Showing posts with label Safety. Show all posts
Showing posts with label Safety. Show all posts

Wednesday 4 October 2023

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.


Wednesday 7 September 2022

Hazards of Flux Fumes

Note:  These health risks are those associated with industrial exposure – frequent and for extended periods.  They do not apply directly to occasional and shorter periods of exposure.

Risks are assessed as acute and chronic.  Acute means immediate reaction.  Chronic means the effects are cumulative and may take years to appear.
 

Composition of Flux

The major components of commercial flux are varying combinations and proportions of zinc chloride (or ammonium chloride), hydrochloric acid, phosphoric acid, citric acid, and hydrobromic acid.  It comes in many forms and many brand names.  It is important to use water soluble flux in stained glass work to enable thorough cleaning.
 
 


 

Zinc Chloride Risks

Zinc chloride inhalation from smoke screen generators or smoke bombs may cause transient cough, sore throat, hoarseness, a metallic taste, and chest pain.  Exposure to high zinc chloride concentrations produces a chemical pneumonitis with marked dyspnoea, a productive cough, fever, chest pain and cyanosis. Pneumothorax and the adult respiratory distress syndrome (ARDS) have been reported. Fatalities have occurred….
http://www.inchem.org/documents/ukpids/ukpids/ukpid86.htm#:~:text=Toxicity%20Zinc%20chloride%20is%20corrosive,anorexia%2C%20fatigue%20and%20weight%20loss.
 

Ammonium Chloride Risks

Exposure to Ammonium Chloride is moderately hazardous, causing irritation, shortness of breath, cough, nausea, and headache. Most exposure is a result of contact with the fume form of this chemical (Ammonium Muriate Fume and Sal Ammoniac Fume), which is a finely divided particulate dispersed in the air. The fumes are capable of causing severe eye irritation. Consistent exposure can cause an asthma-like allergy or affect kidney function.
 
In the event of accidental contact, get immediate medical attention and follow these first aid measures:
·        Skin Contact: Immediately flush skin with water and disinfectant soap and use an emollient on irritated area.
·        Eye Contact: Rinse eye(s) with water for at least 15-20 minutes. Protect unexposed eye.
·        Ingestion: Rinse mouth thoroughly with water. Do NOT induce vomiting.
·        Inhalation: Move to fresh air and administer artificial respiration if needed.
https://www.msdsonline.com/2017/05/05/chemical-spotlight-ammonium-chloride/#:~:text=Exposure%20to%20Ammonium%20Chloride%20is,particulate%20dispersed%20in%20the%20air.
 
 

Hydrochloric Acid Risks

Hydrochloric acid is corrosive to the eyes, skin, and mucous membranes.  Acute (short-term) inhalation exposure may cause eye, nose, and respiratory tract irritation and inflammation and pulmonary edema in humans.  Acute oral exposure may cause corrosion of the mucous membranes, oesophagus, and stomach and dermal contact may produce severe burns, ulceration, and scarring in humans.
 

Acute Effects

Hydrochloric acid is corrosive to the eyes, skin, and mucous membranes.  Acute inhalation exposure may cause coughing, hoarseness, inflammation and ulceration of the respiratory tract, chest pain, and pulmonary edema in humans.  Acute oral exposure may cause corrosion of the mucous membranes, oesophagus, and stomach, with nausea, vomiting, and diarrhoea reported in humans.  [Skin] contact may produce severe burns, ulceration, and scarring…. Acute animal tests in rats, mice, and rabbits, have demonstrated hydrochloric acid to have moderate to high acute toxicity from inhalation and moderate acute toxicity from oral exposure.
 

Chronic Effects: 

(Non cancer): Chronic occupational exposure to hydrochloric acid has been reported to cause gastritis, chronic bronchitis, dermatitis, and photosensitization in workers.  Prolonged exposure to low concentrations may also cause dental discoloration and erosion.  Chronic inhalation exposure caused hyperplasia of the nasal mucosa, larynx, and trachea and lesions in the nasal cavity in rats.  The Reference Concentration (RfC) for hydrochloric acid is 0.02 milligrams per cubic meter (mg/m 3) … The RfC is an estimate … of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without appreciable risk of deleterious noncancer effects during a lifetime.  It is not a direct estimator of risk but rather a reference point to gauge the potential effects.  At exposures increasingly greater than the RfC, the potential for adverse health effects increases.  Lifetime exposure above the RfC does not imply that an adverse health effect would necessarily occur.
https://www.epa.gov/sites/production/files/2016-09/documents/hydrochloric-acid.pdf
 

 
Phosphoric Acid Risks

Phosphoric acid can be very hazardous in the case of skin contact, eye contact, and ingestion. It can also cause irritation if vapours are inhaled. This chemical can cause damage to the skin, eyes, mouth, and respiratory tract. Because of the potential hazards posed by this chemical, it is important to use care when handling it.
 
Repeated or prolonged exposure to phosphoric acid mist can lead to chronic eye irritation, severe skin irritation, or prolonged respiratory tract issues.  In case of accidental exposure to phosphoric acid, follow these first aid guidelines:

Inhalation  Seek fresh air and immediate medical attention.

Eye Contact — Remove contact lenses if present. Immediately flush eyes with plenty of water for at least 15 minutes and get medical attention.

Skin Contact — Wash skin with soap and water. Cover any irritated skin with an emollient. Seek medical attention. 

Ingestion — Do NOT induce vomiting. Never give anything by mouth to an unconscious person. Seek medical attention if any adverse health symptoms occur.
https://www.msdsonline.com/2015/06/17/phosphoric-acid-safety-tips/
 
  

Citric Acid

Citric acid can be a minor skin irritant, causing itchy skin and even minor burns to those that are sensitive to it. Hands should be washed immediately if citric acid comes into contact with bare skin. Protective gloves should be worn during handling to avoid any accidental contact. The acid can also irritate the walls of the throat if ingested or burn the lining of your stomach if ingested in large quantities.
 
Eye IrritationCitric acid is a severe eye irritant. Accidental contact with the eyes can occur … by touching the eyes after the acid has contacted the fingertips. …  Protective eyewear should be worn when working with citric acid under laboratory conditions. Eyes should be flushed with water immediately if they happen to come in contact with the acid.
https://sciencing.com/hazards-citric-acid-8165149.html

Remember that this irritation is equivalent to squirting lemon juice into your eye.  It is not a chronic risk.
 

Hydrobromic Acid (HBr)

Hydrobromic acid and hydrogen bromide gas are highly corrosive substances that can cause severe burns upon contact with all body tissues. The aqueous acid and gas are strong eye irritants and [tear producers]. Contact of concentrated hydrobromic acid or concentrated HBr vapor with the eyes may cause severe injury, resulting in permanent impairment of vision and possible blindness. Skin contact with the acid or HBr gas can produce severe burns. Ingestion can lead to severe burns of the mouth, throat, and gastrointestinal system and can be fatal. Inhalation of HBr gas can cause extreme irritation and injury to the upper respiratory tract and lungs, and exposure to high concentrations may cause death. … Hydrogen bromide has not been found to be carcinogenic or to show reproductive or developmental toxicity in humans.
https://web.stanford.edu/dept/EHS/cgi-bin/lcst/lcss/lcss47.html#:~:text=The%20aqueous%20acid%20and%20gas,gas%20can%20produce%20severe%20burns.
 
 
 

Precautions to be taken by glass workers

The risks outlined above are related to dealing with concentrated amounts of the materials in industrial settings.  Risk levels are much reduced in the craft setting.  The risks are mainly centred on breathing and eye exposure. 
 
It is important to wear masks of the quality that will deal with inorganic fumes.  In Europe these are designated as FFP2.  In general masks rated at N95, P95, or R95 are the level required for filtering out 95% of particles that are larger than 3microns.  Dust masks are not sufficient protection. 
 


Usually overlooked is eye protection.  The risks outlined here show that risks to eyes are equal to - or in some cases greater than – respiratory ones.  Eye protection is as important as breathing filters.  To fully protect the eyes, goggles of some sort are the minimum requirement.  Glasses will not be sufficient to prevent fumes reaching eyes.



 
For a “one stop solution” a full-face mask may be the simplest solution.  The filters on these are long lasting and replaceable.  They can be put on as one unit and are available in various face sizes.
 

At soldering temperatures, there are no lead or tin fumes created.  It is the fumes from the flux that are the risks in soldering.  These risks are small and can be dealt with by using adequate ventilation, masks, and goggles.

Wednesday 29 December 2021

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”.

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.

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.

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.

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.

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.

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.


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. 

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.

 


Wednesday 22 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.  

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.