|Monthly Tech-Tip |
Metal oxide powders are used in ceramics to produce color. But a life time is not enough to study the complexities of their use and potential in glazes, engobes, bodies and enamels.
Metal oxides are what the name suggests. A metal oxide can literally be made by melting and metal and spraying it as a mist into an oxygen chamber. The powder produced is a completely new form of matter with new properties, it leaves the realm of metallurgy and fits comfortably into the ceramic world. A carbonate color could be produced the same way (using CO2 gas of course). Metal oxides are used in dozens of industries, not just ceramics. It goes without saying that some are expensive, some are not, their demand (and price) is often pushed by their utility in technology products.
In ceramics metal oxides impart color to glazes, bodies, engobes and stains (however not all a colorants). Most common glazes used in pottery are a composite of a base transparent with a metal oxide added to color it (with optional opacifer and variegator).
Common metal oxide materials are iron oxide, cobalt oxide, chrome oxide, copper oxide, manganese dioxide, nickel oxide. Some of these have carbonate forms (e.g. cobalt, copper, nickel). Some are stable and predictable in their color contributions (e.g. cobalt, chrome). Some, although named "oxides", are actually a mix of the oxide and hydroxide forms of the metal (e.g. copper oxide). Some metal oxides can exist in multiple colors depending on the metal:oxygen ratio (iron oxide can be red, yellow, brown, green, maroon, black). Some metal oxides are finicky to develop their color, requiring specific chemistry in the host glaze, others require specific firing conditions. Some are very potent, less than 1% giving intense color, while others require much larger percentages. Zinc, lead and tin do not produce color by themselves, but interact with others to develop unique effects. Many metal oxide blends (called systems) exist, they produce completely unique colors. Some have temperature limits, they become volatile and gas off beyond that (e.g. cadmium, selenium). Some colors decompose somewhat during firing and generate gases that can cause glaze defects (e.g. manganese dioxide, copper carbonate). Others decompose and gasify, producing metal fumes (e.g. cadmium, lead, manganese). Some metal oxides contain larger particles that produce specks in glazes (e.g. iron and cobalt oxide). Some materials used in ceramics are not technically metal oxides, but they are minerals having a high metal oxide content (typically a mix of metals). An example is rutile, it both colors the glaze and variegates it (by virtue of the titanium). Other examples are Barnard Slip, Raw Umber.
With metal oxides in ceramics, color development is about chemistry, concentration, particle size and firing atmosphere and firing schedule. The raw color of a metal oxide seldom is anything like the fired color it produces (unlike stains). Mixtures of different metal oxides do not normally produce expected colors (e.g. chrome makes green, tin makes white, but specific mixtures of them can produce pinks and reds if the chemistry of the glaze is right). Stains theoretically eliminate this problem since they have been prefired and mixed with stabilizers to make them refractory. Interestingly, the two main stabilizers are metal oxides that do not produce a color (aluminum and zirconium). Stains also catalog the metal oxide color-systems that history has taught us.
Many people avoid stains and continue to use metal oxides in order to save money. But this may be misguided. People like bright colors, and you will not be able to achieve them as well using metal oxides. Normally, lower percentages of stain are needed to produce a given color. And often glaze layer thickness can be reduced. And stains do not blister and pinhole glazes like metallic carbonates or hydroxides. And stains are safer to handle. And more consistent. And they make it more practical to employ a base-glaze-with-addictions approach, thus minimizing the number of recipes you need to maintain. Stains are definitely better in underglaze colors. It also may be worth learning to apply glazes by spraying, painting or double-dipping so smaller bucket sizes can be maintained.
That being said, some practitioners passionately prefer metal oxides over stains. The best expounded explanation I have heard of this is from Will Robertson: "Ceramic stains would be vastly easier but I think they'd give too much human control - with metal ions in the glass the glass gives a connection from the viewer to the quantum mechanical world of the outer orbitals of the ions in the glass and those in turn give a connection to the ligand fields of surrounding atoms and ions (the exception being the rare earths with their light absorption by inner electron shells). I think there's an excitement and integrity to the creation of colour and to working in an environment that cannot be brought under full human control. I also a bit about using ceramic stains which function as pigments - the metal ions can function more like a dye, dissolving in the glass and giving a clear 3 dimensional coloured space. Humans are used to seeing colours on surfaces, not in 3 dimensions, so hopefully there's some magic in that".
Most heavy metal oxides are hazardous to handle, their fumes are especially dangerous to breathe and their solutes can be toxic. Heavy metal colorant mining and manufacturing operations in the world are widely known to be wildly unethical. Purchasing ethically sourced materials will not be easy, it means you will need to be willing to pay more for already-expensive products. Sometimes it is possible to achieve a desired color my other means (e.g. starting with a natural material that already has some or most of the colorant in the natural form).
Toxicity is a concern for glazes containing metal oxide colorants: Specifically leaching and the hazard of metal fumes during firing. Obviously, the higher the percentage of metal in a glaze or body the greater the potential for fumes. And common sense is a guide for leaching. If a glaze contains 4% manganese, that is one thing, but if it contains 30% that is quite another! Use the GLLE test as a simple way to check. Always be cautious about fired ware having very bright colors, that generally takes a high percentage of metal oxide. Glaze durability against acids and bases degrades as metal oxides saturate the glass. When metal percentages get high enough they can actually precipitate out of the melt during cooling in the kiln, creating crystals. They might look great, but they are very often soluble.
The process he has evolved requires 3 firings. First, the dry porcelain ware is spray-decorated with metal oxides (iron, manganese, copper, chrome and cobalt oxides) and then high bisque fired to cone 9 to bond them on and vitrify the body. Then a cover of lead borosilicate glaze is fired on at cone 04. The lustres (made of clay, which washes off after the lustre firing, mixed wtih silver chloride or silver carbonate) are mostly applied with a brush over the fired 04 glaze and reduction fired on at cone 018. He makes red by blending copper carbonate and bismuth sub nitrate with the clay, and this can be mixed with the silver clay paste to make a ‘gold’. He also sometimes adds silver to the oxide sprays which creates a lustrous underglaze.
You found some recipes. Their photos looked great, you bought $500 of materials to try them, but none worked! Why? Consider these recipes. Many have 50+% feldspar/Cornwall/nepheline (with little dolomite or talc to counteract their high thermal expansion, they will craze). Many are high in Gerstley Borate (it will turn the slurry into a bucket of jelly, cause crawling). Others waste high percentages of expensive tin, lithium and cobalt in crappy base recipes. Metal carbonates in some encourage blistering. Some melt too much and run onto the kiln shelf. Some contain almost no clay (they will settle like a rock in the bucket). A better way? Find, or develop, fritted, stable base transparent glossy and matte base recipes that fit your body, have good slurry properties, resist leaching and cutlery marking. Identify the mechanisms (colorants, opacifiers and variegators) in a recipe you want to try and transplant these into your own base (or mix of bases). And use stains for color (instead of metal oxides).
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
Closeup of a crystalline glaze by Fara Shimbo. Crystals of this type can grow very large (centimeters) in size. They grow because the chemistry of the glaze and the firing have been tuned to encourage them. This involves melts that are highly fluid (lots of fluxes) with added metal oxides and a catalyst. The fluxes are normally B2O3, K2O and Na2O (from frits), the catalyst is zinc oxide (alot of it). Because Al2O3 stiffens glaze melts preventing crystal growth, it is very low in these glazes (clays and feldspars supply Al2O3, so these glazes have almost none). The firing has a highly controlled cooling cycle involving rapid descents and holds (sometimes multiple cycles of these). Between the cycles there are sometimes slight rises. Each discontinuity in the cooling curve creates specific effects in the crystal growth. Thousands of potters worldwide have investigated the complexities of the chemistry, the firing and the infinite range of metal oxides additions.
These have to be seen to be believed, it is the deepest, richest blue we have ever produced. This is Plainsman M340 fired to cone 6. Black-firing L3954B engobe (having 10% Burnt (not raw) Umber instead of the normal 10% Zircopax) was applied inside and partway down the outsides (at the stiff leather hard stage). The incising was done after the engobe dried enough to be able to handle the piece. The glaze is Alberta Slip rutile blue. Firing schedule: Cone 6 drop-and-soak.
Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.
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
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.
|Materials||Copper Oxide Black|
|Materials||Iron Oxide Red|
|Materials||Nickel Oxide Black|