Natural Imperfections in Pottery Clay Bodies - and Why You Should Care

Part of the fun of pottery is that nothing is going to be exactly the same every time. Even if you use the same clay, equipment, and techniques, your final project could come out looking a bit different. Of course, that can also be part of the frustration.

One of the biggest factors that can affect your final project are natural imperfections in the clay body. Whether it’s little black iron specks or discoloration at the top, these imperfections can either “ruin” your project or make it unique—depending on how you look at it. Here’s a quick guide to understanding those natural imperfections.

Where Do Clay Body Imperfections Come From?

To understand where imperfections come from, you have to go back to the very beginning: the Earth.

A pit mine for mining pottery clay, with a bulldozer in the distance.

The materials that make up pottery clay are harvested from the ground in big, open-pit mines. Basically, they dig a big hole, add some roads for trucks and equipment to get down into the big hole, and start scraping off layer after layer of clay and other clay ingredients. All that material is brought to a factory where it’s refined, sent to us, mixed to give it different properties like handling and color, packaged, and shipped to your door, so you can use it to create all sorts of wonderful projects.

As you might imagine, pulling material straight out of the ground isn’t the cleanest process. There’s a lot of different materials that intermingle down there; You’re never going to get one clean swipe of pure clay. There’s just no such thing. There’s a good chance (guaranteed) you’re going to end up with some iron, dirt, rocks, or any other goodies the Earth has to offer. And all those different materials will affect the clay body.

In addition to the various materials you might scoop up with the clay and other component minerals, weather variances will also play a role in the natural imperfections. Since the mines are open pits, all the sun, rain, and runoff will also affect the clay. Even something as simple as an overnight rain could bring a whole bunch of salts and debris down into the mine. The clay mined on the sunny day before might be completely different than the clay mined after the bad weather, even if they’re from the same area!

Higher quality clays get refined after they come out of the ground to remove some of these natural imperfections, but nothing is foolproof. Even the best pottery clay, like those we make at Seattle Pottery Supply, can still have some of these imperfections brought on just from being in the ground.

Most Common Types of Clay Body Imperfections

Clay body imperfections come in many shapes and sizes, but the two most common are scumming from soluble salts and specks from iron or manganese.

Soluble Salts (Scumming)

Salts are everywhere in the ground. Not only are they present in the soil (and, therefore, clay), but they can also wash into the mines after rain. When the salt gets into the clay body, it can cause an imperfection called scumming or, if you’re into more technical terms, efflorescence.

Scumming happens when water migrates to the surface of the clay body and brings the soluble salts with it. When the water evaporates during the firing process, the salts are left on the surface, turning it either whitish or brownish—depending on the mineral content. It’s especially present in low-fire clay bodies.

Although there’s not much you can do about the salt content in the clay body, you can manage scumming with the addition of some Barium Carbonate.

A Seattle Pottery Supply worker manufactures pottery clay in their Seattle warehouse.

Iron or Manganese Speckling

Iron and manganese are two of the most common minerals found in the Earth. Because of that, it’s almost impossible to avoid them when mining clay. These minerals show up in your clay body as black specks, which is called speckling.

Unfortunately, there’s not much you can do about speckling. The only way to really minimize speckling in your project is to buy more highly-refined clay.

Higher quality clay is more refined than its less expensive counterparts - you’re paying for the refining process to take out some of that pesky iron and manganese - leaving your project nice and speckle-free.

You can also refine the clay yourself by putting it into a slip and sieve it smaller and smaller until the specks go away. Of course, that’s an extremely tedious process, so most people just buy highly refined clay if they want to avoid speckling.

Types of Clay with the Most and Least Imperfections

Clay comes out of the ground in three different quality levels - meaning how refined it is naturally. Different types of clay have different levels of imperfections. Typically, the more refined the clay as it’s mined, the fewer imperfections that will have to be refined out during the manufacturing process. Here’s a quick rundown of the level of imperfections in different types of clay bodies.

Most Imperfections: Fireclays

Fireclays will almost always have the most imperfections of any type of clay body. They’re the most impure and the least refined, which means they’ll be full of all kinds of goodies like salt, iron, and manganese. This is the type of clay that’s usually made into brick.

Since it is the least refined, fireclay is also typically the cheapest clay you can find. People who work with fireclay usually expect it to be dirty, so they don’t mind the imperfections as much. After all, you get what you pay for!

Middle of the Road: Ball Clay

Ball clays typically fall right in the middle of the road. They’re a bit more expensive than fireclay, but they also have fewer imperfections. Generally, the price will tell you where a specific type of ball clay lands on the imperfection spectrum. The cheaper the clay, the more imperfections.

Because of their balance between price and fewer imperfections, ball clays are a favorite for both pottery enthusiasts and professionals. While there are dozens of different types of ball clay, OM4 Ball Clay is one of the most popular sold by Seattle Pottery Supply.

Fewest Imperfections: PKaolin

If you’re looking for the top-of-the-line, purest-of-the-pure clay, you want kaolin. Kaolins are the most refined type of clay there is. Although imperfections are still possible, you have a much higher chance of getting finished products that are pure white (or whatever color you choose). That’s why most high-end dinnerware (and creepy dolls…) are made from porcelain clay.

Seattle Pottery Supply's Klamath Yellow pottery clay

As always, because it’s the most refined, kaolin is typically the most expensive types of clay. But with those extra few dollars come less worry of imperfections once your project comes out of the kiln. If you absolutely need a pure, imperfection-free finish on your project, you can’t go wrong with kaolin.

Find Your Perfect Clay Body

There are a lot of variables that can affect the looks of your pottery projects. Because of the way clay is mined, natural imperfections are almost impossible to avoid completely. But you can lessen your chances of imperfections by choosing high-quality clay.

Generally, the more expensive the clay, the fewer imperfections you’ll have to face. Fireclays will typically have the most imperfections, while porcelain and kaolin will have the least. If you want an in-the-middle option, ball clay is a solid choice.

No matter which type of clay you’re looking for, Seattle Pottery Supply has all the products you need to turn your project into a reality. We have a wide selection of clays available, so you can experiment and find the right body to suit your unique style. If you have any questions, our friendly pottery experts would absolutely love to hear about your project and point you in the right direction. No matter your experience level, trust SPS for all your pottery needs!