|Monthly Tech-Tip |
The vast majority of materials used in ceramics are insoluble. But many still present hazards. And you can add hazards (to you and customers of your ware) by the way you use them. Still, there is a need to be realistic about it.
Are you getting suspicious that lawyers write most of the labels that now appear on material packages? Do all the labels look the same? How does one pick out the materials that are the most dangerous when materials thought to be safe are labeled in the same way as the toxic ones? Warnings have lost much of their impact, at times to the point of being ridiculous.
Material Safety Data sheets that outline hazards associated with coming into contact with the material itself, and while these have the same general format, there is a tremendous variety with reference to the information they volunteer, the hazards detail provided, and the degree of harmony between like materials. The average user is overwhelmed by all of these sheets, and reading them tends to make us cynical about how real the warnings are. In addition, these sheets have nothing to do with safety issues surrounding the functional use of glazes made using the materials.
In recent years the subject of material safety and possible safety issues surrounding the functional use of glazed ware with food and drink have become a hot topic on the Internet, in the press, and within educational institutions. There are people putting their heart and soul into trying to educate us about the hazards of many ceramic materials. At the same time there are many, including authors and people of repute, paying absolutely no attention and making glazes from whatever materials and whatever proportions they please. There are some arguing strenuously to defend the use of known poisons and carcinogens because we can't come up with hard data to prove that anyone is being affected by them. There are others campaigning against ceramics in schools and universities, claiming it is just too dangerous.
Surely there is some middle ground we can reach even. I feel the position is best expressed by Monona Rossol, an author, chemist, potter, and activist who summarizes two decades of selfless dedication to this issue on many levels by calling herself an 'industrial hygienist for the arts, crafts, and theatre'. She speaks of the 'ethical' use of materials and formulations. This approach strikes a chord with many who have seen so many cases where the supposed know-it-alls claim total understanding of things and kick down "caution signs" under the banner that no one can produce hard data to justify their existence. Ethical use of materials means we err on the side of safety and the prevailing common sense, and are less swayed by those with hidden agendas and conflicts of interest. If a material like barium, for example, is surrounded in controversy, then glazes that employ it are 'controversial' glazes. Do you suppose anyone would buy your ware if there was a hang-tag on it explaining that industry experts are currently embroiled in heated disputes over the material's safety?
Is the user of your pottery in danger from food or drink dissolving toxic compounds from the glaze surface, from micro-organisms that the glaze surface might provide a home for or from actually swallowing a chip of the glaze that has flaked off the surface? This site has lots of information to help you make your products safe for the end user, it does not require an engineer. There are some really bad glazes out there, we can help you recognize them.
There is some credibility to the statement that silica is the most dangerous material we use since silicosis is the most common ceramic material related health effect in production workers. Silica itself is clearly not a poison, you can eat it with no problems. But quartz crystals can lodge themselves deeply in the air pocket of the lungs so breathing alot of it is obviously not good. But remember, quartz is by far the most abundant mineral in nature. Gravel, concrete, asphat, garden soil, etc are all loaded with quartz. So the minimization of dust is a key component to a safe workplace. Kiln fumes are also obviously potentially hazardous, they need to be vented outside or kilns need to be in a separate room.
There are many articles on material safety on this web site. You can find a list of all of these articles by searching for the word "toxicity" or "safety".As a quick overview following are some general comments. Keep in mind that when I say a material is not generally harmful, I mean in the quantities a person would be exposed to in a clean typical working environment.
An example of a custom-made dust collection hood in the material repackaging area of a supplier. The slots along the front suck particles into the duct, the suction comes from an exhaust fan downstream where the pipe exits the building. It has a wall switch and a sliding damper (where the hood enters the pipe) to enable stopping all airflow (to prevent heat loss in the room during cold days). Notice it is located above the scale and heat sealer where most dust is generated during weighing and packaging. Working in front of a system like this enables me to mix glaze recipes without breathing any dust at all.
These are the results of a detailed elemental composition analysis of a sedimentary clay. The first column of numbers is ppm (parts per million), divide them by 10,000 to get percent. The Fe here, for example, is 50,868 or 5.1%. The second column is +/- error. Notice that this test does not detect boron or lithium, they require a different method. By contrast, the chemical analysis shown on the data sheet of a typical ceramic material shows only the principle ceramic oxides (less than a dozen), but all of these trace elements will still be present.
This designation is an international standard for a general purpose respirator to filter out respirable quartz particles (which cause silicosis). Use one of these when working in a area where ventilation is insufficient to remove all of the dust. Use it also in circumstances where there is temporary generation of large quantities of dust. Do not wear this as a substitute for keeping floors and working areas clean.
These materials have many issues. They can create problems in your glaze slurries (like precipitates, higher drying shrinkage), cause issues with laydown and dried surface and cause fired surface defects (like pinholes, blisters, orange peeling, crystallization). And lithium and barium have toxicity issues (as raw materials). And the lithium, barium and strontium are carbonates, that means carbon burns off during firing (with lithium, for example, 60% of its weight is lost). Yet the oxides that these materials source to the glaze melt, ZnO, Li2O, BaO and SrO can be sourced from frits. In doing that you can solve almost all the problems and get better glaze melting. Fusion Frit F 493 has 11% LI2O, F 403 has 35% BaO, F 581 has 39% SrO and FZ 16 has 15% ZnO. Of course, these frits source other oxides (but these are common in most glazes). Using glaze chemistry you can often duplicate the chemistry of a glaze while sourcing these oxides from frits.
Ceramic glazes can leach heavy metals into food and drink. This subject is not complex, there are many things anyone can do to deal with this issue
Respirable Crystalline Silica
The breathing of respirable quartz particles is a hazard in ceramic ware manufacture and hobby. These particles are of a size that can catch deep in the air sacks of the lungs.
Manganese in Clay Bodies
Manganese is used to stain clays (using black) and to impart fired speckling (as a decorative effect). It is dangerous?
Overview of Material Safety by Gavin Stairs
Toxicology and ceramic materials is a complex subject, what materials and methods pose the greatest dangers, which are the safest?
Crazing and Bacteria: Is There a Hazard?
A post to a discussion on the clayart group by Gavin Stairs regarding the food safety of crazed ware.
Attack on Glass: Corrosion Attack Mechanisms
Max Richens outlines the various mechanisms by which acids and bases can dissolve glass and glazes. He provides some information on stabilizing glazes against attack.
Is Your Fired Ware Safe?
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
How to Liner-Glaze a Mug
A step-by-step process to put a liner glaze in a mug that meets in a perfect line with the outside glaze at the rim.
Are Your Glazes Food Safe or are They Leachable?
Many potters do not think about leaching, but times are changing. What is the chemistry of stability? There are simple ways to check for leaching, and fix crazing.