Cast Iron Mould Risks

There is a lot of concern about the safety of many products used in kilnforming, and much of it is based on hearsay. The best source for understanding the health and safety risks is Gregorie Rawls website.

Another, but more difficult to interpret, source is the SDS for each product.

Cast iron composition and safety

In this case the investigation is cast iron used as moulds. The first element is to know what cast iron is:

Cast iron is a class of iron-carbon alloys having a carbon content of more than 2% and silicon content around 1–3% with a melting point of 1,539°C (2,802°F). [Wikipedia]

The SDS gives the following information on Gray Cast Iron, the material of the cookware commonly used in the kiln:

• This material is rated as NOT HAZARDOUS by OSHA

• Appearance and Odor: Solid Mass, No Odor

• Specific Gravity: 7.86

• Boiling Point: 5000F

• 5 mg/m³ is the Time Weighted Average (TWA*) for fumes over an eight hour day. https://www.cdc.gov/niosh/idlh/1309371.html

These indicate there is no risk from fumes during casting firings as melting point will not be reached and the boiling point of cast iron is much higher than kilnforming kilns can reach.

The real risks are at room temperature, and are from the powder that may be created while grinding or smoothing the metal surface. The TWA* for cast iron dust is 10 mg/m³ over 8 hours. There are two alloy elements that also may be of concern – nickel and chromium. The amounts are low – chrome is from 0.5% to 2.5%, and even less nickel. The amounts are very low, giving little possible exposure.

The health concerns about using cast iron as a mould seems to be one of the misapprehensions of the amount of exposure, and therefore risk, that are common. The precautions are to have ventilation at source, use eye protection, and wear a N95 respirator.

The use of cast iron as a mould material

“Cast iron is a poor heat conductor compared to copper and aluminium, and this can result in uneven heating if a cast-iron pan is heated too quickly or… [unevenly].  Cast iron …[is] capable of storing more heat longer than... stainless steel pans. Slow heating... can lead to a more even temperature distribution. Due to the thermal mass of cast-iron utensils… they can retain heat for a long time...” Wikipedia.

This indicates that slower than usual ramp rates are advisable during the heat up to avoid breaking the cast iron through uneven heating.

Another thing to note is that the expansion rate [CoE] of between 106 and 114. The mould will contract more than glass, so preparing the mould with smooth sides and a sufficient draft is important to being able to remove the glass from the mould.

*Time Weighted Average (TWA) example:

"Rarely is exposure consistent throughout the day. Let’s say you are working in your studio for 8 hours grinding glass and exposure varies throughout the day… [Exposure varies in amounts]. The exposures throughout the day are averaged and the Time Weighted Average is determined. [In the example cited], … the OEL = 10 mg/m3 and the Time Weighted Average is 3.2 mg/m3, so actual exposure is below OEL (Occupational Exposure Limit)."    https://gregorieglass.com/general-information

Kiln wash

People seem to avoid kiln wash. Some reasons are:

• It is messy.
• It requires effort to renew. 
• It needs drying between coats.
• It needs drying before firing anything on it.  
• It has lots of visible brush marks. 
• It must be renewed frequently.  
• It is dangerous.

These notes are to clarify some misconceptions about kiln wash use. Kiln wash is an economical glass separator that requires a little effort to use, but is effective and has less health risks than other separators.  Kiln wash is a separator, not a series of layers built up thickly. Some characteristics to consider in its use.

Thickly applied kiln wash on a mould

The Mix

You mix the powder with water.  Use a thin mix - 1:5 by volume.  There are various descriptions of the thickness of the mix.  Adhering to the 1:5 mix will ensure the right runniness of it.  The mix must be frequently agitated to keep the kiln wash in suspension while you are applying it.  If you do not ensure all the kiln wash is in suspension, you will not be applying enough separator.

Application

Use a soft bristled brush such as a hake or broad squirrel brush to let kiln wash mix flow onto the shelf or mould.  Hold the brush almost vertically and allow the kiln wash to flow off the brush while only lightly touching the shelf with the bristles.  Apply four thin layers in all directions – up/down, horizontal, and the two diagonals - to ensure coverage. 

Gentle application of kiln wash with a hake brush

Drying

No drying between coats is advisable or necessary.  The addition of a wet coat over the dry will wet the previous layer(s) and will lead to clumping.  It is not like painting wooden table that requires drying between coats.  For kiln wash all the coats should be applied without any drying between the directions of brushing.  View this as applying one coat.  And that is all that is needed.

Once the surface has a dull look, it is ready to use, even though not thoroughly dry.  At this stage, or later, you can remove any brush marks.  Place a sheet of paper over the kiln wash.  Smooth it by moving the paper with the palm of your hand over the surface.  Gently remove any dust.

Firing a newly kiln washed shelf or mould with the glass on top will dry the kiln wash before glass is soft enough to stick to it.

Removal

It is advisable to remove the kiln wash once it has been fired to full fuse.  The kaolin in the kiln wash becomes increasingly crystalline as the temperature rises. It is fully crystalline at about 900ºC/1650ºF. At the first full fuse it does not stick to transparent, but often to some opalescent glass. On the second full fuse the kiln wash sticks to all the glass.  At tack fuse temperatures, the kaolin has not fully crystallised, and several firings can be achieved without difficulty.  Experience will show how many firings - at your tack fuse temperature – are possible.

Re-coating

Painting over used dry kiln wash has the same difficulty of clumping as when initially applying.  It is also easier to remove kiln wash that has been fired only a few times. Kiln wash fired to full fuse several times requires much more effort than one fired to full fuse once.

Safety

Kiln wash contains alumina hydrate and most commonly kaolin. The powdered forms of these are irritants, not health hazards.  It is advisable to protect yourself and your work area.  Wear a dust mask when removing the dry kiln wash.  Do this is a well ventilated area or outside to reduce the dust in your studio. Dispose of the used kiln wash in sealed bags to avoid spreading the dust during refuse operations.

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.

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 Irritation - Citric 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.

Mineral Wool Fibres

Refractory Fibres

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

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

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

Credit: Knauf.com

High-Temperature Mineral Wool

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

The three main types of HTIWs include:

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

Alkaline earth silicate (AES) wool

       Calcium magnesium silicate wool

       Calcium silicate wool

       Magnesium silicate wool

Alkali metal silicate (AMS) wool

       Potassium alumino silicate wool

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

       Aluminium silicate wool

       Aluminium zirconium silicate wool

Polycrystalline Wool (PCW)

       Aluminium oxide wool

       Mullite wool

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

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

Alkaline earth silicate wool (AES)

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

Alumino silicate wool (ASW)

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

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

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

Some of the trade names used are:

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

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

Polycrystalline wool (PCW)

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

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

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

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

Credit: Alibaba.com

Kilnforming Refractory Papers

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

Papyros

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

Thinfire

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

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

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

Credit: cdc.com

Fibre Paper – Health and Safety

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

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

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

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

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

ASW/RCF is classified as carcinogen category 1B.

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

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

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

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

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

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

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

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

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

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

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

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

Risks

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

Dimension

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

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

Durability

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

Dose

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

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

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

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

AES (Calcium Silicate);

AES (Magnesium Silicate);

PCW;

RCF. 

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

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

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

 

Handling practices

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

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

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

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

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