This page has been deprecatedPlease click the Glossary heading above for the new section.
Variegation, Reactive Glazes
Variegated or mottled glazes are those that do not have a homogeneous solid color or character (i.e. like a ceramic sink or toilet bowl). They are sometimes called 'reactive glazes'. Variations in color and texture are highly prized by many ceramists. A variety of mechanisms are used to create the variegation. These include crystal growth, addition of speckling agents, phase separation, layering, and thickness variation of translucent glazes.
Out Bound Links
- (Materials)
Rutile - TiO2 - Iron Titanium Mineral
In Bound Links
- (Glossary)
Phase Separation
This phenomenon is responsible for some of the mos... - (Articles)
Variegating Glazes This is an overview of the various mechanisms you can employ to make glazes dance with color, crysta... - (Properties)
Glaze Variegation
In contrast to the typical homogeneous surfaces of...
Pictures
Cone 6 GA6-C variegated blue showing different thicknesses (4% rutile+ 20% frit 3134 in Alberta Slip)
This high boron cone 04 glaze is generating calcium-borate crystals during cool down
Example of variegation and phase separation with about 5% rutile added to a dolomite matte cone 10R glaze.
Example of a variegated wood ash glaze at cone 6 oxidation.
Crystallization in a high MgO matte at cone 10R
A variegated glossy blue ravenscrag slip glaze
Viscosity
The term viscosity is used in ceramics most often to refer to the degree of fluidity of a slurry or suspension (the term 'shear' is often used when discussing viscosity, theoretically engineers understand viscosity in terms of layers particles or molecules that exhibit a friction that resists lateral displacement against each other). Viscosity is the opposite of fluidity, a term also commonly used, viscous slurries are thick and thus lack fluidity. Laboratory instruments that measure viscosity are called viscometers and they express the result in a unit called the poise. Higher poise numbers mean a more viscous slurry. Units of fluidy are taken as 1/poise, thus 2 poise = 0.5 rhe (water has a fluidity of 100 rhe).
The viscosity of a slurry can be reduced by the addition of a deflocculant and fluid slurries of remarkably low water content can be produced. Deflocculants work their magic by imparting electrical charges to the surfaces of particles to make them repel each other. Conversely, the viscosity of a slurry can be increased by the addition of a flocculant that makes it gel. Soluble materials within a powdered mix can impede or block the action of deflocculants and particle properties like size, size distribution, shape, surface area, surface reactivity, density, etc. all affect their action. See the Potters Dictionary under Fluidity for a detailed and easy-to-understand discussion of this (especially relating to the dynamics imparted by flat particles with differing end and flats charges).
Molten glazes also exhibit viscosity, but the term 'fluidity' is normally used.
Out Bound Links
In Bound Links
- (Articles)
A Low Cost Tester of Glaze Melt Fluidity This device to measure glaze melt fluidity helps you better understand your glazes and materials and... - (Glossary)
Thixotropy
Thixotropy refers to the way a slurry's viscosity ... - (Tests)
RHEO - Rheology of a Ceramic Slurry
- (Tests)
AVSC - Apparent Viscosity (cps)
Vitrification
'Vitrification' is a process. As clay is fired hotter and hotter, it reaches a point where, if cooled, it will produce ware of sufficient density and strength as to be useful for the intended purpose. The intended purpose may well require some porosity to gain another more important advantage (i.e. stability in the kiln, resistance to blistering). However 'vitreous' ware is usually functional, water proof, sanitary, hard, and strong. Such products must be fired closer to the melting range of the clay. Ware that has fired dense and strong is also said to be 'mature'. To vitrify some clays requires that they be fired high enough to cause deformation, ware thus needs to be supported in setters or the cross section redesigned to be more stable. Porcelains and very vitreous. Stoneware are generally semi vitreous.
Out Bound Links
In Bound Links
|
