|Monthly Tech-Tip |
Ceramic materials are employed in the ceramic industry to make glazes, bodies, engobes and refractories. We study them at the mineral, chemical and physical levels.
Ceramic materials are employed in the ceramic industry to make glazes, stonewares, earthenwares, porcelains, engobes, refractories, structural products, etc. We study ceramic materials at the mineral, chemical and physical levels.
At first it might seem strange to define this, but it is not as obvious as it seems. In ceramics the concept of a material is different for different people. To a purchasing agent it is a commodity. To a geologist it is a mineral or mix of minerals. To someone most concerned with the physical presence, ceramic materials are an inorganic, non-metallic solid. To a mining crew materials present various challenges in extraction, transport, initial crushing and stockpiling. To a processing department they present sets of issues to grind, separate, purify, size and package. To a production department they are powders that make up part of the recipe of a glaze or body and have associated issues and benefits in process equipment. To a lab technician materials have a physical presence that can be tested and the results of these tests can be expressed on a data sheet. Labs can also deduce the chemistry of a material. To a glaze chemist, materials are "warehouses" of oxides for use in formulations (manufacturers publish their chemistries), each having physical and process side-effects that must be taken into account (recipes theoretically become 'material independent', oxides being supplied from whatever materials are at hand). To a body formulator, ceramic materials offer properties that influence forming and fired properties (e.g. plasticity, fluxing power, thermal expansion.
This material storage area employs a rack to keep pails off the floor so the area can be hosed down easily. The materials in each pail are sealed in plastic bags or the pail is covered with a lid.
Talc is not nearly as dense as many other materials. If this was silica these pallets would be half this height.
This bag will give you a clue as to what manganese dioxide, MnO2, is mainly used for. Staining bricks.
Soda Ash is soluble and is thus not useful in most ceramic glazes. However that very solubility makes it very useful to control the electrolytics of ceramic slurries. This is the dense variety, non-hydrous.
The original bag of this product in 2014.
A bag of magnesium carbonate beside a bag of feldspar. Although the former weighs 25 kg (vs. 22.7 kg for the feldspar), clearly it is a dramatically lighter (per volume unit) material. Lifting that bag of Mag Carb feels like lifting a pillow!
Tony's lab work area of mineral and chemical powders for mixing test glazes and clay bodies. Stoneware and earthenware glazes are made from dozens (even hundreds) of commodity industrial mineral powders.
Each of these eight pallets of kaolin are being sampled to screen them for oversize particles. The 50 gram samples needed can be taken without having to open the bags, they are filled through a valve at the top and it can be opened easily. Kaolins and ball clays especially are suspect and body manufacturers must be vigilant about this (each can tell you disaster stories about making product with faulty raw materials containing grit, carbon and iron particles). The samples will be washed through 70, 100 and 150 mesh screens to spot any particles that could introduce grit or fired speckle into the bodies.
The material is much less dense than most other ceramic materials (that is why these bags are so tall). When moved the powder within becomes unstable and they are prone to falling over.
There is a direct relationship between the way ceramic glazes fire and their chemistry. Wrapping your mind around that and overcome your aversion to chemistry is a key to getting control of your glazes. You can fix problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. Substitute frits or incorporate better, cheaper materials, replace no-longer-available ones (all while maintaining the same chemistry). Adjust melting temperature, gloss, surface character, color. Identify weaknesses in glazes to avoid problems. Create and optimize base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. Create glazes from scratch and use your own native materials in the highest possible percentage.
The cone 6 porcelain on the left uses Grolleg kaolin, the right uses Tile #6 kaolin. The Grolleg body needs 5-10% less feldspar to vitrify it to zero porosity. It thus contains more kaolin, yet it fires significantly whiter. Theoretically this seems simple. Tile #6 contains alot more iron than Grolleg. Wrong! According to the data sheets, Grolleg has the more iron of the two. Why does it always fire whiter? I actually do not know. But the point is, do not rely totally on numbers on data sheets, do the testing yourself.
Some material data sheets show both the oxide and mineralogical analyses. Dolomite, for example, is composed of calcium carbonate and magnesium carbonate minerals, these can be separated mechanically. Although this material participates in the glaze melt to source the MgO and CaO (which are oxides), it's mineralogy (the calcium and magnesium carbonates) specifically accounts for the unique way it decomposes and melts.
This is a quality but expensive material!
Many minerals are just ground up rocks, they were in the ground for millions of years (e.g. kaolin, feldspars, ball clays, bentonite, calcium carbonate, dolomite, talc, kyanite, wollastonite, etc), so the powders should last millions of years as well. Some are powderized man-made glasses and sintered solids, these are very stable (e.g. frits, stains). Other man-made materials are less stable and can hydrate or oxidize (e.g. carbonate colors, plaster), keep them sealed containers. Some materials are organic (e.g. Gum Arabic) and they can go bad in damp conditions, so keep them in a sealed container also.
This term generally refers to the industry that produces the non-metallic objects we use every day (like porcelain, tile, glass, stoneware).
In glaze chemistry, theoretical materials are used to represent what a material would be if it was uncontaminated and perfectly crystallized
International Labour Organization Chemical Safety Database
MatWeb Materials Properties Database