Conditions for Re-firing Kiln Wash

I do not know the exact chemical changes of kiln wash at fusing temperatures.  But I do suspect it has to do with the kaolin.  The aluminium hydrate is stable to much higher temperatures (melting point of 2,072°C/3,762°F).  So, I don't believe that part of kiln wash is changing. 

The Influence of Kaolin

Some reading has led me to learn that by 600°C/1113°F the kaolin begins going through a non-reversable chemical change.  Prior to that, water can rehydrate the kaolin.  In the hydrated state kaolin forms hexagonal plates that can slip over one another.  Once 600°C/1113°F has been exceeded the crystallisation formed cannot be reversed. It is a gradual process.  It does not become fully crystalline until 935°C - 950°C/1717°F - 1744°F. The crystallisation stops the lubricating effect of the kaolin.  I suspect that on the second full fuse firing these crystals (which contain silicon dioxide) interact with the glass (also silicon dioxide) and stick to the glass.  Although it does not fully combine with the glass.  Why this does not happen in the first firing, I do not know.

The fact that the crystallisation cannot be reversed must be the key as to why kiln wash with kaolin cannot be re-used once fusing temperatures have been reached.  The crystallisation at 800°C/1473°F is nearly complete.  It begins to exhibit the "stickiness" to the glass.  

People who consistently avoid contour and full fuse firings find they can get more than one firing from kiln wash.  This will be because the crystallisation is only partially completed.  But it indicates that repeated tack fusing on kiln wash will ultimately fail as the crystallisation will gradually increase with each firing.  The number of firings possible on one coating of kiln wash will be dependent on temperature and times, among a few other things.

However, at slumping temperatures, it appears the crystal formation is so slow as to have no effect with multiple firings.  Many people experience no difficulty with kiln wash sticking to the glass over many firings, when low temperatures are used.  High temperature slumps will reduce the life of the kiln wash (where life is taken to mean the degree of crystallisation).

 

Picture Credit: Amazon

Avoiding Kaolin

There are of course, ways to avoid kaolin. There is a kiln wash, called Primo Primer that does not have kaolin in it. And you could make your own kiln wash from aluminium hydrate. This is known as slaked alumina in ceramics. It can be used on its own with or without a medium to assist the smooth application of the kiln wash.  One good medium is CMC.

When selecting the aluminium hydrate, be aware there are finer and coarser particles.  The grades used in ceramics are usually coarser than glass people want. But it can be made finer by putting it in a rock tumbler with stainless steel ball bearings. You can run the result through a fine screen. Mix with water to brush on, or sprinkle dry over the shelf. Both these can be re-used. Neither provide as smooth results as when the kiln wash contains kaolin.

Chalk, also known as whiting, is calcium carbonate. This is often used as a separator in vitreous paint firings and some forming operations. It has low solubility in water, so cannot be painted onto shelves or moulds. It needs to be used as a loose or compacted powder. It goes through chemical changes too, making renewal after firing advisable. Above 800°C/1473°F calcium carbonate changes to calcium oxide, or quicklime. This corrosive form is another reason it is disposed of after any higher temperature firings.

Kiln wash and calcium carbonate can be fired many times at low temperatures, because their chemical composition remains relatively stable. Once higher temperatures are used, chemical changes occur. This seems to enable them to stick to the glass or form undesirable compositions. This phenomenon requires removal and re-coating of shelves and moulds after full fuse firings.  Their advantages are ease of use and low cost.

Kiln wash beading up

Sometimes kiln wash does not seem to want to stick to the mould.  There are several possible reasons. The main two seem to be a hard spot in the slip cast moulds that we use.  Another is the previous use of boron nitride or other sealant of porous surfaces.

The remedies are different for these two causes.  For hard spots you can add a bit extra kiln wash to the area.  Normally enough separator adheres to the spot to avoid sticking.  This is so even though you can see the spot more clearly than the rest of the mould.

Sealed surfaces present a little more difficulty.  It is possible to carefully sand blast off the boron nitride from the surface using low pressure and very little abrasive.  This works well for textured surfaces, if you are careful.  You can also manually sand the sealant off which works better for regularly shaped smooth surfaces.   The object of both these processes is to remove the sealed surface to reveal the porous material again.  You must remember that you are removing some of the surface of the mould in these abrasive processes.  Once removed kiln wash can be applied as before.

If neither abrasive method works, it does not mean the mould is ruined.  You can continue to use boron nitride.  Or, if you want to avoid the costs of boron nitride, you can sprinkle fine dry kiln wash over the mould.  You should give the mould a final application of boron nitride before using the dry kiln wash.

Kiln Wash Mix

There seems to be a view that the exact consistency of the kiln wash mix is important.  Within limits the mix proportions are not vital.  The general recommendations from manufacturers is one part powder to five parts water – both by volume.  This is a good guide for general use.

 

It is possible to make the kiln wash mix too thick.  If it goes onto the shelf or mould in a pasty fashion it is too thick.  A thick mixture leaves definite streaks and uneven levels that are difficult to smooth and level.  If you get these effects, scrape it off and put it into a jar with more water.  Mix until it is creamy to avoid lumps.  Then add more water until you have a very liquid mix.  It needs only be a little less runny than plain water.

 

Is it possible to have too thin a mix of kiln wash?  I suppose it is, but not likely.  If you feel it is too thin, you only need to add more coats of the mix until the shelf surface is obscured. Often when the mix seem thin, it is because the powder has separated from the water.  It is necessary to stir the kiln wash thoroughly to get all the solids in suspension.  Then frequent stirring during the application is necessary to keep the mix even at both the top and the bottom of the container.  Storing the mixed kiln wash in a clear container will enable you to see if kiln wash is still settled on the bottom.

 

The object of the kiln wash is to provide a separator between the supporting surface and the glass.  It needs to be only a film of separator to be effective.  In fact, if the kiln wash is too thick, it will flake and stick to the back of the glass.  In the case of kiln wash - more is definitely worse.

 

For very absorbent materials such as vermiculite or fibre board, I mix kiln wash thicker – about 1:3.  The idea behind this is to reduce the amount of water the mould absorbs.  With less water in the mould, less drying time is needed, especially with a vermiculite mould, where steam pressure could break the mould.

Hake brushes

Hake (ha-kay) brushes are made from goat's hair. Their advantage over other brushes for applying kiln wash is that they hold a lot of liquid. Proper ones made from joined bamboo work better than the ones with flat handles.

Traditional Japanese hake brush

People often note that these brushes tend to shed hairs. The solution to stray hairs (given to me in a Bullseye workshop) is to invert the new brush and apply super glue at the point where the hairs emerge from the handle.  This holds the hairs in place. It will work on flat handles too.

Inexpensive goat's hair brushes of the hake style.
As can be seen by comparison, there are fewer hairs in these.

Kiln wash application with a brush

Kiln wash is applied thinly in a 1:5 powder to water mix to shelves and moulds.  The object is to get a complete coverage with a smooth surface. 

To ensure full coverage painting four coats is sufficient for excellent coverage.  The kiln was should be applied in four directions – horizontal, vertical, and each diagonal.  This ensures any gaps in one coat will be covered by the others. A broad brush that holds a lot of liquid provides good coverage.  A hake brush is ideal.  The brush should be held almost vertical with the ends of the bristles only touching the surface.

A traditional Japanese hake brush

There is no need to dry each coat before applying the next. It is not like painting your wall. All coats can be applied one directly after the other. No drying between coats is required.  In fact, earlier dried coats tend to make the application clumpy and streaky.

Some people advocate a fifth coat.  I don’t know what the fifth coat is for. What direction other than the four cardinal ones can there be?  It maybe it is insurance that the surface is coated evenly.  This can be checked visually.  The kiln washes used for glass are routinely coloured.  If the shelf shows unevenly through the kiln wash, a little more needs to be brushed onto the more thinly coated area.

It is possible to smooth the kiln washed surface once the kiln wash has a dusty surface – it does not have to be completely dry – you can put a piece of paper between the shelf or mould and your hand.  Gently rub the surface to get a really smooth finish to your kiln washed shelf.

 

More information:

https://glasstips.blogspot.com/2009/08/applying-kiln-wash.html

https://glasstips.blogspot.com/2009/08/smooth-kiln-wash-on-shelves.html

Cleaning Kiln Wash from Glass without Etching

 This is a note from Christopher Jeffree on a piece of research he did on the effects of three chemicals to remove kiln wash and investment residue from glass.  These are the common vinegar soak, my preferred citric acid soak and a tri-sodium citrate soak.  

This latter is a neutralised citric acid. It is widely used in the food, and engineering industries. It is an anti-oxidant. It is used to remove limescale also. Clearly it is an all around useful chemical.  It is edible, widely available, and cheap.

Christopher informs me that "One interesting application for it is for retarding the setting of gypsum plaster, so it is sold by plasterers and building merchants."  It is also available through Amazon, Ebay and sellers of food making supplies.  Typically, it is sold as tri-sodium citrate dihydrate.

Without more introduction, here is Christopher's research and conclusions.

---    ---    ---    ---    ---    ---    ---    ---    ---    ---    

Which etches glass more – 6% vinegar or 6% citric acid? To cut a long story short, a quick experiment shows that it depends on the glass.

·         Both acids etch opal glasses, especially some reds, oranges and yellows, when soaked for 48h, but citric acid etches the same colours more in the same time.

·         Most transparent colours and clears are very resistant to etching, even when exposed for much longer times.

·         The neutralized form of citric acid, tri-sodium citrate, is just as effective as citric acid for cleaning glass of mould material and kiln wash but does not etch either transparents or opals during extended soaks of several days.

·         Bottom line:  to avoid glass etching, long soaks should be carried out in trisodium citrate, not in vinegar or citric acid

 

Samples containing mainly opal yellows and oranges.

Samples containing mainly opal blues and greens. Due to a slight difference in angle of illumination, the etch pits appear bright in this set of sample, but dark in the yellow set above.

 

©Chris Jeffree, December 2021

Glass Separators

Glass separators tend to be in three forms – powdered, liquid or fibre. These are applied to shelves, moulds and other surfaces that might come into contact with the hot glass.

What do they do?

Glass separators keep the glass from sticking to the shelves, kiln furniture and other supports during the higher temperature parts of the firing.  Glass as used for kilnforming reaches its softening point somewhere around 580°C. The glass will begin to stick to all surfaces as it gets warmer.  The separators are stable at high temperatures and do not stick to the glass or the materials used to separate the glass from its supports.

What are they?

       Liquid and powder separators are most often called kiln wash - or batt wash in the ceramics field.  Normally they are supplied in powder form that is mixed with water for painting onto shelves and moulds. 

They normally have a high content of alumina hydrate, some kaolin (also known as china clay) and sometimes a little silica, plus often a colouring agent that burns away on the first firing to indicate fired and unfired shelves.

       A high temperature lubricant, boron nitride, has come into use for kilnforming and has slightly different characteristics than the alumina hydrate-based kiln washes.

Sheet and blanket forms of glass separators are also widely used.  They have the general name of refractory mineral wool. They are often made from alkaline earth silicate (AES) wool, Alumino silicate wool (ASW) and Polycrystalline wool (PCW).  These have different temperature ranges and levels of health risk. The thin sheets are mainly used for covering shelves and other kiln furniture.  The blanket, which starts at about 12mm, is used mainly for insulation purposes.

Thin papers, similar in thickness to cartridge paper have been developed to give a finer texture than mineral wool separators.  These currently have the trade names Papyros and Thinfire, each with their own slightly different characteristics.

Safety

As with all refractory materials, safety precautions are needed.  In the kilnforming world the risks are not those of the industrial environment because the quantities are less, and the time of exposure is much less.  Still, breathing protection should be used. Eye protection is advisable, as the particles are hard and can scratch the eye surface.  Long sleeves and gloves are advisable when handling refractory fibres.

 

Kiln Wash

This blog concentrates on liquid and powdered separators. It draws on information from the ceramics and kilnforming communities.

Basic Kiln Wash Materials

A lot of the kilnforming knowledge of glass separators comes from the ceramics field. A brief look at the development of kiln wash by ceramicists is instructive to kilnforming. 

In order to make a good kiln wash you need to select materials that have very high melting points and that, when combined, do not create a eutectic that causes melting. Knowing a bit about the properties of materials and the principles of kiln wash allows you to choose the ingredients that make the best wash for your specific situation and avoid costly problems. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

The basic materials started as:

EPK Kaolin (which includes alumina)      50%

Silica                                                50%

EPK Kaolin is a high quality, water washed kaolin which is white, has unusually good forming characteristics and high green strength. In mixtures, EPK offers excellent suspension capabilities.  The source of alumina in kiln wash was often kaolin, but now is most often alumina hydrate or alumina oxide.

Silicon dioxide has a melting point of 1710°C and aluminium oxide has a melting point of 2050°C.  A mixture of these two materials will not melt, and will protect the kiln shelves at high temperatures.

This is a good kiln wash for low and mid-range electric firings [for ceramics]. The only problem is that it contains silica, which is a glass-former. So, if a lot of glaze drips onto the shelf, it can melt the silica in the kiln wash and form a glaze on the shelf. Also, when you scrape your shelves to clean them, you create a lot of silica dust, which is a known carcinogen. So, using silica in your kiln wash is not … the best choice.

Another drawback of this recipe is that, if it is used in salt or soda firings, it will most certainly create a glaze on the shelf. This is because silica, as noted above, is a glass-former. When sodium oxide, which is a strong flux, is introduced atmospherically, it can easily melt the silica in the kiln wash into a glass. This is why silica should not be used in a kiln wash recipe for wood, salt or soda kilns. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

For glaze firings a kiln wash with more separator and less glass former is better:

Alumina hydrate            50%

EPK kaolin                    50%

Kaolin has a melting point of 1770°C and alumina oxide has a melting point of 2050°C, so it will not melt, even in a … firing [of 1250°C to 1350°C]. These ingredients are called refractory because they are resistant to high temperatures. … This recipe can be used at all temperatures and in all kiln atmospheres. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

Kiln washes with kaolin, especially if applied thickly, can flake off the shelf after repeated firing.  The cause of this is the shrinking of the drying kaolin - which is a clay – similar to dried out lake beds. Adding at least half the kaolin as calcined EPK kaolin reduces this shrinkage. Calcining involves drying the kaolin at about 1000°C for some time.  This reduces the physical property of shrinkage, but retains the chemical and refractory properties of a glass separator intact.

This gives a kiln wash consisting of:

Alumina hydrate            50%

Calcined EPK kaolin        25%

EPK kaolin                    25%

You can add more calcined kaolin – up to 35% – if you want. You need to keep enough un-calcined kaolin in the recipe to suspend the other materials so that the suspended materials can be applied smoothly.  One difficulty of increasing the kaolin content of the kiln wash is that it tends to stick to the glass - especially opalescent - on a second firing.

It is, of course, possible to do away with the kaolin entirely.  You can mix alumina hydrate with water into a full milk consistency and apply that to the shelf or other kiln furniture.  It is difficult to maintain the alumina hydrate in suspension, though. After the firing you can brush the dried separator from the shelf into a container for re-use.  You do need to ensure that the powder to be reused is free of contaminants.  It is also important to find very fine grades of the alumina hydrate to minimise the texture on the base of the glass.  Most ceramic grades are coarser than wanted for kiln forming.  You can put the powder in a rock tumbler to make what you find finer than as purchased.

There are many variations on these basic kiln wash recipes. To illustrate the wide variety, some potters just dust alumina hydrate on their shelves to protect them, while some wood firing potters use 100% silica and wall paper paste to make a very thick (1/2-inch) coating that protects their shelves from excessive ash deposits. Still others, who have the new advanced nitride-bonded silicon carbide shelves, don’t even use kiln wash at all because the glaze drips shiver off when the shelves cool. Other potters, who are very neat and don’t share their space with others, may not even use kiln wash so that they can flip the shelves after every firing to prevent warping.

Kiln wash is such a ubiquitous material in the ceramics studio that we take it for granted. … There are many recipes to choose from and many solutions to common problems if we just take the time to learn about the materials we use. 

(John Britt www.johnbrittpottery.com ceramicartsnetwork.org › firing-techniques)

Variants on the traditional glass separators

There are variations in the use of alumina hydrate and kaolin, but there are also other glass separators available, although they tend to be expensive.

An example is zirconium. It is a glass separator with refractory properties, as in its zirconium oxide form it melts 2700°C.  In its zirconium silicate form it has a melting point of 2550°C.  These are available under a number of trade names. This can be added to the kiln wash mix in the knowledge that it will be stable throughout the firing.

But you must be careful in the amount you use, as zirconium silicate is used as an opacifier in glass and glazes.  Also, zirconium oxide is one of the hardest substances in the world.

Boron Nitride

Another very popular glass separator is boron nitride.  It has two forms. 

One is cubic boron nitride, a cubic structure similar to diamonds.

     

  

In the cubic form of boron nitride, alternately linked boron and nitrogen atoms form a tetrahedral bond network, exactly like carbon atoms do in diamond.  Cubic boron nitride is extremely hard and will even scratch diamond. It is the second hardest material known, second only to diamond.  Cubic boron nitride has very high thermal conductivity, excellent wear resistance and good chemical inertness, all very useful properties for a material subjected to extreme conditions. Because of its hardness, chemical inertness, high melting temperature (2973°C) cubic boron nitride is used as an abrasive and wear-resistant coating. Cubic boron nitride (CBN) is used for cutting tools and abrasive components for shaping/polishing with low carbon ferrous metals.  (http://www.docbrown.info/page03/nanochem06.htm)

Hexagonal Boron Nitride

The second form, useful in kilnforming is the hexagonal form of boron nitride.  It forms white plates of hexagons one layer thick like graphite.  These plates have weak bonds and so slide easily against one another.

https://www.substech.com/dokuwiki/doku.php?id=boron_nitride_as_solid_lubricant

It is a good insulator and chemically very inert.  It is stable to about 2700°C.

Hexagonal boron nitride (HBN) is used as a lubricant, since the weakly held layers can slide over each other.  Because of its 'soft' and 'slippery' crystalline nature, and its high temperature stability, HBN is used in lubricants in very hot mechanical working environments.  

(http://www.docbrown.info/page03/nanochem06.htm)

The slippery nature and high temperature stability characteristics make this material an excellent coating for moulds and other situations where the glass moves against its supports.

The coating of the moulds needs frequent re-coating because the layers slide from the mould. Boron nitride works very well on solid impermeable surfaces as it adheres easily to smooth surfaces. It can be used on porous surfaces, but does seal those surfaces, meaning that these surfaces cannot be returned to that porous state without significant abrasion.

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The next blog  has notes on refractory mineral wools as separators and health and safety in use.

Removing kiln wash from moulds

“How do I remove kiln wash from a mould that I have decided would work better with ZYP?”

Once coated with kiln wash, slumping or draping moulds do not need to be re-coated until the surface is damaged.  Then it is best to remove all the kiln wash to prepare a new smooth surface for the kiln wash.  You may, of course, as the enquirer above states, want to use a different kind of separator.  The cleaning of the kiln wash from the mould will be the same process whatever you want to do with the mould next.

There are many ways to get the old kiln wash off.  Some of them depend on the material from which the mould is made.

Metal

If the mould is made of stainless steel or other metal, the easiest method is to sandblast with lots of air and a minimum of grit.  You can also use sandpapers or open weave sanding screens. The methods used on ceramic moulds, as described below, can also be used on metal.

Ceramic

Sandblasting is not safe to use on ceramic moulds, as the sandblast medium can erode the surface very quickly and often unevenly.

Preparation for manual removal of kiln wash.

It is best to wear a mask during this process to reduce the amount of dust you inhale. Spread a cloth, newspaper or other covering to be able to easily gather the removed kiln wash and place it in the waste.  Have a vacuum sweeper at hand to remove powder rather than blowing it around the work space.  Of course, if you can do this outside, there is much smaller risk of contamination.

Dry

I suggest that removing the kiln wash while the mould is dry should be the first stage. 

Flat surfaces can be cleaned with a straight edged wooden stick, or wooden clay modelling tool.  Firmly push it along at a slight angle from the vertical to remove most of the kiln wash. 

On curved surfaces you will need a rounded tool such as a plastic burnisher or all nova tool for the coarse work.  This can be followed up by using a stiff sponge to clean up any stray kiln wash still adhered. If the kiln wash is persistently sticking to the mould, you can cut a small piece from an open weave sanding screen and use it to gently remove the most difficult remaining kiln wash.  Do not use more than light pressure, as with heavy pressure, the screen can begin to remove the surface of the ceramic mould.

Texture moulds and those with a lot of detail or right-angle corners need a bit more attention.  You can use a variety of non-metal tools to get into areas of detail.  Some of these are a rounded chopstick, a wooden skewer, a plastic knitting needle, and other similar items with rounded points.  These can be backed up with a small stiff nylon brush.  It is while working on these detailed areas that the vacuum sweeper will be most useful to clear out the debris and enable you to see how well the kiln wash is being removed.

Wet

Some people do not like the idea of the dust created from the removal of the kiln wash being in the air at all.  And sometimes, the dry removal is not complete.

My recommendation is to dampen the kiln wash that is on the surface of the mould.  This will cause some difficulties in removal, because a slurry is created along with the flaking of the baked-on kiln wash.  The same tools can be used to clean the mould as when dry.  The vacuum sweeper will not be of use though.  Once the kiln wash appears to be cleaned away, the mould needs to dry to enable removal of the remaining kiln wash.  Once dry, you can use dry sponges, or the small nylon brush to clean the remaining film of kiln wash from the mould.  This cleaning may reveal areas where the kiln wash is still adhering. These can be dealt with wet or dry, although I prefer dry.

Soaking or washing the mould does not remove the kiln wash as easily as you might think.  It is especially to be avoided where the mould has an internal hollow, as it may take days to dry sufficiently to apply other separators.  To put it in the kiln risks breaking the mould by the steam build up during the initial rise in temperature.

If you must soak the mould, I recommend that you use a 5% solution of citric acid because it has a chelating action on some of the components of kiln wash.

Remember that once the mould or shelf has been coated with boron nitride, it is almost impossible to go back to kiln wash again.  The boron nitride fills the porous parts of the ceramic making it difficult for the kiln wash to adhere to the mould.

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

Hake brush

Bamboo handle hake brush

The hake (pronounced hah–kay) brush was developed in the far east.  It has several variations – the original consisted of a group of bamboo brushes bound together in a line.  These are still made and used. Many modern hake brushes have a broad wooden handle with a wide line of hairs.  These brushes are made of very fine, soft hairs - often goat hair is used. 

Flat wooden hake brushes

The flat hake brushes are most often cheaper and in a wider variety of sizes than the bamboo ones.  I prefer the bamboo for the feel in the hand that the broad handle gives.  With the longer hairs, it holds more moisture and delivers even amounts of kiln wash even with long strokes. 

Use

These brushes can hold a lot of moisture and deliver it evenly.  This makes it good for laying  down large areas of even colour in watercolours, and in glass painting. The same characteristic makes it very good for coating shelves with kiln wash.  The brush should be filled liberally with the paint or kiln wash. The brush should be gently shaken to remove any excess. Hold the brush nearly vertical and let the bristles barely touch the surface as you move along in smooth sweeps across the surface.  This allows the kiln wash to be evenly spread with very few brush marks.

Maintenance

One drawback of these brush is that the fine soft hairs are difficult to bind into the ferrule.  This results in the brushes often shedding hairs onto the shelf as it is being coated. A tip I learned from Bullseye is to treat the new hake brush with superglue at the base of the hairs. It does not have to be super glue.  It can be any runny glue, or diluted PVA.  I prefer super glue, even though it is reported to have some sensitivity to moisture. You can work the glue into the centre by using a needle to poke at the hairs to move the glue toward the centre of the bristles.  The glue binds the hairs in addition to the binding at the ferrule, and so keeps the brush from shedding. 

I did this on my bamboo handle hake brush a couple of years ago and it is not yet shedding hairs during applications of kiln wash.

Make sure you clean the bristles immediately after using to avoid any material drying among the hairs and causing them to break when next used.  To clean the brush, you only need running water run through the bristles.  Do not scrub the bristles against anything.  The hairs are delicate.  Set the brush aside horizontally to allow water to drip off and the hairs to dry.  Setting the brush upside down when wet allows water into the bindings of the hairs.  Putting it with the hairs down onto a surface deforms the hairs, making it difficult to straighten them later.

A hake brush is among the most useful tools to put kiln wash onto shelves and moulds because it holds so much moisture.  It does require maintenance to ensure the hairs do not shed and that the delicate hairs are not broken.

Citric Acid Cleanser

Christopher Jeffree has kindly outlined the reasons for the effectiveness of citric acid as a cleaner for removing refractory mould residue and acting on kiln wash stuck to glass.  This is his work (with a few personal notes removed).

"Citric acid works well for removing the plaster scale that builds up in vessels used to mix plaster, and it helps to remove traces of investment plaster and kiln wash from glass.  Its metal-chelating properties probably help with dissolution of calcium deposits, but I am less clear why it is so good at removing kiln wash.  The constituents of kiln wash are kaolin and alumina hydrate, neither of which I would expect to be soluble in dilute acids.  Equally, the refractory materials in investment formulae I would expect to be insoluble.  However, kaolin forms layered structures in which flakes, molecular layers, of alumina hydrate and silica interact through hydrogen bonding. It is possible (I am guessing here) that citric acid can disrupt those hydrogen bonds, thereby disaggregating the clay.  All we can say is that empirically, it works.

"I prefer to use citric acid partly because it has a defined composition, but also because it is safe and pleasant to handle – no odour, and comes in the form of easily-dissolved dry crystals like granulated sugar.  Vinegar stinks, and glacial acetic acid is  an aggressive flammable, corrosive liquid with a chokingly acrid smell.

"Calcium sulfate has low solubility, but is not completely insoluble in water - gypsum (calcium sulfate dihydrate) has a solubility of about 2.5g per litre (0.25%)  from 30-100 C. Its solubility is retrograde, meaning that it decreases, rather than increasing, with temperature.  Natural gypsum is an evaporite, a type of rock that often forms by evaporation of lake water in a geological basin with little or no outflow. It can also be produced hydrothermally in hot springs, when water containing sulfuric acid passes through limestone.  

"Calcium citrate is not very soluble either, only in the order of about 0.85g per litre, but the important thing from our point of view is not to get the material into solution but to separate its crystals and make it detach from the glass.

"In other contexts, warm citric acid is used by jewellers and silversmiths as a pickle for dissolving copper oxide (firestain) from silver and gold alloys  after heating / soldering.  It is a safer alternative to the traditional jeweller's pickle of 10% H₂SO₄.

"Citric acid also dissolves rust from iron, without much etching the iron itself, so is good for cleaning rust off tools etc.

"These pictures show a plaster mixing bowl with (presumably) CaSO₄-rich deposit on the surface, cleaned by soaking with 5% citric acid for 4 hours,

and flash from the pate de verre castings with tightly adhering kiln wash, cleaned using 5% citric acid soaked for 4 hours, and vinegar (white wine) soaked for 24 hours.

"I'm not sure about reaction products - I was speculating a lot there, running through hypotheses that I can't support. We don't really have data on the composition of the layers that are stuck to the glass, or a clear idea of why they sometimes stick and sometimes don't (e.g. the differences between transparent and opal glasses in this respect). Maybe this would be a topic to discuss with technical people at Bullseye."

Hope this helps
Best wishes

Chris Jeffree

Subsequent to this work Christopher has done more work and found that Tri-sodium citrate is an even better chemical for cleaning glass of kiln wash and mould material.

Drying kiln wash

“Dry your kiln wash between coats and before firing.” 

This is a frequent statement when talking about renewing kiln wash on shelves and moulds.  The main reason given seems to be that there will be less risk of creating bubbles by evaporating moisture.  The air drying will reduce moisture in kiln is a second reason.

There are some difficulties with this statement and reasons.

Drying between coats of kiln wash means you are applying liquid over powder. This can promote clumping and streaking through a too rapid absorption of water by the dry kiln wash. Also, it makes applying kiln wash a lengthy process.  It is not like painting a door or even a floor, where you must allow drying to avoid streaks. 

Credit: Ceramicartsnetwork.org

Applying kiln wash by brushing is smoothest if all coats are done at once.  This is what happens if you spray kiln wash on your shelves and that gives a smooth surface.  If it were otherwise, drying between coats would apply to spraying too.  Drying between coats promotes streaks in the applied kiln wash that needs to be smoothed before use.  This of course, does need to be done after the kiln wash has dried.

Drying before using the shelf or mould is unnecessary. The evidence I have to offer is that I frequently fire within an hour of applying fresh kiln wash to a cleaned shelf. I have had no problems with creating bubbles or glass picking up the kiln wash. The shelf dries, with a moderate rate of advance, long before the glass settles into the texture of the surface.  It is only as the glass settles into the contours of the kiln wash that it seals air, or any other material, under the glass.

The pigment in most kiln washes is to tell you which shelves have not yet been used.  If they are fired dry at even moderate temperatures, the pigment disappears.  Then you have removed that indicator of freshly prepared shelves or moulds.

Drying of kiln wash before use in not necessary.  If you wish to be cautious, air drying will be enough to avoid any problems with moisture.

Materials for making dams

Rectangular or straight sided shapes

Broken shelves

Accidents happen to mullite shelves causing breaks or cracks.  Rather than throwing them out, you can cut them into rectangles or 50mm strips with a tile saw.  The resulting shapes need to be kiln washed to keep glass from sticking.  They can be used flat or stood on their edges with supports on the outside.

Thick ceramic tiles can be used in much the same way.  You do need to remove the glaze from the tile to make sure they don’t stick to the glass.  Or you could use the unglazed side toward the glass. Again, the tiles need to be kiln washed.

Stainless steel can be used as a dam.  It will need treatment with a separator such as boron nitride or kiln wash.  In addition, it needs to be lined with refractory fibre paper to cushion the force of the greater contraction of steel than glass.

These materials cannot easily be adjusted in length to fit the size of the glass piece being dammed.  Instead, arrange them in a swastika like formation. 

This photo also shows how shorter lengths can be incorporated to make the whole dam.

Vermiculite board is a refractory material that can be used to form dams by cutting with a wood working saw.  The saw you use to cut the vermiculite will be dulled and only be useful for cutting vermiculite in the future.  Do not use any expensive cutting equipment!  

Credit: Bullseye Glass Company

Refractory fibre board is available in many thicknesses.  It can be cut with craft knives even though it dulls the blades quickly.  The thicker boards can be used without rigidising.  This avoids the need to kiln wash and allows adjustments in length.  If you do rigidise fibre board, you must coat it with a separator such as kiln wash or boron nitride.

Weighted fibre paper can be used.  It is sometimes the quickest and easiest to use, as there normally is a stash of scraps around the studio.  It is easily cut with a craft knife.  You can build up the thickness of the dam by layering pieces on top of one another.  Sometimes people put metal wire or pins in the layers to ensure there is no movement between the layers. I’ve found that if weighted, the fibres interlock enough that the layers do not shift.  But you need to line the layered fibre paper dams with vertical strips of fibre paper, so the glass does not take up the layered dam profile on its edge.

Note that you need to use breathing protection when cutting all these materials.

Curved and circular pieces

Many times, the shape to be dammed is not formed of straight lines.  Different materials need to be used in these cases.

Formed stainless steel is a good durable and reusable material.  You need to line the shape with fibre paper if it completely contains the shape, because it contracts more than the glass and can crush the piece.  It is expensive to have made and so needs to have multiple uses to justify the cost.  A cheaper alternative is to make your own shape using stainless steel strapping as used for shipping crates and pallets. 

Fibre paper is an excellent material for damming irregular shapes.  It can be cut into complicated shapes, and it can be layered to attain the required height. You can weight it if you are taking things to a high temperature and fear that the glass will flow under the fibre paper.

You can also use the thicker fibre papers upright by backing up with multiple pieces of kiln furniture to maintain the shape you desire.

Vermiculite board is a good material for making shapes, although not as complicated ones as possible in fibre paper.  Vermiculite can be shaped with wood working materials, but cheap ones should be used as they are quickly dulled. You can rough out a shape with a jigsaw and refine it with various wood working tools, including coarse sandpaper.  Because it is a relatively rigid material, a lot of inventiveness can be used in forming the edges by altering angles from the vertical, incising designs into the edge, etc.  Be certain that you have adequately kiln washed or put other separator on the board, as it will stick to the glass if left bare.

Fibre board is a less rigid material than vermiculite, but is easier to work with simple craft tools.  It is simple to use for a unique one-off shape. It only needs smoothing and does not have to have a separator applied because it does not stick to the glass.  If you create a shape that you will want re-use, you can rigidise the board after shaping, but it will require separators then.

Note that when working with refractory materials, you need to wear respiratory protection and clean surfaces with a HEPA vacuum or by dampening dusty surfaces and wiping them clean.  Dispose of cleaning materials safely.