All Glossary

Powder Carrier

A powder carrier is the liquid vehicle that enables a ceramic powder mixture to be brushed, sprayed, poured, printed or otherwise applied. The term is especially useful for underglazes, engobes, brushing glazes, ceramic inks and screen-printing pastes.

It should be distinguished from the ceramic medium. A ceramic medium is the inorganic formulation that hosts the stain or colorant and remains after firing. It can contain frit, feldspar, clay, silica, zircon or other ceramic materials to control melting, adhesion, opacity, thermal expansion and surface character. The powder carrier controls application and drying.

A typical colored product can thus be considered as having three parts:

* The stain or colorant, which produces the fired color.
* The ceramic medium, which bonds that color to the ware and determines its fired properties.
* The powder carrier, which gives the wet mixture the application properties needed to put it there.

In ceramic ink terminology, the powder carrier is often called the vehicle. The carrier must temporarily transform a powder into a usable liquid, paste or ink. Depending on the application method, it may need to:

* Wet and disperse the particles.
* Keep particles from settling.
* Produce the required viscosity and yield value.
* Flow under a brush or squeegee but stop flowing afterward.
* Level sufficiently without bleeding beyond the intended area.
* Control the drying rate.
* Bond the dried layer to the ware.
* Give the dry layer enough hardness and flexibility to survive handling.
* Permit subsequent coats without lifting or redissolving earlier ones.
* Release cleanly from a screen, pad, stamp or applicator.

No single carrier additive necessarily performs all these functions. For studio underglazes and brushing glazes, the liquid part is normally just water (vs oils, glycols, resins or organic solvents used in industry). The carrier can contain:

-CMC gum to improve dry adhesion, hardens the applied layer, slows drying and increases viscosity. Other similar products are also used (acrylic binders and natural or synthetic polymers).
-Gelling agent: Bentonite, Veegum, hectorite, xanthan gum (in small additions) enable a slurry to remain stationary and suspended in the container while becoming fluid when brushed or stirred.
-Dispersant: A dispersant permits more powder to be carried in less water. It can be especially valuable in high-solids inks and pastes.
-Flocculant: A small addition of a soluble salt or acid can create a gel structure in an otherwise fluid suspension.
-Wetting agent or surfactant: This lowers surface tension, helping the carrier wet particle surfaces.
-Defoamer: High-speed mixing gums and surfactants can entrain air.
-Preservative: To stop microbial growth.

The simplest carrier may just be water (e.g. a dipping glaze). Others are carefully balanced systems of binder, gellant, dispersant, surfactant and preservative.

-Brushing glazes or underglazes, for example, must behave somewhat like paint. They can be high or low in solids.
-Screen-printing pastes, by contrast, are high in solids and thus much thicker. They must become fluid under the shear of the squeegee, pass through the mesh and release from it cleanly. After printing, viscosity must recover quickly enough to preserve fine edges and prevent spreading. The carrier must also resist drying in the screen during use.
-Inkjet ceramic inks require extremely fine particles, low viscosity, controlled surface tension and resistance to nozzle blockage.
-Spray-applied products normally need lower viscosity and less gum so they atomize without stringing or plugging the nozzle.
-Dipping and pouring glazes generally need less or no binders, because fast drainage and drying are priorities.

A practical development sequence to tune a DIY carrier is:

1. Formulate the stain and ceramic medium for the intended firing temperature and fired result.
2. Add water and, where needed, dispersant to achieve a workable solids loading and specific gravity.
3. Add a fully hydrated gum solution in small increments to obtain the required brushing behaviour and dry hardness.
4. Adjust suspension and thixotropy separately using a gelling agent or controlled flocculation.
5. Test wetting, application thickness, leveling, drying time, cracking, dry adhesion and resistance to handling.
6. Fire test the product at its intended thickness and schedule.
7. Record the specific gravity and the dry percentages of gum and other additives.

Unless blender mixing, CMC is normally easier to incorporate as a prepared solution than as dry powder (avoiding persistent lumps).

DIY glazes and underglazes permit the ceramic medium and the powder carrier to be optimized independently. The ceramic medium can contain only enough stain and melter to produce the required color, opacity and fired adhesion. The carrier can then be tuned for the actual application method instead of accepting the compromises necessary in a commercial product.

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