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 word “medium” is also commonly used for a liquid painting or printing vehicle, so the qualification “ceramic medium” is important when referring instead to the fired inorganic formulation.

What does the carrier do?

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 additive necessarily performs all these functions.

What is in a powder carrier?

For studio underglazes and brushing glazes, the liquid is normally water. Screen-printing and industrial decorating vehicles can also employ oils, glycols, resins or organic solvents.

The carrier can contain several types of additive:

Binder or gum: CMC gum is common because it improves dry adhesion, hardens the applied layer, slows drying and increases viscosity. Other cellulose gums, acrylic binders and natural or synthetic polymers can perform similar functions.

Gelling or suspending agent: Bentonite, Veegum, hectorite, xanthan gum and similar materials develop yield value. This enables a slurry to remain stationary in the container while becoming fluid when brushed, stirred or forced through a screen. A good gel is often more effective at preventing settling than simply making the liquid continuously viscous.

Dispersant: A dispersant can lower viscosity and permit more powder to be carried in less water. It can be especially valuable in high-solids inks and pastes. However, excessive dispersion can destroy yield value and allow rapid settling.

Flocculant: A small addition of a soluble salt or acid can restore gel structure to an overly fluid suspension. Flocculation and gum addition are not the same thing: one controls particle relationships, while the other primarily modifies the liquid phase and dry bond.

Wetting agent or surfactant: This lowers surface tension, helping the carrier wet stain particles and spread over dense, smooth or previously coated surfaces. Too much can cause foaming, bleeding or excessive penetration.

Defoamer: High-speed mixing, gums and surfactants can entrain air. A defoamer may be needed when bubbles interfere with printing or produce craters in the dried layer.

Preservative: Water-based mixtures containing organic gums can support microbial growth. Spoilage can produce odour, gas, mould and loss of viscosity, so commercial products often contain a preservative.

Not every carrier needs all these additions. The simplest may contain only water and gum. Others are carefully balanced systems of binder, gellant, dispersant, surfactant and preservative.

The carrier is temporary—but not always insignificant

Water evaporates during drying, and organic gums and binders should burn away during firing. They are not intended to supply useful ceramic oxides or become part of the fired glass.

However, this does not mean that they cannot affect firing. A thick layer containing excessive organic material can generate enough gas or carbon during burnout to contribute to crawling, pinholing, blistering, poor adhesion or discoloration, especially during rapid firing. Clean burnout is therefore another property of a good carrier.

Mineral gelling agents create an overlap between the carrier and the ceramic medium. Bentonite and Veegum perform a carrier function in the wet slurry, but they do not burn away. Their fired residue becomes part of the ceramic layer. Although the percentage is normally small, it should be included when calculating the dry ceramic recipe and its chemistry.

Different application methods need different carriers

A brushing glaze or underglaze must behave somewhat like paint. It should load readily onto the brush, spread without dragging, remain wet long enough to join adjacent strokes and deposit the intended thickness. After drying, the first coat should be hard enough that application of the next does not lift it.

Screen-printing pastes are normally high in solids and much thicker than brushable products. 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 are very different. They require extremely fine particles, low viscosity, controlled surface tension and resistance to nozzle blockage. Thus, it is not accurate to describe all ceramic inks as thinner versions of brushing underglazes.

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 binder than brushing products because fast drainage and drying are priorities.

Specific gravity is not viscosity

Specific gravity indicates how much solid is being carried in relation to the water, but it does not describe how the slurry flows. Two products having the same specific gravity can behave very differently because of differences in particle size, particle shape, surface area, clay content, stain type, soluble salts, dispersion and gum content.

A higher specific gravity generally deposits more ceramic powder in each coat while introducing less water into the ware. This can enable a thicker layer with fewer coats and reduce drying shrinkage. It is particularly valuable when decorating unfired ware, where excess water can soften or slake the surface.

However, increasing specific gravity too far can cause brush drag, poor leveling, cracking, screen blockage or difficulty mixing the product. Adding gum simply to make a dilute slurry appear thick does not compensate for inadequate solids loading. Such a mixture may look creamy yet deposit very little ceramic material and shrink excessively during drying.

Tuning a DIY carrier

The best carrier is not defined by a universal percentage of CMC gum. Different stains and ceramic media have radically different particle sizes, surface areas, densities and water demands. Gum grade also matters: two CMC products can produce very different viscosities at the same percentage.

A practical development sequence 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 rather than relying only on spoonfuls or volumes of prepared solution.

CMC is normally easier to incorporate as a prepared solution than as dry powder, which can form persistent lumps. It is wise to begin with less gum than expected. More can be added, whereas an over-gummed product may need substantial dilution or additional ceramic powder to correct it.

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 intended to brush well, remain stable in storage and work across a wide firing range.

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