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|GSPT - Frit Softening Point||2550C|
Zircopax is a generic brand name of zirconium silicate or zircon (see Zircon for more information). It is primarily used in ceramic to opacify glazes. In North America, the most popular zirconium opacifiers fall under the brand names of Zircopax Plus, Superpax, Zircosil and Excelopax. These vary according to particle size, the finer the size the greater the scattering of light (and thus the better the opacification). In addition, the finer sized materials contain a little extra silica for maximum whiteness.
Of course, the amount of zircopax in a glaze determines the opacity. Small amounts (1-3%) may give no noticeable difference but are sometimes employed to improve glaze hardness. Since zircopax is refractory, the more that is added the more the degree of glaze melting (and melt viscosity) is going to be affected. Up to 15% or more might be needed to fully opacify a glaze. If higher amounts are needed the glaze formulation may need to be adjusted to reduce the amount of SiO2 or increase flux (to melt the glaze better).
Zircopax affects glaze melt viscosity, surface smoothness, thermal expansion and color development and can be implicated in a range of glazes faults associated with these. Please read the page on zircon for more information.
Zirconium prices are have increased rapidly in recent years, reflecting the world supply situation. Even though the material is very expensive, Zircopax is added to the body recipe of “porcellanato” tiles. It is a very effective body whitener, especially for casting porcelain. Excessive additions give the porcelain an artificial "white plastic" appearance.
Formula: ZrSio4 Weight: 183.1
Linear Coefficient of Expansion (25-700C cm/cm/degreeC): 42 x 10 -7
Specific gravity: 4.5
Particle Size: Zircopax Superpax Excelopax
Microns, Ave. 1.3 .74 .55
Bulk density lbs/cu ft 100 75 75
Surface area, m2/g 4.5 9.8 12.8
Left: An example of G2571A cone 10R magnesia matte. Right: with 10% added zircopax (zirconium silicate). The zircopax version is a very bright pasty white compared to the original.
Opacifying a cone 10 reduction magnesia matte glaze. On the left: G2571A dolomite matte, a popular recipe (from Tony Hansen). Right: 10% Zircopax has been added. Both are on a buff stoneware (H550 from Plainsman Clays).
Only 3% Veegum will plasticize Zircopax (zirconium silicate) enough that you can form anything you want. It is even more responsive to plasticizers than calcined alumina is and it dries very dense and shrinkage is quite low. Zircon is very refractory (has a very high melting temperature) and has low thermal expansion, so it is useful for making many things (the low thermal expansion however does not necessarily mean it can withstand thermal shock well). Of course you will have to have a kiln capable of much higher temperatures than are typical for pottery or porcelain to sinter it well.
Right: Ravenscrag GR6-A transparent base glaze. Left: It has been opacified (turned opaque) by adding 10% Zircopax. This opacification mechanism can be transplanted into almost any transparent glaze. It can also be employed in colored transparents, it will convert their coloration to a pastel shade, lightening it. Zircon works well in oxidation and reduction. Tin oxide is another opacifier, it is much more expensive and only works in oxidation firing.
On Plainsman H443 iron stoneware in reduction firing. Notice Tin does not work. Also notice that between 7.5 and 10% Zircopax provides as much opacity as does 15% (Zircon is very expensive).
G2934 cone 6 matte (left) with 10% zircopax (center), 4% tin oxide (right). Although the cutlery marks clean off all of them, clearly the zircopax version has the worst problem and is the most difficult to clean. To make the best possible quality white it is wise to line blend in a glossy glaze to create a compromise between the most matteness possible yet a surface that does not mark or stain.
These crucibles are thrown from a mixture of 97% Zircopax (zirconium silicate) and 3% Veegum T. The consistency of the material is good for rolling and making tiles but is not quite plastic enough to throw very thin (so I would try 4% Veegum next time). It takes alot of time to dewater on a plaster bat. But, these are like nothing I could make from any other material. They are incredibly refractory (fired to cone 10 they look like bisqued porcelain), a have amazing resistance to thermal shock. I could pour molten metal into them and they will not crack. I can heat one area red hot and it will not crack. I can throw the red hot piece into water and it will not crack!
This melt flow tester demonstrates how zircon opacifys but also stiffens a glaze melt at cone 6. Zircon also hardens many glazes, even if used in smaller amounts than will opacify.
Even commercial dinnerware can suffer cutlery marking problems. This is a glossy glaze, yet has a severe case of this issue. Why? Likely the zircon opacifier grains are protruding from the surface and abrading metal that comes into contact with it.
An example of crawling in a zircon opacified glaze on a tile. The immediate source of the problem is likely at the decoration stage. The water from the blue overglaze is rewetting the white under glaze, expanding and reshrinking it. This compromises the white glaze's bond with the body, resulting in cracking and lifting of the edges of the cracks. A number of things can be done to improve the situation: Adding a binder to the white glaze, reducing the clay content or using less plastic clays in its recipe, reducing the water content of the overglaze, heating the tiles before glazing and/or decorating so they dry faster and reducing the surface tension of the glaze melt.
This is a melt fluidity test comparing two different tin oxides in a cone 6 transparent glaze (Perkins Clear 2). The length, character and color of the flow provide an excellent indication of how similar they are.
It is 5 mm thick (compared to the 17mm of the cordierite one). It weighs 650 grams (vs. 1700 grams). It will perform at any temperature that any kiln that I have will generate and far in excess of that. It is made from a plastic body having the recipe 80% Zircopax Plus, 16.5% 60-80 Molochite grog and 3.5% Veegum T. The body is plastic and easy to roll and had 4.2% drying shrinkage at 15.3% water. The shelf warped slightly during drying, so care is needed. First-firing at cone 4 yielded a firing shrinkage of 1%). Notice that cone on the shelf: It is not stuck so no kiln wash is needed! Zircopax is super refractory! It is held together by sinter bonding, so the higher the temperature you can fire to the stronger it will be.
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