Gloves for the Kiln

If you handle hot pots from the kiln, then you need gloves with good contact heat resistance. Woven Kevlar gloves are best for this, and are available at ratings of up to 500C, though the higher the rating the thicker and less sensitive the glove, so you may have problems handling smaller pieces. Note that these gloves are not waterproof so must be kept dry, or you may find your hand in a blast of super-heated steam.

Barium

Barium can create some great colours and matt glazes, but is it safe to use?

In the studio, barium carbonate has very low solubility in water, but is more soluble in acids, like your gastric juices. So it can be absorbed into the body by swallowing, but doesn't harm the lungs or enter the body by that route, and doesn't affect the skin. But although barium poisoning is nasty, we are pretty resistant to it. 5g/day will not harm you, and to kill you the LD50 dose is 100g, so no problem unless you put it in your coffee. Note some sources give much lower toxic levels - this is for fully soluble compounds like the chloride, not the carbonate potters use.

As far as food safety is concerned, by mapping the safe daily intake to leaching tests, we get to a safe leach level of 1mg/l. A number of tests of factory made ceramics all came under this, but tests of about 60 pieces of tableware from studio potters told a different story. 2/3 of them contained barium, and only 35% of these leached less than 2mg/l, with the highest reading 80 mg/l😮. Small amounts (some say up to 5% of the glaze) form part of the silica network and don't leach easily, but don't use high barium recipes in tableware and cookware.

So no problem in the studio, but only use a small amount in food safe glazes.

Microwave safe?

Apart from the normal thermal stress from heating and cooling (which I'll cover another time), there are specific issues in making a piece suitable for the microwave.

Before going into them, a quick summary of how microwaves work. Microwaves are an electromagnetic field, so polarised molecules will align with that field. If the field is rapidly changing, as in a microwave oven, then they will rotate to align with the changing field. If they collide into adjacent molecules as a result, that will set them into motion as well, and the energy manifests itself as heat. Water is the primary molecule in food that is heated by microwaves, but oils, fats and sugars are also heated to a lesser extent.

So what about pots? Well there are four main considerations.

First, any metallic ions in the silica network, and oxides of many of our colourants, are polarised. This can cause the pots themselves to heat up, sometimes quite dramatically. A moderate amount of this isn't a problem, but too much and the user may burn their hands or lips.

Secondly, if there are any thin films of metals, such as lustres from raku, enamels or overglazes, or metal foils, these act as an antenna and heat up very rapidly, causing local thermal stresses that can break the pot.

Third, if the metal has any spikes, sharp points or corners, or is rough or crumpled, this may cause sparks or arcing. A few sparks probably won't harm the microwave but won't appeal , which is seriously bad news! Just search on metal in a microwave in YouTube to find out why 🙄😁.

Finally, what if there is water in the clay? Whatever the type of clay, if it hasn't been fired to be fully vitrified then if the glaze is crazed, or there are unglazed areas like the base or foot ring, it may well have absorbed water. This will obviously heat up in the microwave. If it stays liquid, it will (just) heat the pot. But, with enough heat, it will turn to steam and the pot will probably explode, just like bisque firing pots that haven't dried. Not the best route for customer satisfaction!

There are EU and US standards for this - both require checking for visual damage, and checking the temperature of the pot. The maximum handle temperature after 1 minute is 56°C in the EU, though Americans have tougher skin as higher temperatures are allowed. So what can you test as a potter? Well place your piece (empty) in a microwave, alongside a small bowl of water so you aren't running the microwave empty. Put on maximum power for a minute, and see if your piece has heated up. Also, check the porosity of your clay in the firing cycle you use, and if it absorbs over 3% water check that it is fully sealed with your glaze.

Uranium

This material was widely used as a glaze colourant until the 1940s, whereupon it was deemed better to employ it in nuclear weapons and power stations. But since the end of the cold war it has become available again, though not through your local potter's store. It is now available as the insoluble oxide U3O8, and the uranium is depleted, so it has about half the radioactivity of natural material.

For the potter, if uranium oxide is breathed in over the long term then it can cause damage to the lungs and respiratory tract, though studies have not sufficiently differentiated between the pure oxide and its mixture with silica, radon and other uranium compounds. If ingested, only 0.2% of uranium oxide is absorbed into the body, and of this 2/3 is passed out through the urine within a day, so it has little chance of being toxic. It can attack the kidneys, but the amount to be ingested for this is much higher than is likely to be encountered in a studio pottery.

The level of radioactivity of uranium is very low. An analysis has been done for a dishwasher in a restaurant, washing nothing but uranium glazed crockery. Their total exposure over a year is similar to that of a single X-ray. So the effect on the potter is likely to be similarly small.

As far as the end user is concerned, having uranium glazed pots around the house will not significantly increase your exposure to radioactivity. The risk comes from uranium leaching out of the glaze, which can be particularly high in lead glazes. The leached uranium is, of course, soluble, and so 1-2% is absorbed through the gut into the body. In some areas where there are high levels of uranium in the drinking water, food and drink and take the user up to 3/4 of the safe daily allowance, and a heavily leaching plate will tip their intake over the edge. So it is not a good idea to use uranium glazes in areas in contact with food or drink.

You may think that there is unlikely to be uranium in the water, but uranium is more common than you might think - at one stage most European countries had at least one uranium mine (in Cornwall in the UK).

PPE for Wood Firing

Packing my bits up for an Anagama firing in Oxford starting on Wednesday, so here's the safety gear (AKA PPE - Personal Protection Equipment) that I'll have with me.

For stoking, I'm trying out these Firemaster Alpha gauntlets from Southcombe. These are basically all Kevlar/Aramid fibre, with a breathable but waterproof middle layer. Many wood firers use leather welding gloves - it'll be interesting to see how these compare. Southcombe also do a range of leather gloves and gauntlets, all to the relevant EU standards for heat resistance, mechanical durability and firefighting use.

I've also got 2 different goggles to protect from the UV light when looking into the kiln, a lighter pair from Delta Plus and a darker pair from Bolle. Different people recommend different levels of protection, so it'll be good to compare them.

I'll report back next Monday!

Wedging

I didn't do many posts over the last week as I put my back out (again), and I've only been able to move around without things going into spasm for a couple of days.

So today's health and safety topic is backs and wedging - how my studio is set up and how I should do it. Wedging is kneading the clay to homogenise it and remove any air bubbles.

Here you see my current setup, with my wedging slab sitting on top of the worktop. As you can see, my arms are bent so all the effort comes from my arms and shoulders, and my back muscles need to counteract my arms to keep things in balance.

What I should do is lower the height of the wedging table so the top is level wih my knuckles with my arms by my sides. Then I can wedge using my body weight instead of my muscles, by rocking from my legs or hips. My arms and back would be straight, and not subjected to any undue loads.

So, time to listen to my body and make time to change things like this.

I talked earlier about crazing with respect to food safety, but there is also the bigger issue of glaze fit (whether crazed or not) and the strength of the piece. The user doesn't want it to chip or break in their hands.

Getting the glaze right can increase strength four fold compared to an unglazed piece, or a badly fitting glaze can weaken the piece. Note that there's no universally good glaze, you need to look at the combination of glaze, clay body and firing cycle.

The primary effect of a glaze is to fill or bury all the surface defects in the clay that act as stress raisers, which can trigger a crack propagating through the piece. The glaze is smooth, with fewer, smaller defects. And once the clay defects are covered by clay, they need double the force to start a crack.

But also for maximum strength the glaze should be in tension, putting the clay body into compression. This pre-stressing means that it takes a larger force to make the piece go into enough tension to fail (both the clay and glaze are much stronger in compression, so we don't need to worry about compression failures). If the glaze tension is too high, it will break the bond between clay and glaze, and chunks of glaze will come off (shivering) though this is quite rare.

More common is when the glaze is in excessive tension, weakening the piece. If the tension is high enough, the glaze will crack, resulting in crazing. Now the glaze cracks act as stress raisers, making the piece even weaker.

So check you have a good fit between your clay and glaze. And if you must have crazing for your aesthetics, make the clay thicker to compensate for the weakening effects of the crazing.

Kiln Fumes

Some kilns will give off more fumes than others. And I am not sure how well a kiln vent would work with this baby!

Here I'm looking at vapours from boring old electric kilns, so not gas or wood fired or raku, with their smoke from reduction firing.

When fired, organic substances burn out at low temperatures, together with sulfur if present. Then, as metals in the glaze get hotter, they can start to vaporise - for example with lead it can start to vaporise at 500C.

But how much is given off, and how harmful is it? Sticking with lead, the only mention I can find on vaporisation from glazes is in Hamer & Hamer, and different entries contradict each other. As the metal is bound in to the frit or glaze melt over the whole firing range, I would expect the vaporisation to be less than when just heating lead oxide. I have an experiment planned to measure this, more of which in due course.

Then how much escapes from the kiln? Assuming the bungs are in (and the kiln isn't vented) it is fairly well sealed, but with rising temperature the air expands and will leak out into the room. In firing from 500 to 1200C, about a third of the kiln air will enter the room. There it will mix with the existing air, diluting any vapours.

The trouble is that whilst the various lead compounds and frits in their raw state are too large to fully penetrate the lungs, so a minimal amount is ingested, vapours are micron sized particles that penetrate deep into your lungs, where over 90% is absorbed into the blood.

Apart from needing to know how much lead may be given off, the following points should be considered :
- ensure there is air circulation in the room when firing, and ventilation if it is a small room
- if the room is part of or connected to your house, consider how you can prevent fumes entering the house
- don't inhale over the kiln

Unless you find the kiln fumes an irritant, or use a small broom cupboard for your studio and kiln, or deal a lot with toxic materials that give off significant fumes when fired, having a kiln vent system is probably overkill. As always, when reading an article recommending one, look who wrote it and ask what their interest is.

For example, in the UK the Health and Safety Executive don't recommend their use, even in schools. There is only one paper I can find on the topic - Google "Exposure to Pottery Kiln Emissions" by Robert Douglas Hirtle

Food safe?

Here I'm talking about the leaching of potentially toxic chemicals from the glaze into food or drink - the potential hazards of crazing are a separate post.

Almost all of the food and drink we consume are neutral or acidic (with a few exceptions such as birds' nest soup, conch and egg white). The most acidic foods are probably lemons and limes, with a pH of 2.

In an acidic environment, leaching occurs by hydrogen atoms in the liquid swapping places with metal atoms in the glaze. At first this starts at the surface, but slowly eats further into the glaze (though this also slows down the rate of leaching). A strong acid or high temperature both increase the rate of leaching, as can the presence of some metals in some cases, e.g. copper greatly increases the rate of leaching of lead. Also, immersion in an alkali environment such as a dishwasher can attack the top level of silica in the glaze, thereby exposing more metals to be leached next time the pot has food put in it.

Lead leaching is the main concern for most people, as it is cumulative in the body and many of us have already had high exposure through leaded petrol and lead in house paints. But Barium is often leached at levels that mean the maximum recommended daily intake is exceeded, and other metals can also be leached to excessive amounts.

The other effect is that the glaze is weakened, with the metal atoms being replaced by much smaller hydrogen atoms. This both reduces the physical strength of the glaze, and its resistance to chemical attack.

So what to do? The following all help:
- avoid the use of lead, barium or other metals in quantities that may be toxic in glazes on food or drink contact surfaces
- high fired glazes are inherently less likely to leach than low fired, so be extra careful with low fired pots and glazes
- don't use glazes that are matt due to being underfired. Test by firing 100C higher and compare. - use as much aluminium (especially) and silica as possible in the glaze
- don't use more alkali metals than necessary, as these are very prone to leaching (lithium, sodium, potassium)
- avoid copper as it destabilise many glazes

Only lead and cadmium leaching is tested in most countries, of which studio potters only really use lead. But with the current maximum approved leaching levels it is easily possible to exceed the maximum recommended level of lead ingestion. In the light of this, the EU are currently drafting new laws, which are also likely to cover other metals as well. They also have regulations for materials in contact with food, which bans any release of materials into food at toxic levels. The official tests are too complex to replicate in the studio, but what can be done is to make two identical bowls, and fill one with lemon juice. After a week, rinse it out and dry it, then compare the glazes looking for any change in colour or loss of gloss.

Beryllium

Not an everyday substance used by potters, but some have experimented with it, and it continues the thread of alkali earths.

Although not cheap, it affects glaze colour (as in the gems emerald and aquamarine), acts as a glaze modifier like aluminium, and hardens lead glazes.

For the potter, it is pretty safe. The only significant route of exposure is breathing, and fine particles can stay in the lungs for years, causing lung damage, but the exposure level for this is much higher and longer than a potter will experience. Similarly, although it probably increases the risk of lung cancer, this is only with long, high exposure levels.

Some have expressed concerns about beryllium fumes when firing, as these are ultra fine articles that can be inhaled. There is no evidence whether or not beryllium fumes are given off from a glaze, but a kiln vent or a kiln room vented to the outside makes sense as a precaution.

But if you have cuts or abrasions that could let it get under your skin, this can cause irritation and be quite nasty, so gloves make sense whether the material is wet or dry.

As far as food safety is concerned, there doesn't seem to be any data on leaching rates, though because of its chemistry these are likely to be lower than other alkali earths. But even if it leaches heavily, less than 2% of swallowed beryllium is absorbed into the body, so the amount is well below any risk of toxicity.

Frits

Frits are made by melting together a number of raw materials, and then grinding them up.

They are used in glaze recipes for a number of reasons:
- protecting potters from toxic materials that can enter the body in raw form, but not in a frit, e.g. Lead bisilicate enables potters to work with lead glazes safely
- making materials available in an easier to use form, e.g. with Boron we find Gillespie Borate is slightly soluble and so the glaze mix varies with time, whereas Colemanite can give off its water explosively during firing, damaging the glaze surface
- offer glaze components in proportions that cannot be achieved using standard insoluble glaze materials
- the frit may have a lower melting point than the raw materials, or need less heat work to melt, as the energy to create intermediate compounds has been put into the system when making the frit
- ease of use and consistency. Using frits can mean fewer components to mix up in the glaze; the frit generally has a more consistent formulation

Obviously the first of these is key to our theme of health and safety.

But, contrary to what I've seen some people claim, it doesn't make any difference to the finished product. Assuming the glaze chemistry is the same, you get exactly the same result whether you use, say, lead bisilicate frit or white lead and silica. Using a frit won't solve any issues of glaze durability or leaching.

Rice husk ash

A commonly used glaze component in oriental glazes, composition varies according to where it comes from but is generally mostly silica, with a couple of percent potassium, and the rest just traces of elements.

The silica in the rice husks is amorphous, not the crystalline sort that can cause silicosis, and this can be seen as just a dust hazard.

However on burning the husks to create the ash, the silica crystalises to cristobalite and tridymite from about 800°C (a lower temperature than pure silica due to the potassium acting as a flux), so rice husk ash should be treated with the same respect as silica and most other silica containing materials. In fact, it is more hazardous than many, as the silica particles are particularly fine, and so a greater proportion will descend down deep into your lungs.

Dust masks on!

This piece is on lead in the studio, and risks to the potter (not lead glazes on functional ware, and risks to the user, which merits a separate post). The effects of lead poisoning have been known for centuries, but in pottery only came to a head with the mass manufacturing of Wedgwood and others in Stoke-on-Trent, when people's whole work may be glazing. This resulted in about 400 deaths a year, and many more severely sick.

The employer's approach was to pay the glazers an extra 2.5p a week, but it prompted parliament to pass its first health and safety legislation.

Nowadays, the old mainstays of lead oxide and lead carbonate are not permitted in potteries. A significant amount of lead is absorbed through the respiratory and digestive systems (though not the skin), and their use caused lead poisoning in time as the body excretes very little lead.

Instead, a lead frit, lead bisilicate, is now used. The frit is a lot less soluble, further improved by adding traces of aluminium and titanium, and covering the frit in silica. With this, only 0.2% of the lead dissolves into the water, and with good studio practice only a small proportion of this may be absorbed. Bristol glazes and boron in glazes were also developed to get rid of lead. Combined with better workshop practices, over fifty years the number of deaths from lead poisoning amongst potters dropped to zero.

If you need a lower melting point or less silica, you can use lead sesquisilicate, though this is more soluble.

If using lead, you must use good practices in the studio: wipe or wash everything down to eliminate dust, use ventilation to get dust out of the air, wear a good dustmask and clean tightly woven clothes that don't pick up dust, and no food or drink in the studio. If working with lead a lot, it is probably also worth having a blood test every 6 months to check your lead levels.

So, by following good practice, there is no reason why you should worry about using lead in the studio.

Lithium

I'm back to the chemicals we use, looking at the alkali metals, and beginning with Lithium.

Lithium is popular both as a flux for medium to low firing ranges, and also in reducing crazing. However it has never been cheap, and the increased demand for its use in batteries has pushed the price up further over the last few years. As well as batteries, it is also known for its treatment for bipolar and other mental health issues.

The maximum daily medical dose for adults is 2.4g, and this is roughly where toxicity effects kick in. This is a lot of lithium to breathe in or swallow, especially as in a study of some 60 small potteries only 2% had detectable levels of lithium in the air. As regards a lethal dose, this is over 80g for adults.

Turning now to the user and levels of leaching, I have only found one glaze test result for leaching of Lithium, at 0.48mg/l. With a daily total intake of about 4l of water, this would correspond to about 2mg of lithium a day. To take a worst case scenario, we could multiply this by 10 to give 20mg/day.

To put this into perspective, people ingest up to 4mg a day, but this can easily be doubled if they are taking mineral supplements or in an area with lithium in the soil.

So, for those not on lithium medication its use in the studio only presents the usual dust hazard. For the user the levels leached are about 1% of the safe daily dose, so again not significant. If a potter is on medication, then there is a slight increase in risk. However they will be getting regular blood tests, and will probably be familiar with the side effects of excessive lithium, and so they will be able to detect any problems from increased exposure.

The greatest potential risk is for the end user on medication, as they probably won't realise that their tableware is leaching lithium, and their medical dose may be close to the toxicity level. But we are talking about increasing their daily dose by a maximum of 1%. If we consider that the tablets (in the UK) are available in 250mg and 400mg doses, I think it is reasonable to say that a 20mg variation of daily intake of lithium (max) is not too significant.

Cracking pots

With the last frosts having gone a couple of weeks ago, I looked round the garden at pots like this that have cracked during the winter, and started looking into why it happens. If you understand that, then you can do something about it.

The standard story is that it is just water that has permeated the pot expanding (by about 10%) on freezing, but that doesn't explain why some stoneware pots, with low porosity, fail, and other very porous ceramics (like terracotta or bricks) don't.

It turns out that simple water expansion is just one of several failure mechanisms, and generally the least likely.

I'm still researching this, with papers on ice damage to bricks, concrete, rocks etc. (though many are behind the academic pay wall 😠). An interesting mechanism is the ice lens. Water in narrow, curved pathways can stay liquid down to -10C, and with a temperature gradient across the pot wall the supercooled liquid moves towards an initial ice formation, forming an ice lens parallel to the surface, until its pressure splits off the pot surface, which is what may have happened here. The same thing happens at a much larger scale in the soil at high latitudes, which can be pretty impressive.

Whilst, for example, the tile industry has European, US and ISO standards for frost resistance, they are just based on simple freeze/thaw cycles and don't properly model the real world causes, which is why many people complain that "frost proof" products can still fail. Similarly with the rule of thumb used by many potters, of porosity under 0.5% being frost proof, and under 3% frost resistant.

More on this later as the research unfolds...

Calcium

At last, something that isn't toxic!

For the potter, it is normally added as Whiting, i.e. ground up chalk. This, along with dolomite, bone ash and Colemanite (which add Magnesium, Phosphorous and Boron respectively) are just dust hazards, as are more complex sources like Gerstley Borate. Wollastonite, or calcium silicate, needs a bit more caution due to the presence of silica, with its greater risk of lung damage through silicosis.

For the end product, if some sodium or potassium can be replaced by calcium, it will increase glaze durability. Only small amounts of calcium can enter the glaze matrix at low temperatures, though this increases as the temperature rises. Excess calcium acts as an opacifier, giving a matt white glaze, but this also devitrifies the glaze, making it more prone to leaching and weathering. Although leaching of calcium won't harm anybody, if the glaze contains other materials that are toxic this may be an issue, as well as any adverse effect on the appearance of the glaze.