Glazes Flashcards
What is frit?
Water-insoluble source of PbO and B2O3
What is whiting?
CaCO3
What is white lead?
2PbCO3 * Pb(OH)2
What are glazes made of?
frits (PbO and Ba2O3), white lead (2PbCO3 * Pb(OH)2), whiting (CaCO3), quartz, clay, water, sometimes organic binder
What substitution is being made in glazes and why?
B2O3 for PbO as there are health risks associated with PbO ingestion
What fine crystals are devitrified during the formation of matte glazes?
> anorthite: CaAl2Si2O8
wollastonite CaSiO3
What’s a fritted glaze?
Used in semivitirious wares and hotel china
What are qualities of lead containing glazes?
high brilliance and flow well during firing
How are fritted glazes, including lead containing, fired?
They are first bisque fired: without glaze, then glost fired: with glaze
What is a crystalline glaze?
> specialty glaze that fosters devitrification of large spherulitic willemite (2ZnO*SiO2) crystals
How are crystalline glazes formed?
By overheating and then cooling to a low temperature soak
Why do glazes need opacity?
to hide body defects
What is opacity caused from?
scattering of light from fine crystalline particles dipersed within the glassy matrix, particularly if they have a high index of refraction difference with the glass
What is the most effective opacifying particle?
Zircon (ZrSiO4)
What causes cracking in glazes?
too much clay or glaze particles that are too fine = excessive water adsorption
What is whiting?
washed powdered chalk
What are frits and why are they used?
pre-melted, quenched in water to form a glass, used as a water-insoluble form of PbO and Ba2O3
What happens as the silica content of the glaze increases during highest firing temps of glaze?
The liquid viscosity increases, preventing gravitational movement of the glaze
What effect do bubbles have on firing glazes?
–> porosity is trapped as bubbles in the viscous liquid.
–> decomp of CaCO3 (carbonates) further forms bubbles
–> at maturing/soaking temp, liquid viscosity is low enough for most bubbles to escape from surface
What reactions between the glaze and body improve adherence?
–> different phases on the surface of the body have different solubilities, forming ROUGH INTERFACE
–> MULLITE NEEDLES from SiO2 and Al2O3 grow from the body into the glaze
What happens to bubbles in glazes while cooling?
Remaining bubbles shrink, causing minor dimples in glaze surface, which slightly degrades surface reflectance
What CTE do you want for the glass and glaze?
You want the glass/glaze to have a lower coefficient of thermal expansion than the body so at room temp the glaze is in compression
What is crawling?
If glaze has high surface tension and/or doesn’t wet the body due to high interfacial energy; it will bead up rather than spread
How do you increase the CTE of a glaze? (which you don’t want to do)
increase the alkali content of the glaze
