Muriatic acid as a cleaner of kiln wash

Muriatic acid is a common name for hydrochloric acid.   Let’s look at what is being cleaned off first.

The main components of kiln wash are hydrated aluminia, kaolin, and colouring. Colouring burns away, hydrated aluminum is inert at kilnforming temperatures, Kaolin begins a non-reversable change from hexagonal plates to a crystalline form at about 600C/1100F and completes it by 900C/1650F. Now consider the characteristics of each element. 

Aluminium Oxide

Aluminium oxide is widely used for its hardness and strength. It is only slightly softer than diamond. In its hydrated form it is a separator between glass and supporting structures. It has excellent refractory characteristics with a melting point of 2,072 °C/3,762 °F. But it is insoluble in water and all solvents. It is largely impervious to acids. 

Kaolin

Kaolinite structure, showing the interlayer hydrogen bonds in white.
Source: Wikipedia

 

Compared with other clay minerals, kaolinite is chemically and structurally simple. It consists of layers, each bound together by shared oxygen ions. The layers are bonded via hydrogen bonding between oxygen on the outer face of one sheet and the other. … The close hydrogen bonding between layers also hinders water molecules from infiltrating between layers, accounting for kaolinite's non-swelling character.

When moistened, the tiny plate-like crystals of kaolinite acquire a layer of water molecules that cause crystals to adhere to each other and give kaolin clay its cohesiveness. The bonds are weak enough to allow the plates to slip past each other when the clay is being moulded, but strong enough to hold the plates in place and allow the moulded clay to retain its shape.   Source: https://en.wikipedia.org/wiki/Kaolinite

It is this slipperiness that makes it a good carrier of the aluminium hydrate. However, kaolin begins a non-reversable change from hexagonal plates to a crystalline form at about 600C/1100F and completes it by 900C/1650F. It is the crystalline form that sticks to glass. So, it is the clay (kaolin) that needs to be removed from the glass.

Hydrochloric acid as a cleaner of kiln wash

Glass is almost impervious when it has a minimum of modifiers. Glass which has a minimum amount of [modifiers] and is almost entirely SiO2 is remarkably chemically inert and reacts only with very strong alkaline (bases) materials.   Source: https://www.quora.com/How-come-hydrochloric-acid-does-not-burn-through-the-glass-bottle-that-its-stored-in

Note that coloured and fusing glass have a significant level of sodium and potassium modifiers. This means that fusing glass is subject to attack by hydrochloric acid. 

Safety notes on hydrochloric acid

Being a strong acid, hydrochloric acid is corrosive to living tissue and to many materials, but not to rubber. Typically, rubber protective gloves and related protective gear are used when handling concentrated solutions. Solutions of less than 25% cause skin irritation, serious eye irritation and respiratory irritation. Over 25% causes severe skin burns and eye damage. It is also a precursor of many illegal drugs. Serious safety gear is required to handle even 10% solutions. 

Even then:

“Clays are not truly soluble in HCl acid, [but] exposure to HCl acid does affect the structure of clay minerals. Hydrochloric acid cleans clay minerals by removing free iron oxide from the surface. … The dissolution of kaolinite clay in hydrochloric acid solutions has been carried out in the presence of fluoride ions. Leaching in the presence of fluoride ions activates the clay for leaching, making higher extractions possible at lower roasting and leaching temperatures. Acetic acid [vinegar] is less effective.”   Source: Stability of Clay Minerals in Acid, by D E Simon and M S Anderson. https://onepetro.org/SPEFD/proceedings-abstract/90FD/All-90FD/SPE-19422-MS/68436 

This piece of research shows that hydrochloric acid is most effective in combination with fluoride and heat.

Other reported research from Researchgate shows:

“Kaolin and other clays are partly soluble in acidic solutions (organic or inorganic acids in water) but the … solubility is never complete. Increasing the acid content doesn't … increase the solubility.” Philip G Jessop, Queen's University. 

       “Potassium hydroxide … will get kaolinite dissolved with a white residue for selective leaching. … The most aggressive solvent is hydrofluoric acid which "kills" almost all silicates [including kaolin]. … For the kaolinite group … use hydrazine as solvent.” Harald G. Dill, Leibniz Universität Hannover. 

Hydrazine is highly toxic unless handled in solution. Hydrofluoric acid may dissolve the kaolin, but it also dissolves the minerals in glass. Both these chemicals are extremely dangerous. 

Conclusion

It is not advisable to use hydrochloric (muriatic) acid as a cleaner of the kaolin in kiln wash from glass. 

There are other much safer methods which use a chelating action rather than attempting to dissolve the almost insoluble kaolin. These are citric acid for brief (less that 24 hours) soaking, or trisodium citrate for longer periods.

Kiln Wash Sticking to Glass

Causes and avoidance

 

Photo credit: Immerman Glass

 In general, kiln wash for glass is made up of aluminium hydrate with kaolin (China clay) as a carrier. 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.

 Some reading has led me to learn that by 600°C/1113°F the kaolin begins to go 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 crystallisation cannot be reversed. It does not become fully crystalline until 935°C - 950°C/1717°F - 1744°F. The crystallisation stops the lubricating effect. I suspect that on the second firing these crystals (which contain silicon dioxide) interact with the glass and stick, although not fully combining 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 used in a previous firing. It also indicates that repeated tack fusing on kiln wash will ultimately fail as the crystallisation will gradually increase with each firing.

 However, at slumping temperatures, it appears the crystal formation is so slow as to have no effect on multiple firings.

 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, although the incorporation of  binders makes the application easier. The grades used in ceramics are usually coarser than kilnformers want. But it can be made finer by putting it in a rock tumbler with some stainless steel ball bearings. You can run the result through a fine screen to remove the ball bearings. Mix with water to brush on, or sprinkle dry over the shelf. The aluminium hydrate can be re-used, if they are kept free of contaminants. Aluminium on its own does not 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.

Kaolin provides significant advantages in the smooth application of kiln wash.  Caution needs to be exercised in using it after it has been fired to fusing temperatures, although it can be used at low temperatures for indefinite numbers of firings.

 Methods for removal of kiln wash are in this blog post.

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.

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.