Zlakusa

Pottery, ceramics or porcelain

Pottery and porcelain are not opposites — both are ceramics. Ceramics is the umbrella term for everything shaped from clay and fired, and within it the kinds differ by the temperature they were fired at and by how porous the body remained. Earthenware is fired lowest and stays porous; porcelain highest, and keeps no pores at all.

Ceramics is the umbrella term

In everyday speech "ceramics" has become the name for glazed, colourful ware in a shop window, and "pottery" for old earthenware. The trade does not divide them that way. Ceramics is everything fired from clay, from a roof tile to an insulator on a power line. Pottery is one kind of ceramics — the kind a potter shapes on a wheel.

So the question "is this pottery or ceramics" has no answer in that form. The question that does work is: which kind of ceramics is it, and that is read off the firing temperature.

Three kinds by firing temperature

  • Earthenware — fired lowest, roughly 900 to 1150 °C. The body stays porous and takes up water, so it is usually glazed in order to hold liquid
  • Stoneware — fired around 1200 to 1300 °C. The body has partly vitrified, takes up almost no water, is opaque and rings when tapped
  • Porcelain — fired highest, roughly 1300 to 1400 °C, from a kaolin body. Fully vitrified, and translucent where it is thin

The boundaries are not sharp and the figures vary from body to body; what does not change is the order. The hotter the firing, the more of the fine particles in the body have softened and closed the pores — the process is called vitrification, and it is the real difference between these three.

Porosity is what you notice

A porous body takes up water, fat and smell. That is why pottery is washed without detergent, why it is greased before first use, and why it must dry completely before being put away. Porcelain does none of this: it takes up nothing, washes with anything and asks for no preparation at all.

At the table, porcelain is the more practical. On a fire it is worthless — and that is where the comparison stops being academic.

Why ordinary ceramics cannot go on a flame

A vessel on a flame takes heat at one point and has to carry the difference between the heated and the cold part of its wall. Most ceramics will not: a vitrified body expands more under heat than it can bear, and a glaze over it expands at a different rate from the body beneath, so coating and base fight each other. Which is why a porcelain dish may say it is oven-safe and never that it may go on a hob.

Zlakusa pottery gets around that through its composition rather than its firing temperature. Ground calcite is added to the clay, in roughly equal measure: it reduces shrinkage during drying and firing and gives the body resistance to thermal shock. On top of that there is no glaze — no second layer expanding at its own rate. The result is a vessel that is earthenware by every measure, porous and low-fired, and yet takes a direct flame. That is an exception, not a rule, and it is the reason this craft was inscribed by UNESCO at all.

Telling them apart at home

  • Light — hold a thin part up to a bulb. Porcelain lets light through; stoneware and earthenware do not
  • Sound — tap it with a finger. Porcelain and stoneware ring long and clear, earthenware answers with a shorter, duller note
  • The foot — turn the piece over. On earthenware the unglazed foot ring takes up a drop of water within seconds; on porcelain the drop just sits there
  • Weight and wall — at the same volume earthenware is thicker and lighter, porcelain thinner and denser
Ceramics
the umbrella term — everything shaped from clay and fired
Earthenware
about 900—1150 °C, porous body, takes up water
Stoneware
about 1200—1300 °C, partly vitrified, takes up almost none
Porcelain
about 1300—1400 °C, fully vitrified, translucent
Fit for a flame
as a rule none of the three — barring specially built bodies
Zlakusa pottery
porous and unglazed, but with calcite — takes a direct flame

Frequently asked questions

What is the difference between pottery and ceramics?

Pottery is a kind of ceramics, not its opposite. Ceramics is the umbrella term for everything shaped from clay and fired, and pottery is the ceramics a potter shapes on a wheel, fired at lower temperatures so that the body stays porous.

Is Zlakusa pottery ceramics?

It is. By every measure it is earthenware: fired at a lower temperature with a porous body. It is unusual only in composition — ground calcite in the clay gives it resistance to thermal shock, so it takes a direct flame, which ordinary earthenware cannot.

Why can ordinary ceramics not go on a hob?

Because on a flame or a ring the heat arrives at one point, so the wall has to carry a large difference between its heated and its cold part. A vitrified body expands more than it can bear under that, and a glaze over it expands at a different rate from the body beneath.

How do I tell porcelain from stoneware and earthenware?

Hold a thin part up to a light: porcelain lets light through, the other two do not. Tap it: porcelain and stoneware ring long and clear, earthenware answers more dully. A drop of water on an unglazed foot is taken up by earthenware within seconds.