reported are two layers of 1mm fibre paper as a separator for the same processes.

These practices are excessive and wasteful.

 

Kiln wash   

·         Once fully covered, the shelf does not need additional layers. 

·         Stir the kiln wash mix each time you dip the brush.

·         Apply thinly.

·         Use only enough coats to evenly cover the shelf.

 

Fibre and shelf papers are not recommended to be placed on the shelf for high temperature processes.

·         The shelf papers can become incorporated within the glass as it moves along the shelf.

·         Fibre papers inhibit the movement of the glass in unpredictable ways.

 

If you do use fibre papers anyway:

·         Use only one layer.

·         Place a clear sheet of glass over the fibre paper to allow better flow during the firing.

·         A disc of clear glass also helps to separate opalescent glass from the shelf.

 

More layers of kiln wash or fibre paper does not make a better separator.

Is there a best separator?

 Is there a best separator?

Kilnforming separators

Separators for kiln forming come in various forms. Chemicals, liquids, sprays, refractory fibre paper, kiln wash, and others I suppose. Which is best?  Each separator has its uses. No one is useful in all circumstances.  Some will be best for one circumstance and others for another.

Boron nitride

Boron nitride (BN) is a high temperature lubricant. It can be sprayed or brushed onto the mould. It adheres to smooth non-absorbent surfaces.  BN is among the most expensive of separators for glass. It seems most useful on detailed, texture and casting moulds. BN is often recommended for steel moulds as it adheres to it better than kiln wash.  Although kiln wash will work as a separator on steel, boron nitride is easier to apply.  Various conditions apply to its use. 

Kiln wash

Kiln wash works well on slightly absorbent surfaces – ceramic moulds, and shelves, for example. It is the least expensive form of separator.  It is shipped as a powder to which five parts water is added to one of the powder.  This makes a liquid that can be applied to any appropriate surface.  It can be sprayed or brushed. The mix can be with less water on very absorbent surfaces, showing some of its flexibility. 

Almost all kiln washes contain kaolin which helps keep the alumina hydrate in suspension.  But most importantly, allows the solution to be applied evenly.  However, the same kaolin also slowly changes to a crystalline substance by 900ºC/ 1650º that sticks to glass. It needs to be re-applied after every full fuse.

Refractory fibre paper

Shelf paper works well on flat surfaces and simple moulds.  It is a moderately priced separator.  Two of the popular trade names are Papyros and Thinfire.  They both contain alumina hydrate but with different binders.  They provide a smooth surface for the shelf and cylindrical shapes. They are not so good at separating glass from irregular surfaces and incised details.  The shelf paper disintegrates after firing. Although it can sometimes be used several times if undisturbed.  The resulting powder is an irritant and should be disposed of carefully.

There are thicker refractory fibre papers.  These normally range from 0.5mm to 6mm.  Thicker versions are called blanket.  These have the same characteristics as shelf papers, although coarser.  They also do not use binders to keep them together.  These are most useful in forming moulds and insulating glass from rapid temperature changes.

 

The general statement is that there is not one separator that is best in all circumstances.  Each has its strengths.  Knowledge of the objective of the firing and its conditions will help in choosing the right one.

Fibre Paper Re-use in Kiln Carving

“I would like to use 1/4 inch Fiberfrax to impart texture on the back of transparent glass.  Is there a way to make it reusable?  I tried mould hardener on a small piece of it, but the hardener wasn’t absorbed.  I’m afraid the fiberfrax will lose its structure if I pre-fire it (to burn out the binders) before removing it to apply hardener.”

 

It is difficult to reuse refractory fibre paper after moving it between firings, but not impossible.  I have used two processes. One is to place the glass over the cut fibre paper. This works for small pieces. The fibre paper was placed on thinfire to allow air migration out. I used a long low temperature bubble squeeze to ensure the binder was completely burned out.

The other arrangement I have used for larger pieces.  This is to assemble and fire the fibre paper to burn out binders.  There is a large chance that not enough air will get to the centre of the fibre paper when large glass is placed on top.  Binder not burned out leaves a brown mark on the fibre paper and stains the glass grey. Turn off the kiln once all the binder is burned out as evidenced by the paper returning to white.  As soon as the temperature in the kiln is comfortable, you can place glass on top of the fibre paper.  It is strong enough that it will not be compacted by the weight of the glass.

Using un-hardened fibre

But there is no logical reason for these processes, although they work. Firing to about 500ºC/930ºF with a suitably long soak will clear those gasses before the forming of the glass begins.  You will know when there no longer is a smell of burning paper, or on more recent fibres a chemical smell.  Make sure you vent the kiln during this burnout to allow the smoke to escape.  For a large area, a soak there may need to be hours long.  Another check is when the fibre has turned white again, the binders are gone.  A further protection against bubbles in any area is to place the whole assembly on a bed of fibre paper.

It is possible to use un-hardened refractory fibre without a separator, as the older versions do not stick to glass easily.  However, if you are using the current eco fibres, they will stick in many areas.  Kiln washing any fibre before firing is the best protection against lengthy clean ups.  It also allows the best chance to remove the un-rigidised fibre for re-use.

A smoother surface can be given to the refractory fibre, if you want. Do this by smoothing powdered kiln wash over the bed layer and any other layers the glass will be touching.  An alternative to powdered kiln wash is to put Thinfire or Papyros cut to shape over each layer.

After firing, slide the fibre onto cardboard or another flat stiff surface. Then place into a large pizza or similar box. I have stored fibre in such a way for several firings.

Using hardened fibre

Of course, the fibre can be hardened by use of colloidal silica.  Make up the whole stack of fibre paper for the kiln carving.  Harden the whole stack at once. This helps to bind the layers together.  Brush on the hardener to the exposed part of each layer.  Cover both horizontal and vertical surfaces. Hardener does take time to soak into the fibre paper.  Give it time.  You can add more hardener at intervals.  Be careful to avoid overdoing it.  Fully wet fibre is difficult to move and takes a long time to dry - days.  The objective is to harden the surface of the fibre, not to harden the whole by soaking it.

Allow the carrier of the hardener to evaporate for hours or a day.  When you can move the fibre, fire to at least mid-700’s ºC/ 1300ºF to 1400ºF.  After firing, it must be covered with significant amounts of kiln wash. This can be as a liquid or as powder. I prefer liquid.  The kiln wash is required over all edges and surfaces to keep the glass from sticking to the fibre.

Storage of the rigidised fibre paper can be in the same way as for the fibre without binders or hardener.

Damming for Exact Shapes

 Many times, exact dimensions of the final piece are not critical.  When they are and the piece is 9mm and thicker, or has irregular amounts of glass near the edge, damming is required.

 If the dimensions are rectangular, you can use straight edged refractory materials, usually sawn up broken kiln shelves, vermiculite, or fibre board strips.  

 These need to be kiln washed and lined with fibre paper.  The dams should be lined with 3mm fibre paper that is 3mm narrower than the final height of the piece.  This allows a bullnose shape at the edge to form.

 If the shape is a circular or irregular shape the dams can be made from thick fibre board or vermiculite.  The lining of the dams is the same as for rectangular shapes.  

 The use of 3mm fibre paper means that you have to make rectangular shapes 6mm bigger in each direction to achieve the exact final dimensions.  For circular or irregular shapes, the edge will need to be only 3mm larger.  This is because the edge goes around the whole shape, rather than only one side.

 

Mineral Wool Fibres

Refractory Fibres

The general name that includes refractory fibre is mineral wool. It is any fibrous material formed by spinning or drawing molten minerals and ceramics.  These are used as thermal insulation, filtering, soundproofing and as a hydroponic medium, in addition to high temperature insulation as in kilnforming and furnaces.

The initial manufacture of mineral wool was in Wales in the mid-19th century, but the process was so dangerous that it was abandoned. The first commercial production was in 1870’s Germany, manufactured by blowing air through a fall of molten slag metal.  At the end of the century an American developed a technique to turn molten rock into fibres, so initiating the rock wool industry.  The high temperature versions were developed during the second world war, but not commercially available until the 1950’s.

Current manufacturing involves a flow of molten minerals (at ca 1600°C) through which air is forced.  This creates fibres of amorphous structure that can be compressed together without binders.  More advanced production rapidly spins molten minerals similar to the production of candy floss, or cotton candy. This results in a mass of fine, intertwined fibres with a typical diameter of 2µm to 6µm (microns).

Credit: Knauf.com

High-Temperature Mineral Wool

High temperature mineral wools are rated for about 650°C to 1600°C and are made in similar ways to the lower temperature versions.  However, they are more expensive and so are used in refractory circumstances including kiln forming.

The three main types of HTIWs include:

Low Bio-persistent (LBP) Wool, including Alkaline Earth Silicate (AES) wools and others:

Alkaline earth silicate (AES) wool

       Calcium magnesium silicate wool

       Calcium silicate wool

       Magnesium silicate wool

Alkali metal silicate (AMS) wool

       Potassium alumino silicate wool

Alumino Silicate Wool (ASW), also known as Refractory Ceramic Fibres (RCF)

       Aluminium silicate wool

       Aluminium zirconium silicate wool

Polycrystalline Wool (PCW)

       Aluminium oxide wool

       Mullite wool

The main forms that kilnformers are interested in are blanket, paper and board.  The paper and board normally contain binders ranging from latex to cellulose. There are other forms: bulk fibres, modules or blocks formed ready for installation, vacuum formed shapes, cement mastics, textiles, yarns and ropes.

A brief description of these kinds of refractory mineral wools are:

Alkaline earth silicate wool (AES)

AES wool consists of amorphous glass fibres that are produced by melting a combination of calcium, magnesium oxides and silicone dioxide.  Products made from AES are generally used in equipment that continuously operates and in domestic appliances. AES wool has the advantage of being bio-soluble—it dissolves in bodily fluids within a few weeks and is quickly cleared from the lungs and so has been excluded from carcinogenic classifications. It is generally rated up to 1200°C.

Alumino silicate wool (ASW)

This is also known as refractory ceramic fibre (RCF), again consisting of amorphous fibres produced by melting minerals and blowing air across the flow.  In this case, a combination of aluminium oxide and silicon dioxide.  It has a low thermal conductivity, and good resistance to chemicals. Alumino silicate wool is generally used at temperatures from 600°C to 1300°C  for intermittent operation, making it good for kilnforming. 

This was classified in Europe as a carcinogen category 2 – “Substances that should be regarded as if they are carcinogenic to humans” under the Dangerous Substances Directive in 1997. This was translated under CLP Regulation into a carcinogen category 1B “Known or presumed human carcinogen; presumed to have carcinogenic potential for humans, classification is largely based on animal evidence”.

https://www.ecfia.eu/materials-alumino-silicate-wool-asw/

Some of the trade names used are:

• Kaowool®, a high-temperature mineral wool made from kaolin. It was one of the first types of high-temperature mineral wool and continues to be used. It can withstand temperatures to 1250°C. 
• Cerablanket®, is a spun blanket manufactured from a high purity blend of alumina-silica and is classified up to 1315°C.
• Cerachem® and Cerachrome® provide chemical stability and strength and have acoustic as well as thermal insulation characteristics. They are classified to 1426°C.

There are bio-soluble fibres produced under trade names such as Superwool^® with temperature ratings of 1300°C and 1450°C.  Superwool® fibres are exonerated from carcinogen classification within Europe and not classified as hazardous by IARC or under any national regulations throughout the world.

Polycrystalline wool (PCW)

Polycrystalline wool was commercialised in the 1970’s and consists of fibres that contain more than 70% aluminum oxide. It is produced by sol–gel method from aqueous spinning solutions. The water-soluble green fibres obtained as a precursor are crystallized by means of heat treatment. This is produced in small quantities for specialised applications.  Its characteristics are that the fibres are of regular defined dimensions, it is chemically and thermally stable, with low shrinkage and high tensile strength, all with less dust produced in handling.  It is a more expensive process than producing RCW papers and blankets.

The polycrystalline wool is generally used at temperatures above 1300°C.  One trade name is Denka Alcen with a temperature rating up to 1600°C. Denka blankets are more resistant to acid and alkaline solutions than conventional alumino-silicate fibre blankets and have good thermal insulation characteristics.

Other than kilnforming, applications are in the ceramics, metals, petrochemicals, aerospace and automotive industry sectors. Typical PCW applications include use as support mats in catalytic converters and diesel particulate filters to reduce exhaust emissions, and as insulation in industrial high temperature furnaces for energy conservation, particularly in high temperature and/or chemically aggressive environments.

https://www.ecfia.eu/materials-polycrystalline-wool-pcw/

Credit: Alibaba.com

Kilnforming Refractory Papers

There are two fibre papers widely used in kilnforming: Papyros and Thinfire.  These are special cases of the RCF papers and deserve particular attention, although they are subsets of the previously described RCF wools.

Papyros

This is a fibre paper similar in thickness to cartridge paper.  It consists of  aluminium hydroxide, hydrated magnesium silicate (hazard classification: irritant), alumina borosilicate glass (hazard classification: irritant), wood pulp and resin (both binders).  None of the materials used in the composition of Papyros are classified as a possible carcinogenic substance.  It is recommended that eye, breathing and skin protection be used when handling the fired residue to reduce any irritation.  Washing after handling the dusts is recommended.

Thinfire

This fibre paper is also like cartridge paper in thickness and has a slightly finer texture than Papyros.  Its constituents are aluminium hydroxide, glass fibre, polyvinyl alcohol, cellulose, and polyamide resin.  Only the glass fibre is classified as an irritant.  The dust can be an irritant to eyes and skin.  If either are irritated, wash with large amounts of water. It is sensible to use breathing protection while handling the fired residue.

The materials used place both these fibre papers in the AES group of refractory fibres, which are biosoluble.  The use of hydrated magnesium silicate in Papyros gives an extremely small increased health risk over Thinfire.

https://occup-med.biomedcentral.com/articles/10.1186/s12995-019-0235-z

Credit: cdc.com

Fibre Paper – Health and Safety

Mineral wool fibres and refractory ceramic fibres have been  classified as "possibly carcinogenic to humans" (Group 2B).  In contrast, the more commonly used vitreous fibre wools produced since 2000, including insulation glass wool, stone wool, and slag wool, are considered "not classifiable as to carcinogenicity in humans" (Group 3). The International Agency for Research on Cancer (IARC) elected not to make an overall evaluation of the newly developed fibres designed to be less bio-persistent such as the alkaline earth silicate (AES) or high-alumina, low-silica (ASW) wools. 

Bio-soluble fibres are produced that do not cause damage to the human cell. These newer materials have been tested for carcinogenicity and most are found to be non-carcinogenic.

Due to the mechanical effect of fibres, mineral wool products may cause temporary skin itching. To diminish this and to avoid unnecessary exposure to mineral wool dust, information on good practices is available on the packaging of mineral wool products with pictograms or sentences. Safe Use Instruction Sheets like safety data sheets are also available from each producer.

People can be exposed to mineral wool fibres in the workplace by breathing them in, skin contact, and eye contact. … The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 5mg/m3 total exposure and 3 fibres per cm3 over an 8-hour workday [the highest existing standard].  The equivalent European standard is set by the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH).

AES, ASW and PCW have been registered before the first EC deadline of 1 December 2010 and can, therefore, be used on the European market.

ASW/RCF is classified as carcinogen category 1B.

AES is exempted from carcinogen classification based on short-term in vitro study result.

PCW wools are not classified; self-classification led to the conclusion that PCW are not hazardous.

Based on the total experience with humans and the findings of scientific research (animals, cells), it can be concluded that elongated dust particles of every type have in principle the potential to cause the development of tumours providing they are sufficiently long, thin and bio-persistent. According to scientific findings inorganic fibre dust particles with a length-to-diameter ratio exceeding 3:1, a length longer than 5μm (0.005 mm) and a diameter smaller than 3μm (WHO-Fibres) are considered health-critical.

High-temperature mineral wool is processed into products containing fibres with different diameters and lengths. During handling of high-temperature mineral wool products, fibrous dusts can be emitted. These can include fibres complying with the WHO definition.

https://en.wikipedia.org/wiki/Mineral_wool#Kaowool

There is concern about the silica content of refractory fibres.  The silica that is of concern is of a crystalline structure.  The method of production does not produce crystalline silica. The process used to create the fibres is:

Amorphous high-temperature mineral wool [fibres] (AES and ASW) are produced from a molten glass [or mineral] stream which is aerosolised by a jet of high-pressure air or by letting the stream impinge onto spinning wheels. The droplets are drawn into fibres; the mass of both fibres and remaining droplets cool very rapidly so that no crystalline phases may form.

https://en.wikipedia.org/wiki/Mineral_wool#Kaowool

The potential effects on health of the materials in refractory fibres have been tested and found to be non-hazardous.

In after-use high-temperature mineral wool crystalline silica crystals are embedded in a matrix composed of other crystals and glasses. Experimental results on the biological activity of after-use high-temperature mineral wool have not demonstrated any hazardous activity that could be related to any form of silica they may contain.

https://en.wikipedia.org/wiki/Mineral_wool#Kaowool

Thus, no crystalline silica is produced and the risk of silicosis from refractory fibres does not exist.  Certain sizes of any fibre present other risks.

Risks

Consideration of risks and therefore precautions, relate to three factors: Dimension, Durability and Dose.

Dimension

Fiber dimensions are critical, as only fibres of a certain size can reach the lungs…. Mineral fibres with a diameter greater than 3 microns are, in humans, “non respirable”. … Even below this respirability threshold only the finest fibres may be deposited into the gas exchange region of the lungs.

While respirability is determined by fiber diameter, fiber length is also important. Short fibres behave as if they are compact particles and can be cleared by the normal mechanisms which involve cells called macrophages. However long fibres [greater than 5 microns] frustrate this mechanism and, for some still unknown reason, are more biologically active.

Durability

Durability in this context describes the ability of a material to persist in the body and so is more accurately called “bio-persistence”. …  Fibres can dissolve or they may break into shorter pieces which can then be removed to the airways or through the lymphatic system. The rate of removal of different fibres is typically measured … and expressed as their “half-life” – that is the time it takes to reduce the number of fibres in the lungs by 50%.

Dose

The [dose] is the result of [dimension and durability] and is often referred to as “lung burden”.  With chronic exposures the lung burden is the result of … [continued exposure] and … bio-persistence. If the exposure is high enough and clearance slow then a sufficiently large dose will accumulate for adverse health effects to result.

http://www.htiwcoalition.org/health.html

The scientific knowledge about fiber toxicity allows comparison of fibres in terms of their toxicological potency and has also driven several initiatives to reduce potential risks in the workplace.  This has led to development of manufacturing processes for thicker fibres, although this is limited by the lesser thermal efficiency of thick fibres.  Thicker fibres are also more likely to cause skin irritation.  A lot of effort has been put into the development of bio-soluble fibres such as the AES wools which are increasingly available.

Recent research has shown a gradation of increasing bio-persistence is in the order of – least to greater –

AES (Calcium Silicate);

AES (Magnesium Silicate);

PCW;

RCF. 

This same research shows that fibres longer than 20 microns cannot be easily cleared from the lungs.  Breathing protection must filter out all particles larger than 20 microns. 

https://occup-med.biomedcentral.com/articles/10.1186/s12995-019-0235-z

The WHO research shows that lung health effects can be produced by particles down to 3 microns. This means that filters used must be able to eliminate particles larger than 3 microns to provide effective protection against high exposure.

 

Handling practices

Sensible precautions when handling refractory fibre papers are eye, breathing and skin protection.  This can be safety goggles, dust mask (see filter size above), and long gloves and long sleeves.  Higher levels of protection can be used, but are not indicated as necessary by the research and classifications of health and safety organisations in the western world.

During clean-up the fibres should be dampened before any brushing of the residue, or vacuumed with HEPA filters to reduce the movement of fibres into the air.  You should also wash exposed skin after handling any of the dust.  Clothes should also be cleaned and washed frequently. 

Do not smoke, eat or drink in areas where the fibre dust is present.

More detailed information is available in the e-book: Low Temperature Kilnforming.

The understanding of the composition and manufacture of refractory fibre papers and blankets should help assess the small risks of using these materials, and the precautions that should be taken in handling both the un-fired and fired forms.

 

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.