How porous is too porous?
Updated: 3 hours ago
There are good reasons why contemporary standards for ceramic tableware specify very low water absorption, and when I am developing a body for restaurant use, I take those requirements seriously.
But I also find the subject fascinating because, historically, the story of ceramics is full of vessels that are deliberately porous.
So where does that leave us? Is porosity something we should always try to eliminate, or is it simply a property of clay that can be useful in some circumstances and undesirable in others?
Porosity isn't necessarily a problem
A porous clay body contains tiny interconnected spaces that can absorb water. Depending on the level of absorption and how a piece is used, this can contribute to staining, the absorption of oils and food residues, lingering smells or, in colder climates, damage caused by water freezing inside the clay.
But that doesn't mean that a porous clay body is automatically unsuitable for food.
Traditional earthenware is a good example. The food-contact surface is often glazed, creating a continuous glass-like layer between the food and the porous clay underneath. The body can remain relatively absorbent while the surface itself is suitable for everyday use.
And sometimes porosity is not just tolerated. It is the whole point.
Take the traditional Spanish botijo, or càntir in Catalonia: an unglazed earthenware vessel designed to keep drinking water cool. Water slowly moves through the porous clay to the outside surface, where it evaporates. The energy needed for evaporation comes from the water inside, creating a natural cooling effect.
In this case, making the clay completely impermeable would actually remove the property that makes the vessel work.
That doesn't mean a botijo and a restaurant plate should have the same material specification. Quite the opposite. They have completely different jobs.
Standards are there for a reason
This isn't an argument against standards, or against low-absorption stoneware for contemporary tableware.
A restaurant plate has a very different life from a traditional water vessel. It might be washed hundreds of times, go through a dishwasher repeatedly, experience changes in temperature and come into contact with oils, sauces and acidic foods. It needs to remain stable, durable and easy to clean throughout all of that.
This is where a well-vitrified, low-absorption stoneware body has a real advantage.
As clay vitrifies during firing, the amount of interconnected pore space decreases. The result is a denser material that absorbs less water and is generally better suited to the demands of modern functional tableware.
But I think this is where it is useful to distinguish between a standard and an absolute.
A water absorption figure is a useful technical measurement. It gives us something objective to test and compare, and standards exist for good reasons. But the number doesn't tell us everything about a piece of ceramics, nor does it exist independently of the use the object is intended for.
The clay body matters. The glaze matters. Glaze fit, crazing, surface durability and resistance to repeated washing all matter too.
Is 3% really the magic number?
You will often hear figures such as 3% absorption mentioned as though they represent a clear dividing line: below it, good; above it, bad.
It isn't quite that simple.
Water absorption is a useful measurement, but it is only one part of the story.
The glaze matters enormously. So does whether it is properly matured, whether it crazes, how durable its surface is and how it behaves with repeated washing and exposure to food.
A very low-absorption clay with a poorly fitting or unstable glaze isn't necessarily a better piece of tableware simply because the number on the absorption test is lower.
Equally, a well-made glazed earthenware vessel can function perfectly well despite having a much more porous body.
So why do we still look for low absorption?
For my own work, particularly when I am making pieces for restaurants, I do prefer a relatively low-absorption stoneware body.
Not because I think that anything above a particular percentage suddenly becomes unsafe, but because I want my pieces to cope with contemporary use.
I want them to be able to withstand repeated washing, everyday handling and the demands of a professional kitchen. I want the clay body to be dense and stable, while still retaining the qualities that made me choose it in the first place: its colour, texture, strength and the way it responds to my glazes.
And that last part is important.
There is no perfect clay body
The more I work with clay, the more I realise that ceramics is rarely about finding the "best" material.
It is about finding the right material for the job.
A clay body with a little more porosity might have exactly the colour or texture I am looking for. Another might vitrify beautifully but lose something of the character I want in a particular piece.
There are always compromises.
And perhaps that is what I find so fascinating about ceramics. The material isn't simply something we impose our ideas upon. We have to listen to it, test it, understand its limits and work with what it wants to do.
So, does stoneware really need to be non-porous?
Not quite.
What it needs is to be appropriate for its intended use.
And for the kind of tableware I make, particularly when it is going into a restaurant and is likely to be used and washed again and again, low absorption is one of the things that helps me make that promise.
But it is only one piece of the puzzle. Perhaps the more useful question isn't “How porous should clay be?” but “What properties does this particular piece need?”
Comments