All Glossary

Permeability

In ceramics, the permeability of clay slurries and plastics determines the rate as which water can move through the matrix

Key phrases linking here: permeability - Learn more

Details

The term permeability is used in several ceramic contexts. Most often, it describes how readily water can move through a clay—either through a compacted powder, a dried clay body, or a layer deposited from a slurry.

Particle size and surface area have a major influence. Bentonites consist of extremely fine particles having enormous surface area and are among the least water-permeable clays. They also dry very slowly. When bentonite powder is wetted, the outermost particles can swell and form a gel layer only a few millimetres thick. That layer may become nearly impermeable, preventing additional water from penetrating into the dry powder beneath it.

Kaolins generally have much larger particles and lower surface areas. Some are relatively non-plastic and highly permeable to water, enabling them to wet and dry much more quickly. Certain non-clay minerals can also resist water penetration, so permeability is not determined by plasticity alone.

Interestingly, a clay that has already been thoroughly mixed into a plastic mass is normally quite impermeable. The plate-like particles are packed closely together, and the remaining pore channels are very small. After that same clay has been completely dried, however, shrinkage cracks and interconnected pores permit water to enter. The piece can then slake rapidly as water penetrates between particles and breaks it apart.

Permeability is especially important in slip casting. Water must pass from the slurry through the growing clay layer and into the porous plaster mould. A highly permeable casting slip builds thickness quickly, whereas a slip containing more extremely fine clay particles generally casts more slowly. Those fine particles may be desirable because they increase plasticity and dry strength, but they can also reduce casting rate and make dewatering more difficult.

Manufacturers therefore need a repeatable way to measure permeability. One method is the Baroid permeability test, performed in a small pressure vessel fitted with filter paper or a membrane and a liquid outlet. The slurry is placed under a specified pressure for a set time. The amount of liquid expelled is measured, and the deposited filter cake can be removed, weighed and measured for thickness. The test provides useful information about both filtration rate and cake-forming behaviour.

The very low permeability of bentonite is also valuable outside ceramics. In agriculture, pond construction, landfill liners and geotechnical applications, compacted bentonite can swell to seal pores and greatly restrict the movement of water.

Related Information

Permeability of Texas and Montana talcs:

Larger round particles vs tiny platelet particles

This picture has its own page with more detail, click here to see it.

Texas talc (left) quickly absorbs all the water poured on it. Montana talc (right) resists whetting of the particles much more, the water is just sitting on top and has not penetrated at all.

The Montana material is more pure, and it is a platy talc. Talc platelets expose broad basal water repellent surfaces that are difficult for water to wet. When these fine plates settle parallel to the surface, they can form a tightly packed, water-repellent layer with very small pore entrances. Surface tension and trapped air then prevent water from entering, so it remains pooled on top.

The Texas is chemically and physically different. It contains appreciably more calcium-bearing material and other non-talc components. It also has rounder, fluffier and substantially coarser particles. These characteristics provide larger, better-connected pore passages and more readily wetted mineral surfaces. Once water enters those pores, capillary suction quickly pulls it through the powder.

A stock pile of raw, unground ball clay

This picture has its own page with more detail, click here to see it.

Plainsman A2 ball clay. In spite of three times the normal rainfall this summer, its natural high impermeability made it shed almost all of the water. But there is a back story. When mined, this clay was quite wet, about 10% moisture - some of the lumps are microwave-oven-sized! The outer ones dried over a period of weeks and, in the process, fractured down to what you see here. But the inner section of the pile takes years to dry out. By now it has broken down so fine that a completely dry sample put into water will slake in minutes to produce a smooth slurry. We can thus say that the pile has been "weathered".

A few drops of water on top of a tiny pile of bentonite powder.

This picture has its own page with more detail, click here to see it.

Notice the water just sits there in a little lake. It does not soak in because the bentonite gels in contact with the water and that gel acts as a non-permeable barrier. This water-barrier property of bentonite is a key to its use in many products. When ceramic bodies and glazes contain bentonite, they dry more slowly. In industrial settings, this can slow down production. But in studio settings, slower drying can enable more time to work with the material and more opportunities to intervene to make sure drying is happening evenly throughout.

Links

PayPal	No tracking, No ads, No paywall, No transient content! Just organized, concise information constantly updated and improved. Was this helpful? Consider supporting me.

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk