Left to right: Manganese dioxide, manganese and iron, and iron oxide wash
Glazes are, by and large, concoctions of powdered minerals mixed with water.Some of these are relatively benign, others are categorically lethal.There are dangers from carcinogens, heavy metal toxins and even – if you throw all caution to the winds – radiation.
But don’t let that stop you from using most glaze ingredients (with the exception of uranium, because no, just no), as potters have been using them safely for years and our understanding of both the minerals and the precautions necessary has never been better.Every potter should make it their business to know the potential dangers of each ingredient they use, because it’s not just their safety at stake, but also anyone who may come in contact with the raw glaze, the firing fumes or the finished functionalware.Glaze safety doesn’t start and end with the process of glazing, the potter is responsible for it every step of the way.
The number of possible minerals used to make glazes is nearly as varied as the number of ingredients used to make biscuits.Are they oatmeal or chocolate?Sprinkles or nuts?A gooey caramel centre or crunchy honeycomb bits?You get the idea. The below selection of photos show the number of stages of glazes and minerals involved in one piece. The most dangerous, Manganese Dioxide, I did first so it would be covered by the others and I wouldn't risk exposure while handling. (Hover over to see the other ingredients involved).
I’ve written an article detailing the dangers and uses of some of the more common colourants Ж, but to address all the possible ingredients individually is slightly beyond me at this point in time.
There are, however, broad statements and precautions that apply to all ingredients that will help form a good understanding of the dangers involved, and it’s these I intend to discuss.
Unless you’re buying all premade glazes (and even then, they become dusty when dry) you will be dealing with minerals in powder form.ЖAll dust is an irritant, even the stuff that collects just out of reach on top of our bookcases.Breathe in enough of that and it will cause inflammation in your lungs, leading to coughing, sneezing and shortness of breath.This is at the most benign end of the spectrum of dust, and for some mineral powders such as iron oxide, this is as far as it goes. Others, like Rutile and Tin Oxide, can collect in your lungs as black deposits, and while these look and sound intimidating, there’s little evidence so far to suggest that they cause much more than a slightly reduced pulmonary function.
A step up from that is a much thrown-around term ‘Metal fume fever’, which can result in a whole host of temporary flu-like symptoms (including nausea, muscular and joint pain, fever, chills and headaches), but the long-term effects are unknown.Minerals that cause metal fume fever are relatively common, and while the ailment itself isn’t believed to be particularly serious, many of the minerals that cause it also have additional dangers to be aware of.
Beyond metal fume fever we start getting into the nasty stuff, specifically, those that are carcinogens, neurotoxins and those that cause pulmonary fibrosis.There are plenty of other possible side effects of dangerous mineral exposure, such as gastrointestinal haemorrhaging and coronary disease, but the three below represent common themes in ultra-high risk minerals.
Carcinogens can result in cancer, often appearing in the lungs or respiratory system due to inhalation of the dust.Known carcinogens in pottery include chromium, nickel, cobalt (suspected) and even silica (particularly in fibre form and after being fired). Ж
Neurotoxicity describes a mineral’s affect on the brain and nervous system, and some heavy metals such as lead and manganese dioxide can cause significant damage.Due to the complexity of the central nervous system and the many different ways its impairment can present as symptoms, it can be quite difficult initially to identify.Heavy metals often build up slowly within the body, but can be very hard to shift once there, so the full consequences of prolonged exposure may take time to be realised.
Pulmonary fibrosis is the scarring of the lung tissue and can lead to death.The causes of fibrosis aren’t always clear, but certain compounds are well documented risks.The mineral is inhaled and begins by killing off the lung tissue, which reacts by scarring over.The scar tissue is too thick and heavy to perform the function of gas exchange, meaning there is a significantly reduced respiratory function which usually presents as constant shortness of breath, wheezing and coughing.There is no cure for pulmonary fibrosis and the damage it causes cannot be reversed.
A lot of the above is pretty scary, but it needn’t be a deterrent so long as common sense and adequate precautions are in place.I remember signing a disclaimer when going for a massage that included on its list of possible unintended side effects “paralysis and death” – seriously!That’s never going to stop me from getting a massage at every available opportunity, I just have to trust the masseuse to know enough to not crush my larynx or snap my spine.
Similarly, sensible precautions can go a long way towards minimising the risk of any of the above.Dust masks are the obvious first step.If you’re not working with particularly dangerous minerals, the disposable ones may be sufficient, but for the higher risk ingredients you might want to look into a proper unit with replaceable filters.Keep in mind that the risks go beyond the task of glazing, and the same potentially dangerous minerals can be airborne when stacking or unloading a kiln, particularly if you use silica sand or kiln wash powder as a shelf buffer. Ж
Secondly, care in handling should be a no-brainer.Don’t blow it around the place, scratch your nose with contaminated hands, or get it on anything that you might take into the living areas of your home.This leads on to taking precautions with your clothing.Mineral dust is likely to embed itself into fabric and be difficult to remove completely, even in the wash, so I’d recommend having a dedicated set of clothes just for glazing.Ideally, these would be washed separately from the rest of your clothes and not brought into your house any more than necessary.It may also be a good idea to wear an apron over the top of these clothes to further minimise any contamination.I’ve read a reported case of two family members falling ill with respiratory disorders which were linked back to the husband’s clothes.He was an industrial worker and came into contact with silica powder on a regular basis, and when his clothes that were washed in the same load as the rest of the family it became a risk to both his wife and daughter.So it can happen and it’s just not worth risking if you’re working with any ingredients that are dangerous.
Liquid/Dermal contact and absorption:
Liquids aren’t as mobile as airborne dust, but the same minerals are involves so we still have to take steps to prevent any of them from entering our body.There may be the perception that once it’s in liquid form it’s safe because it’s bound up in the water and so long as we don’t drink it, it can’t hurt us.In some instances that’s relatively true and certain minerals won’t go much beyond giving you a bit of an itch if you come into direct contact, but others can be absorbed directly through your skin and cause as much damage as if you ingested them.
Many of the possible disorders are already listed above in the Inhalation section, so I’ll not repeat them here, aside to say that there is – obviously – less of a direct danger to the respiratory system and more to the skin, although the consequences can extend far further if the toxins are absorbed through the skin into the bloodstream.
Gloves are definitely recommended.If you’re using light latex ones, have your nails well trimmed and be careful putting them on.I’m not sure if anyone noticed, but during one of my videos I’d let my nails grow too long before oxiding and stabbed a hole through one of the fingers.My left glove filled up completely (luckily just iron oxide, which is about as benign as you can get), and when I stripped the glove off I was literally caught ‘red handed’ and had a bit of a job scrubbing it off afterwards.Had it been anything more dangerous than iron oxide I would have stopped and remedied the situation immediately, but some still would have been absorbed into my skin even so.If you’re glazing large vases in big vats you may need elbow or shoulder-length gloves, which I believe can be bought from veterinary suppliers. Gloves may feel a little cumbersome and cause a larger fingerprint mark than bare skin, but if you’re handling toxic minerals it’s in your interests to do everything you can to prevent them from coming in contact with your skin.Finger sheathes should still be able to be worn with gloves to reduce the mark from where you were holding the piece of pottery, just be careful when you’re putting them on that one of the stiff ridges doesn’t pierce through the glove.
My mother's vent system was a little more elaborate than mine, but she was dealing with more dangerous minerals
So far the dangers highlighted have been during the glazing process and therefore mostly restricted to affecting the potter.But at the point of firing we have to expand the circle of responsibility to include anyone in the vicinity of the kiln.
Glazes grow molten under intense heat, some of them bubble and most will fume to some extent.Those with texture usually fume more than those without, although that’s just a rough rule of thumb and may not always be the case.Either way, contaminants are released into the air through the process of firing and can affect people nearby.
Any kiln instruction manual will talk of the need for adequate ventilation, and this is mostly designed to protect you, the potter.Left unchecked, potentially toxic fumes will build up to dangerous levels in an enclosed room.Opening a window is probably not going to be sufficient, ideally you should have a way of directing the fumes out safely.Emphasis on ‘safely’.Pumping them out into your neighbour’s back yard is definitely not recommended.
Here are a couple of things to consider when setting up your kiln room:
Is it attached or close to your house?
Is there a way to seal any entrances or windows that would otherwise allow firing fumes to enter on that side of the house?
If your kiln is in your work studio rather than being separate, do you intend to work while it is firing?If so, do you have an adequate ventilation setup, such as a Kilnvent, to direct the fumes away?
Where are they going to go once outside?
Which direction are your prevailing winds and are there any neighbours close by that may have the fumes blowing directly into their windows/doors?
Some fumes are definitely going to be worse than others, as with the other sections, but the precautions we take for ventilation are usually much more set in stone than those to protect against dust or skin contact.We can, for instance, easily buy better gloves or dust mask if we shift from using benign ingredients to the more dangerous ones, but if our kiln is in a position where it’s pumping fumes directly into your bedroom window, it’s a major exercise to move it if you suddenly have a need to.
Some people may question ingestion being a significant danger in pottery.We’re not, after all, likely to sprinkle chromium carbonate on our cereal like sugar, or chow down on a few spoonfuls of titanium dioxide to start our day.Those who do aren’t likely to be in the ceramics business – or any business – for very long.But regardless, ingestion is still something that falls under our purview, although in this instance the dangers are primarily to our customers instead of ourselves.This section specifically relates to functionalware, any bowls, plates, mugs or platters that have a specific use involving food or drink.Vases, ornaments or display pieces can be excluded and don’t have to be worried about unless you have reason to believe your customers will be beset with the uncontrollable urge to lick them repeatedly.Stranger things have happened.
Once fired, glazes look and feel solid.They’re hard and glassy and make a nice ‘ping’ when you tap them with a fingernail.But appearances can be deceiving, just like how coal is technically classified as a liquid – who would have believed that from looking at it?!Glazes look solid and immutable, but it’s not actually the case.The long and the short of it is that minerals leech out of glazes under certain – often quite common – circumstances.
Foremost of these is being left for a prolonged period in some sort of acidic solution, which isn’t as unlikely as you may think.A citrus juice in a mug, a fruit salad in a bowl or a tomato-based meal cooked in a baking dish.All of these are quite reasonable, expected even, yet they’re close to the worst case scenario in terms of a dangerous glaze.
Some glaze ingredients are infamous for it, which is why there are now regulations on using lead in functionalware.But too many potters seem to believe that so long as they stay away from lead they’re fine, that nothing else could possibly leech out.This is not the case.Lead, as a neurotoxin, is a particularly high-profile example, especially as its dangers have come under considerable scrutiny in other areas such as with paints and soldering.But it’s hardly the sole perpetrator.
Chromium oxide and cobalt (both oxide and carbonate) are two mineral colourants that are also potentially dangerous when used on functionalware.Some potters say there are stable glaze bodies that prevent leeching whereby ingredients such as these can be used safely, and that may be the case (I’m no expert on specific glaze chemistry), but I strongly urge you to check and get your glazed pieces tested for leeching before advertising them as food-safe.At the very least, do some research and know exactly what you’re putting into a glaze, particularly if it’s going to come into contact with food or drink.Keep in mind that even small alterations can drastically change the safety of a glaze in this regard, as some minerals such as copper work to make glazes more soluble and therefore more dangerous.
I may sound like a fearmonger, that I’m overcomplicating ceramics and taking excessive precautions, but when working with anything dangerous I feel we have a responsibility to err on the side of caution.Customers aren’t going to research, or even know, each of the minerals we put into our glazes, they’re just going to trust us that it’s safe to use with food.My rule of thumb is to never send anything out that I wouldn’t be happy eating off myself.And considering I’m rather fond of my life and good health, I want to make damn sure that neither of those will be compromised by something I’m eating off every day.