Chemicals, Metals & Oxides used in Ceramics, Stoneware, Porcelain, glazes & enamel:

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What is in Ceramic, Stoneware and their glazes & enamel?  How is it different from MEC’s clay? 

Ceramic clay, porcelain, stoneware and the glazes and enamel used for the coating are a composition of different chemicals, oxides and metals.  The percentage of natural clay (which is usually secondary clay) is anywhere from 4 to 9%.  Given below are some of the ingredients in this type of ware.

100% of Miriams Earthen Cookware is pure and all-natural primary (purest form of clay), clay.  No chemicals or additives are used in the making, finishing or firing process.  The process of making the pots may be the least attractive to most commercial cookware makers because of the time and effort required.  It is our 1st choice because it produces the healthiest and most non-toxic pots.  Here is how it’s made.

The complete composition of our clay is tested before harvesting and the list of nutrients present can be found here:  Testing and certification for MEC Clay.


Raw materials used in ceramics, glazes, stoneware, enamels:

LEADED SILICA. CADMIUM: to obtain dark coloring and resist glaze penetration, IRON CHROMATE: FeO l Cr2O3 (m.p. 3000o F/1725o C) used in glazes (typically <2%) to produce several colors. In combination with Tin, it produces pink/red; with Zinc, brown; in Alkalies, grey; and in Potash and Spodumene glazes it can produce mottled greys. In Engobes it also develops grey color. POTASH FELDSPAR G-200, PETALITE: an ore of lithium. SILICON CARBIDE, TIN OXIDE, TITANIUM DIOXIDE, UMBER (Burnt) – an oxide consisting of Limonite (FeO.H2O) and Manganese Dioxide, VEE GUM CER – It is used in glazes to optimize the surface hardness of un-fired glazes, to stabilize viscosity, and to enhance glaze suspension,  WAX RESIST – is a wax emulsion, available in an oil or water base, and is used to resist glaze water penetration during the application of glaze, WOLLASTONITE – CaSiO3 (m.p. 2804o F/1540o C) is Calcium Silicate, mined principally in Willsboro. ZINC OXIDE – ZnO (m.p. 3272o F/1800o C) is derived from Zinc Sulfide ores. It is an important component of many glaze types and is used as a flux, opacifier, and color modifier. ZIRCOPAX – ZrSio4 (m.p. >3600o F/2000o C) is a Zircon opacifier used in glazes and slips VEE GUM T – hydrated Magnesium Aluminosilicate. used as a plasticizer and as a suspension agent in glazes.  VANADIUM PENTOXIDE – V2O5 (m.p. 1270o F/690o C) is derived from Vanadium ores in. It is used in a glaze as a colorant in combination with Titanium. UMBER (Burnt) – is Limonite (FeO.H2O) and Manganese Dioxide. RUTILE – TiO2 (m.p. 3452o F/1900o C) is an economical source of TiO2 used in ceramic clay to promote and dark tans in oxidation,  in glazes.  PYRAX HS – (m.p. 2790oF/1530oC) is a high Sericite (a fine grained muscovite mica; K2Ol3Al2O3l6SiO2l2H20) pyrophyllite blend produced (at this writing) by the R.T. Vanderbilt Company, Inc. It is made up of 35% foliated (platy) Pyrophyllite, 35% Quartz, 25% Sericite, and 5% Kaolinite. To increase strength in vitreous bodies. ALUMINA HYDRATE – Al(OH)3 (m.p. 3722o F/2050o C) is used primarily in glazes as a source of Alumina. It is often favored over the oxide (Calcined Alumina) form due to its promotion of glaze adhesion and its capacity to remain suspended in a glaze. It is also used in salt glaze, bungs, and kiln wash. ANTIMONY OXIDE – Sb2O3 (m.p. 1166o F/630o C) used in glazes as an opacifier and colorant (rarely, due to cost). However, its primary use is as a colorant (cone 06-1). In the presence of Lead or Iron, it will produce yellow. It is slightly soluble and very toxic.  BALL CLAY – is so named because it was first sold in England in the shape of a ball. It is a fine particle size secondary clay containing montmorillonite as its chief clay mineral constituent. It is also used as a source of Alumina in high temperature glazes (cone 8-13), and it helps to keep a glaze slip in suspension.  BARIUM CARBONATE – BaCO3 (m.p. 2480o F/1360o C) is used in glazes as the typical source for Barium. It is reactive and highly toxic. In low fire glazes (cone 06-5) it promotes matt (sometimes dull) finishes. However, at high temperatures (cone 8-13), it is a powerful flux. It is used in clay bodies to control scumming by rendering sulfates insoluble. BONE ASH – 3CaOlP2O5 Bone Ash is prepared by calcination of selected animal bones (especially cow bones; some CaCO3 contamination) and then ground to a predetermined particle size. When Phosphate is added to a glaze it tends to cause opacity, too much is likely to cause crawling or blistering. It is used as a flux in higher fired glazes (cone 8-13), and also as flux in porcelain bodies where it is known to enhance translucency.
Synthetic: Bone Ash is produced from other non-bone Calcium Phosphate sources such as; the mineral apatite [Ca5(PO4)3 (OH,F)].
BORAX – Na2Ol2B2O3l10H2O (m.p. 1366o F/741oC) also known as Sodium Tetraborate. Granular: Borax (5 and 10 mol.) has a coarse particle size (99.9% <2.4 mm), and is readily soluble in water and can, therefore, have limited use in an unfritted form. It is used in a glaze as a source of both Sodium and Boron, it can increase the fluidity of the glaze to help heal over defects. BORIC ACID – H3BO3 is soluble and has limited use in glazes due to other insoluble sources; i.e. fritted forms. When used as the only flux in an alkaline glaze it is less likely to cause crazing than Sodium fluxes. Boric Acid can be used in a clay body when introducing alkalies.
CALCINED ALUMINA – Al2O3 (m.p. 3700oF/2040o C) is produced by calcining a hydrated alumina (see alumina hydrate-Al(OH)3) at temperatures of 1200-1300C to convert it to alpha-Al2O3. Alpha-alumina is the most stable form of aluminum oxide. Ceramic clay and porcelain bodies (325 mesh). It reduces shrinkage, increases thixotropy (shear thining characteristics), minimizes firing warpage, and adds fired strength.

C & C BALL CLAY – used principally in clay bodies to add plasticity and improve strength. In a glaze, it is a source of Alumina and will enhance glaze suspension CHROME OXIDE – Cr2O3 (m.p. 4418oF/2270o C) is used  as a refractory. Used in combination with Tin, it will produce pink, and with Zinc Oxide it will produce brown. Bright reds are possible in low-fire lead glazes that are low in Alumina content. COBALT CARBONATE – CoCO3 (m.p. 1661o F/905o C) is used as a colorant to produce blues in glazes (also see Cobalt Oxide). This compound is sometimes preferred over the Oxide form due to its particle size, thus eliminating the necessity for ball milling. Cobalt Oxide is used with Manganese, Iron Chromate, or Nickel to produce strong blacks.  COBALT OXIDE – Co3O4 (m.p. 1661o F/905o C) is the major oxide used for producing blues (also See Cobalt Carbonate It’s a very strong colorant. CORNWALL STONE – (Cornish Stone) is produced in Cornwall, England. It is a Feldspathoid used frequently in clay bodies and glazes feldspathoids are chemically similar to Feldspars.  COPPER CARBONATE – CuCO3 (m.p. 2100o F/1149o C) is the most common source for Copper Oxide and is used (2%-5%) as a major colorant to produce greens, turquoise, and copper reds. The carbonate form is preferred over the oxide due to its finer particle size. The finer particle size enhances the production Alkaline glazes: turquoise in oxidation, and copper red in reduction (also see Copper Oxide). It is volatile above cone 8 and may require a sagger to protect other ware and kiln.  COPPER OXIDE BLACK – (CuO) RED (Cu2O) (m.p. 2100o F/1149o C) are the two forms of Copper Oxide. Red Copper Oxide can be precipitated during reduction firings as a component of a copper red glaze finish is used with Barium. Copper Carbonate can be substituted for Oxide if the amount is doubled.  CRYOLITE – Na3ALF6 (m.p. 1828o F/998o C is used in a glaze with Aluminum or sodium. CUSTER POTASH – is used in glazes and clay bodies as a source of K2O. It contains 10% Potash and 2-3% Soda, and can be interchanged with G-200 Feldspar with modest adjustments (see also Feldspar).  DARVAN #7 – is a high molecular weight, long chain, Sodium Polyelectrolyte, is used as a general-purpose deflocculant (dispersing agent) in casting and glazing slips.  DOLOMITE -MgCa (CO3)2 (m.p. 4800oF/2650o C) is used as an inexpensive source of Magnesia in glazes. It is a double Carbonate of Calcium and Magnesium, and a true Dolomite is typically 56% CaCO3 and 44% MgCO3 by weight. Impurities of Alumina, Iron, and Silica are common. It can be used in clay bodies to promote longer and lower firing ranges (below cone 9).   FLINT – (SiO2) (m.p. 3119o F/1715o C) is the dark porous variety of chert that contains organic matter FRIT – is commercially prepared by combining water soluble fluxes such as; Alkali Carbonates, Nitrates, Borates, etc., with Silica, which are then fused and ground to a predetermined particle size to render the fluxes insoluble. Harmful compounds such as Lead are also fritted to reduce their toxicity. Frits can be used in both clay bodies and glazes when introduction of water soluble and/or toxic elements is desired.   GLAZE CLAY #1 – is a fine ball clay (< 55%, <1 micron) used in clay bodies to add plasticity and improve strength. In a glaze, it is a source of Alumina and will enhance glaze suspension. GERSTLEY BORATE – 2CaOlB2O3l5H2O (m.p. 1225o F/660o C) is a hydrated Calcium Borate with Sodium as an impurity (< 5%). Its water solubility is minimal and, therefore, it is one of the only quasi-insoluble sources of Boron other than fritted form. GLOMAX LL – Al2O3l2SiO2 is a non-plastic calcined Kaolin  used to control firing cracks and shrinkage. In high temperature glazes (cone 8-13) it is used as a source of Alumina..   GROLLEG – is the direct alteration of Granite and, therefore, higher in alkali, e.g. Potassium, than the secondary Kaolins.  HAWTHORN BOND. HELMER KAOLIN contains kaolinite and halloysite (m.p. 3100o F/1700o C) clay minerals producing a white burning fire clay resembling china clay. Halloysite has the same chemical formula as kaolinite Al2O3l2SiO2l2H2O however, the crystal structure is different. GOETHITE (FeOOH) a dark iron mineral. The Geothite is not separated but blended with in the clay.   is used in glazes as a colorant to produce tans and buffs, and it can also be used in granular form to produce mottling. IRON CHROMATE – FeO l Cr2O3 (m.p. 3000o F/1725o C) is used in glazes (typically <2%) to produce several colors. In combination with Tin.  IRON OXIDE – (m.p. 2818o F/1548o C) Red: Fe2O3 Iron Oxide is the most commonly used form of Iron as a colorant in glazes and clay bodies. It is used most often to produce tan to brown colors. When used in amounts greater than 4% it can react as a flux and increase the fluidity of the glaze.  BLACK: FEO IRON OXIDE as a colorant will create a variety of colors depending on the glaze base used. Its effect is most often the same as Red Iron Oxide. However, it is preferred for producing celadon glazes in reduction firing.
YELLOW #5060 – is a manufactured product that can be relied on to produce consistent results from year to year.

KAOPAQUE 10S is a water washed secondary delaminated Kaolin. Its median particle size is 2 micron, and is used in clay bodies for extrusion.  KYANITE – Al2O3lSiO2 (m.p. 3300o F/1800o C) can be added to clay bodies to stabilize shrinkage (due to its tendency to expand in firing) and improve strength. Its principal use is in the manufacturing of high temperature refractories for kiln bricks and furniture (see also Mullite.  LITHIUM CARBONATE – Li2CO3 (m.p. 1330o F/720o C) is the most common form of the alkali Lithium. It is a major flux for higher temperature (Cone 5-13) alkaline glazes. MAGNESIUM CARBONATE – MgCO3 (m.p. 5072o F/2800o C) is used as a source of MgO in glazes. In glazes (cone 5-10), it can develop a butterfly surface favored for utility ware. At high temperatures (cone 9-16) it has a similar effect as Alumina; it will increase the viscosity of a glaze and hence reduce the fluidity; e.g. crystalline variety, in which Alumina is not desired.  MANGANESE CARBONATE – MnCO3 (m.p. 3000oF/1650oC) MANGANESE is a vigorous flux (cone 5-10) in glaze and clay bodies (in a clay body use <5%), and it is also a strong oxidizing agent in glazes. The powder and granular forms differ by particle size and reactivity. The granular form can be used at higher temperatures where the reduced reactivity will develop dark speckles in both clay bodies and glazes. MOLOCHITE – 3Al2O3l2SiO2 (m.p. 3150o F/1760o C) is a high temperature calcination of high purity kaolin to maximize the conversion of kaolinite to mullite is used as a low impurity grog in porcelain and white clay bodies grog improving drying shrinkage and thermal shock resistance.   MULLITE – 3Al2O3l2SiO2 (m.p. 3300o F/1800o C) is calcined Kyanite. Is added to clay bodies to stabilize shrinkage, improve strength, and modify thermal expansion.
NEPHELINE SYENITE – K2Ol3Na2Ol4Al2O3l9SiO2 (m.p. 2200o F/1200o C) is a Feldspathoid used frequently in clay bodies and glazes (Feldspathoids are chemically similar to feldspars, however they contain only 2/3 the amount of Silica and are therefore, richer in Alkalies). Its unique characteristic is: the molecular equivalence of K2O and Na2O is greater than a typical Soda or Potash feldspar in relation to the Silica content. It can be used in a clay body when lowering the maturing temperature of the body is required. In a glaze, it can reduce glaze defects such as crazing and extend the working range of a glaze in the cone 011-5 vicinity.