Sodium and potassium

The most common fluxes used by potters.

In the studio, these are mostly used in feldspars, and some frits.

These are insoluble and not toxic in studio quantities, so only cause a dust hazard. However sometimes they are used as the carbonates - soda ash and potash or pearl ash. These are soluble in water, giving a pH of 10 or 11. This is quite caustic, so it is advisable to wear gloves.

For the end user, there is no risk if either of these two are leached out of the glaze. However if they are leached out then this weakens the glaze matrix, making it more likely that other substances may be leached out too.

It's worth mentioning the mixed oxide effect here. If several fluxes are used in a glaze, rather than just one or two, it becomes much more resistant to leaching.

Titanium

Now on to the transition metals, the big block in the middle of the periodic table, beginning with titanium.

This is used either as the pure titanium dioxide, or as the mineral rutile which also contains iron, as well as some niobium and tantalum.

These materials are mostly harmless, to quote Ford Prefect. They aren't absorbed through ingestion or the skin, and don't irritate the skin. The risk to the potter is in breathing the dust, as this can cause irritation to the lungs, and possibly lung cancr after long term exposure. The risk is not through the material's chemical properties, but rather through the size and physical properties of the particles. During the manufacturing process, whatever size grade is being made, it inevitably includes a proportion of nano particles which get buried deep in the alveolii of the lungs where they cause inflammation and possibly cancer, and from where they take an awfully long time to be expelled.

For the end user, titanium isn't harmful, and incorporation of up to 1% in glazes can toughen the glaze, so there may be some benefit in including a small proportion.

Boron

With boron being used more in lower temperature glazes, I thought I would have a look at it from a health and safety viewpoint.

By far the biggest sources of Boron are the insoluble sources of Colemanite, gerstley borate and borax frits. Their low solubility means that the body does not absorb them to any significant extent, so they are no more hazardous than other dust sources.

The soluble forms, such as borax and boric acid, are used to a much lesser extent. They are readily absorbed if inhaled or swallowed, and can be harmful in large doses. Also, they can cause irritation to the skin and eyes. So dust mask and gloves should be worn.

As far as the end product is concerned, if low amounts of boron are in the glaze (up to 12% B2O3 by weight) the glaze is very tough chemically, so highly resistant to leaching and weathering. But as the Boron increases beyond this point it becomes ever less durable, to the extent that you can make a porous glass by using high boron and then dissolving it out once cooled. If used for tableware, high boron glazes will leach out other metal oxides which may be toxic, so bear this in mind. A small amount of boron may also be leached out, but the quantity is insignificant in terms of health and safety.

So you can't just ramp up the boron to make glazes for low fired work going outside, and if making tableware with a high boron glaze you need to be sure not to include any toxic colourants in food toxic areas. Instead, consider lithium or zinc, or just fire to a higher temperature.

Crazing - is it food safe?

Whilst crazing has always been seen as a flaw in commercial ceramics, some studio potters like the aesthetics of a crazed glaze, and often tacitly assume it is food safe as nobody has produced evidence to the contrary.

Here is the centre of a commercially produced earthenware dinner plate, at least 10 years old, that has developed crazing over the years. Whilst it just looks like a little bit of crazing when it comes out of the cupboard, this is what it looked like after being in the oven at 220C for 20 minutes - all the lines are dark coloured greasy gunk that has come out of the crazing.

Whether or not it is harmful to health, would you want people to eat off one of your plates if it did this?

The porous earthenware has soaked up this stuff over the years. A fully vitrified clay, such as a high fired stoneware, won't be able to absorb this, and so is a better option if you decide to use a crazed glaze.

For me, this is enough evidence to tell me to avoid crazing on tableware, no matter how good it looks!

Vanadium

Vanadium is great for producing yellows in earthenware glazes, and lustre in stoneware.

Some potters worry about the toxicity of Vanadium, both in the studio and for producing tableware. But this turns out to be a red herring (a distraction, for those who aren't fluent English speakers). Less than 1% of the most soluble vanadium compounds are absorbed through the digestive system. In the studio, vanadium pentoxide has low solubility, so this isn't an issue. And if any vanadium leaches from your glaze, even though it will probably combine with sodium to create a more soluble compound, the levels will be too low for any toxic effects.

A greater risk in the studio is vanadium dust, as it can damage and irritate the respiratory system (the particle size is generally too large to penetrate deep into the lung, where it can be absorbed). But the dust concentration to have any harmful effects is much higher than you are likely to achieve when mixing or applying glazes.

So this is one of the good guys, and you needn't worry about using it.

But if you want some fun, try it in Egyptian Paste or something else with a lot of soda ash. They react, frothing and bubbling away, creating sodium/vanadium compounds and giving off carbon dioxide.

Also, note that vanadium is slightly soluble in water (8g/litre at room temperature), so just mix these glazes up when you need them, or some of your vanadium will soak away into the clay body.

Dishwasher Safe?

What can go wrong when placing pots in the dishwasher, and how can you improve things?

The inside of a dishwasher is a pretty unpleasant environment. Hot water (up to 85°C), caustic water (pH 9 - 10), full of food particles, being jetted against the pots to clean them. At least most countries have banned the more corrosive phosphates, due to their harmful effect on the environment.

First, let's look at the effect this has on the glaze. In a strong alkaline environment, the predominant reaction is dissolution of the silica network by the hydroxyl ions in the water. So your glaze is being eaten away each time it goes into the machine and, unlike leaching, it doesn't slow down with time.

To test for this, we can take the European standard EN12875 as a starting point (and the less nerdy can simplify this further). Roughly measure the surface area of the pieces to test, and use at least 1 litre of water per 130sq.cm. of pots. Put the water into a stainless steel pan (aluminium will be attacked), and add 0.5% of dishwasher liquid. Heat to 75°C and add in the pots. Keep at this temperature for 16 hours. Then take out, rinse and inspect for colour change, loss of gloss etc. Repeat at least once more, with a fresh solution. If there is no change, it can be assumed to be dishwasher proof (there are also similar ASTM standards in the USA). To make the glaze more durable, you can add lead, zinc or zircon. I suggest a couple of percent of zircon - it will improve the glaze but not make it opaque.

Tho other problem is water absorption - which only affects pots that are not fully vitrified, and so are porous. The clay absorbs water and expands. This puts the glaze into more tension, which can cause crazing. This both weakens the piece and lets it absorb food juices, which may cause a health hazard. Also, in an oven or microwave, the water can turn to steam and the pressure breaks the pot if it can't escape quickly enough. So, if using earthenware or a low fired stoneware, glaze the bottom and foot ring, and don't use a crazed glaze. If in doubt, fire the clay unglazed and test for porosity.

Oven Safe?

So, what happens when using your pots in the oven? Dry, hot air heats the pot and its contents. Not much to worry about, surely?

The problem starts with ceramics being poor heat conductors, so temperature differences take quite a time to even out. This causes hot areas to expand more than cold, setting up thermal stresses. The bigger the temperature difference, the greater the stresses. But ceramics are also brittle, so high stresses cause cracking - either crazing of the glaze and micro cracks in the pot, or catastrophic failure.

Micro cracking reduces the strength of the pot, and the bigger the temperature difference between hot and cold, the greater the loss of strength. Tests on porcelain show a moderate loss of 2.4% for a difference of 180°C, rising to 75% for a 600°C difference. But after a few cycles of heating and cooling, there is no further loss in strength.

Possibly more important for the user is if a large crack forms, breaking the pot. They may be burnt by hot food, or at least have a ruined meal and a mess to clean up. More on this later... Users should be advised to minimise large, sudden temperature changes. Let food thaw before putting it in the oven; don't put a hot pot under the tap; put it into a cold oven, not a preheated one, and don't put it on a hotplate or use a blowtorch on food in it.
Much can be done in the design to minimise stresses: thin walls, rounded edges and corners, no sudden changes in thickness, and making it smooth.

Turning now to making, the best thing you can do is choose a clay with a low rate of thermal expansion - the less expansion, the lower the stress. The glaze must also be well matched to the body, so it doesn't craze.

Now you can choose one of two approaches: fully vitrified or low fired. For the first approach, you are high firing the clay to make it as strong as possible even though it becomes more brittle and so cracks are more likely to be catastrophic. This needs a stoneware with low thermal expansion, low grog for maximum strength, and fired to be fully vitrified.

The alternative is to low fire the clay (of whatever type) so it is flexible enough to absorb the thermal stresses. The pores in underfired clay, together with a good dollop of grog, help prevent cracks from growing. Glazing helps strengthen the pot, so long as it is glazed on both sides and doesn't craze. A low Si:Al ratio toughens the glaze against thermal stress, as does the inclusion of Titanium.

To test your pot, heat it up in your oven at top temperature, then quickly remove it and put it into a sink of cold water. Repeat this for 10 times, and inspect for cracks or crazing.

Magnesium

Beloved for matt glazes, and as a high temperature flux.

For the potter, it is normally added using dolomite, talc, or magnesium carbonate. Dolomite and the carbonate are not significant hazards, but talc is a different story.

So what harm can talc do? Most trivially, it is an eye irritant. More seriously, it is only cleared from the lungs very slowly (a particle deep in the lungs may stay there for a decade). So long term exposure to even low doses can cause talcosis - inflammation of the lungs leading to scar tissue and bronchitis,often long after exposure has stopped. For example, one person breathed in a puff of talcum powder every morning for 4 months, and developed talcosis 10 years later. It can also cause lung cancer, typically over 20 years after exposure starts. So beware, unless you are in your last decade of life.

As if that weren't enough, a good proportion of mined talc is intermingled with asbestos (talc from some mines has asbestos, whilst that from others doesn't). Although industry finally removed the asbestos from cosmetic, food and pharmaceutical grades of talc (some 50 years after they learned of the risks), it may be found in the industrial grade of talc used by potters (which may have up to 25% of impurities). Asbestos is seriously nasty stuff, very difficult to expel from the lungs, and primarily causing lung cancer and mesothelioma, but also adversely affecting anywhere else along the respiratory and gastrointestinal systems.

So always wear a face mask and gloves when using talc, and minimise dust.

For the user, things are much better. Any asbestos will have been destroyed in the kiln, and leached magnesium won't do any harm.

Food Safe Labels

If you make functional pieces such as tableware or items for the kitchen, it pays to tell the user how it can (or cannot) be used. And remember that if the item is bought from you as a gift, or bought through a shop or gallery, you won't be able to tell the user at the time of the sale. And even if you do, how long will they remember it for?

I've decided to use ceramic transfers on the bottom of pieces to apply use symbols. These have long been available for over glazes, but are now also available for underglaze, so there is no longer the need for a separate firing.

The symbols here are for food safe, microwave safe, oven safe, a and freezer safe. The only fully standardised symbol is the fork and glass for food safe. The designs of the others vary between manufacturers, but some common elements such as the wavy lines for a microwave mean that they should be readily recognisable. And a diagonal line through a symbol means not, e. g. not freezer safe. Also, some jurisdictions such as California can require specific symbols or wording.

Gloves for Wood Firing

For wood firing, consider fireman's gloves. These protect against contact, radiated and convected heat better than leather gauntlets, protect against wood splinters, and are also waterproof unlike leather gauntlets.

Before buying, read up on the relevant standards so you understand the glove spec - ANSI 105 in the USA, and EN388 and EN407 in Europe.

Lifting

Having had to load and unload the car with crates of pots on both Saturday and Sunday, a few words on lifting seem appropriate.

The art of lifting is to keep your body balanced, the spine upright, and your legs doing the work - they are much stronger than your arms or back.

To lift from the floor, space your feet to be stable and bend your legs, keeping your spine upright and looking ahead. Hold the item in both hands, and as close to you as you can. Now straighten your legs, keeping the object close to your chest and your elbows tucked in. The object needs to be balanced, so you aren't having to twist or lean to one side.

If you can avoid lifting, e.g. by sliding a sack truck underneath, then so much the better. High handles also help reduce the amount of lifting - you are at your strongest lifting between mid thigh and mid chest height.

Wooden fuels not to use

Whether your thing is pit firing or an anagama, you should avoid these as fuel, because of toxic fumes. From top to bottom:

Composite woods such as MDF, chipboard and plywood use an urea-formaldehyde adhesive (up to 20% of the weight of MDF). When burned, it gives off formaldehyde and cyanide fumes, amongst other things, both toxic.

Preserved (or tanalised) wood has often been treated with an arsenic containing compound (CCA) that gives off toxic arsenic fumes when burned. Although use in residential and domestic use was restricted in the EU and the USA in the early 2000s, other uses are still permitted, some wood will predate this ban, and it is still permitted in many other countries.

A lot of older gloss paint has high proportions of lead in it. When burned above about 1000C the lead will start to vaporise, and the very small particles can descend right into the depths of your lungs, from where they are absorbed into your body. In time, this can give you lead poisoning. Also, the lead can appear as a grey film on your pots.

Whilst the odd piece of any of these in amongst your fuel is unlikely to cause any health problems, you should aim to steer clear of them all as much as possible.

Lanthanoids

This group of elements inhabits one of the orphan rows at the bottom of the periodic table. I've grouped them together as they are not used by most potters, and they are mostly harmless - though as they are generally not widely used, they haven't been researched as extensively as more common chemicals.

Of the whole group, the ones used by potters (as colourants for glazes) are cerium, praesodymium, neodymium, holmium and erbium.

The only harmful effects found to date are that inhaling large amounts of Cerium over time can cause growths in the lungs - and the necessary level is well above that likely to be found in a potter's studio.

So all of this group of chemicals can be used in the studio with just normal dust precautions, and can be used perfectly happily in food safe glazes.

Strontium

Moving on from Barium it makes sense to look at Strontium next, as many use it as a "non-toxic" barium substitute (as a start, use 75% of the weight of barium carbonate) - though the non toxic label to some extent reflects a lack of testing.

For the potter, strontium carbonate has very low solubility in water, and so is not absorbed to any significant extent through breathing or the skin (though most inhaled dust will be swallowed). However it is a lot more soluble in acidic gastric juices, and prolonged exposure can cause a rickets like effect in the bones, as well as affecting the thyroid and kidneys. The ILO give a very conservative tolerable daily intake of 0.13mg/kg body mass/day (i.e. 7.8mg/day for a 60kg person), whereas the EU's ECHA give what seems a more realistic DNEL (derived no effect level) of 0.8mg/kg/day (48mg/day @ 60kg). So dust masks on when mixing glazes, and don't put it in your coffee.

For the customer, strontium will leach out of glazes, just as barium does. I haven't found any test results on strontium leach rates from glazes or glass, though. But we can compare the safe daily doses or barium and strontium, at 0.051mg/kg/day for barium against 0.8 for strontium (as TDI and DNEL are broadly comparable). Allowing for the lower molecular weight of strontium, this suggests that glazes can safely leach 21 times more (molar) than barium.

But tests showed that bad barium glazes exceeded the safe limit by up to a factor of 80, so bad strontium glazes will exceed it by a factor of 4. So we cannot just assume that the use of strontium makes a glaze food safe.

And, if you have vowed never to use any barium, this also means no strontium, as strontium carbonate contains 1 - 3% barium carbonate. Not enough to affect the toxicity, but significant if you stick to absolute red lines.

Lead Fumes

I was asked about emission of lead vapours when firing. Lead vapours are potentially nasty as, unlike frits or lead compounds, the particles are small enough to penetrate deep into the lungs, where virtually all the lead is absorbed into the body.

What I didn't have was any data on the quantity of lead fumes emitted. I thought I would have to do some tests, but in reviewing Industrial Ceramics I found the data I needed! If fired to 1000C and held for an hour (I.e. a much more severe test than a real life glaze firing) lead frits gave off 0.4 to 0.7% of their lead by weight - this could be divided by 10 to represent a normal glaze firing. With aluminium added to make it more like a normal glaze, this dropped by a further third. Another paper tested the atmosphere round the kiln, and found no lead fumes - any emitted were presumably diluted to below the detectability limit.

Lead glazes can be harmful, but the cases of lead poisoning that occur are found to be due to underfired or badly formulated glazes, or with copper, which causes massive leaching. A well formulated and fired glaze leaches very little lead, and there are papers with glaze recipes and leaching tests for you to use.

The hazards from lead have primarily been from leaded paint (up to 50% lead) and leaded fuel, which were basically poisoning the entire planet.

Those at risk are pre-teens children, and women of child bearing age. For these groups, the safe dose is very low to zero. I wouldn't use lead in the studio if either of these groups were present, to play safe. Also, for functional ware you cannot control who uses it, so I strongly suggest always using a lead free glaze on the inside.

Cutlery Marking

After a few posts that have concentrated mostly on issues in the studio, let's look at a problem that can affect the user - glaze hardness and cutlery marking.

If you make tableware, the chances are that sooner or later your plates will acquire grey marks from metal cutlery. So what causes this, and what can be done about it?

The marks occur because tiny amounts of metal are scraped off the cutlery and deposited on the glaze. For this to happen, the glaze must be harder than the metal, and also have some roughness.

On the other hand, if the glaze is too soft, softer than the cutlery, the glaze may be scratched by the cutlery. In this case, there will be a scratch mark, but no grey metal deposit.

Smaller diameter fluxes and glass formers, such as Lithium and Boron, make the glazes harder, whilst large diameter ones such as Lead make glazes softer (I won't discuss the suitability of lead glazes for tableware here). Also, Zircon is often used as an opacifier and is a very hard crystal; in time two things happen: the softer glaze erodes away to reveal the zircon, and then the forces on the zircon creates micro cracks which get filled with the metal. Minimising the amount of Zircon, or using tin or another opacifier, will avoid this.

Matt glazes are inherently rough, so will cause and retain more cutlery marks than a high gloss, smooth glaze. The possible exception to this is when the glaze is matt because it has been underfired, in which case it won't be at full strength and may just crumble away.

For testing against cutlery marking, I have this idea of using an old record player, fixing the arm in one place, and strapping a weighted knife to it.