An Update on EPK (Edgar Plastic Kaolin) - December 2025

As many of you know, the source mine for EPK (Edgar Plastic Kaolin) is once again at a standstill. Unfortunately, it now looks like EPK may not be available again any time soon. Eek! We are currently updating our operations to account for this shortage.

We know that this issue will impact potters at many levels. In an effort to let you in on our thought processes our team has put together a brief "need to know" about this change.

Happy reading!

Understanding the Shift: From EPK to Wilco UPF

As we transition from EPK (Edgar Plastic Kaolin) to Wilco UPF, it is helpful to understand exactly what these materials are, why EPK was a studio staple for so long, and how this new Georgia kaolin compares.

What is Kaolin?

At its most basic level, kaolin is a clay consisting primarily of the mineral kaolinite- Al₂Si₂O₅(OH)₄. It is formed through the weathering of feldspar and other rock-forming minerals under humid conditions.

In ceramics, kaolins are essential because they provide the necessary alumina Al₂O₃. and silica SiO₂, to clay bodies and glazes. Depending on how they were formed, they are generally categorized in two ways:

Primary Kaolin:

Found at the site of their formation (residual).

Secondary Kaolin:

Transported from their original site by water, often picking up impurities but generally gaining finer particle sizes and better plasticity.

The Legacy of EPK

For decades, EPK was the industry standard in North America, largely because of its unique balance of properties. While kaolins are generally less plastic than ball clays, EPK was known for being distinctively more plastic than primary kaolins (such as Grolleg).

EPK was used so widely that it became a baseline for comparing the workability of other clays. For example, potters often used EPK to gauge how much bentonite (a plasticity agent) was required to make a clay body workable on the wheel.

Introducing Wilco UPF

With EPK no longer available, we are recommending Wilco UPF, a kaolin mined in Georgia.

Geologically, Georgia is famous for its vast deposits of kaolinite. These clays are widely utilized in ceramics because they offer high purity and consistent particle size distributions. Just like EPK, Wilco UPF is processed to serve as a refractory source of alumina and silica in your recipes.

Summary of the Switch

The Similarities (Why it works as a substitute):

Mineralogy:

Both EPK and Wilco UPF are kaolinites. They share the same fundamental two-layer crystal structure (silica tetrahedra sheets linked to alumina octahedra sheets).

Chemistry:

Both provide the essential refractory backbone for glazes and clay bodies. In a glaze recipe, they perform the same chemical function: supplying Al₂O₃ and SiO₂ to stiffen the melt and stabilize the glaze.

The Differences Between EPK and Wilco UPF (What to watch for):

Plasticity:

EPK was renowned for its specific plastic feel. While Georgia kaolins are excellent, different kaolins interact with water and plasticizers (like bentonite) differently. You may notice slight changes in how a clay body throws or how a glaze slurry suspends.

Particle Size:

Natural kaolins from different regions have unique grain size distributions. Georgia kaolinites are distinct from other deposits (such as those in Japan or the UK). This may result in subtle differences in shrinkage or drying behavior.

What we have done to replace EPK with Wilco UPF:

When switching to Wilco UPF in a clay body recipe, we tested a small batch first. If you find the plasticity slightly different from what you are used to with EPK, remember that kaolins respond very well to augmentation; a small adjustment in the amount of bentonite can usually match the plasticity you previously enjoyed with EPK.

We are also undergoing testing of all products that contain EPK (clay, glaze, slip, etc.). In 2023 we experienced a pause on EPK mining. The silver lining of this pause was that we got a chance to redevelop our products that contained EPK. This gave us a little bit of a head start to reducing our reliance on EPK.

Testing still continues, and product changes are expected in the coming year. We are committed to communication in this tricky time. Our team is small but mighty and we want to ensure the best quality products that we can.

Be well, and keep making art,

The Seattle Pottery Supply Team