Clay Making Deep Dive

Written by Slip-Casting Specialist, Joshua Kochis

Contribute to our ongoing Clay Mixer Repair Campaign

Thought you’d heard it all about Pewabic’s clay-making process? Think again! You may be aware that we’ve got some big-time repairs to our trusty 113-year-old Patterson system in the works, thanks to former Executive Director Steve McBride's post outlining the clay mixer repair campaign. If you’re a long-time fan of the blog here, you might even know how much it means to us to carry on the legacy of the original Pewabic crew, complete with photographic fashion inspiration for your next one-piece jumper, courtesy of Jerome. True fans, Detroit natives, or visitors to the historic pottery would have heard the spiel delivered by one of our talented docents on a guided tour. But for those of you too far away or too busy to come see us in person, this article will cover all of that good stuff and then some. 

We think it's worth shouting from the rooftops because the clay-making process is such an important part of what we do, and have been doing since 1903. So today we’re taking a deep dive - literally! As you’ll soon find out, more than one part of the process takes place underground. Where’s my smock?

This photo from 1981 shows the original clay-making system installed in 1912, before a few modifications were made as certain steps in the process became unnecessary or needed updating.

She bought an upgraded Patterson system in 1907, equipped with a small clay-mixing apparatus, pottery wheels, glaze mills, and material sifters, all on one connected system of electrically-powered cast-iron shafts, gears, and leather belts that serviced the entire pottery studio. By 1912, Pewabic had reached capacity in terms of how much clay could be made on this particular setup, which was not enough for the organization’s growing ambitions. Just five years after the initial system was installed, an addition was built onto the north side of the building to accommodate what was essentially a much larger version of the baby clay maker. This behemoth takes up a significant amount of space and is still churning out that sweet, sweet mud today.

Once you have your clay body formulated, and there is enough demand for your product that you need 1100 pounds of clay at a time, you are finally ready to scale up to the kind of system that Patterson hooked us up with in 1912. 

The on-site clay-making magic begins with a big mixing chamber called the Blunger Mill. It has a set of stainless steel mixing paddles mounted on a cast-iron shaft, rotated by two giant gears driven by a system of belts. It is a thing of beauty to see - and hear - this baby fire up in a symphony of metallic clanks and the hum of canvas moving around pulleys. The Blunger Mill has a red-brick interior, with one conspicuous brick missing about halfway up the chamber on the left side as you look down from its square opening at the top. The bottom of that missing brick serves as a water fill line for Cameron Hodge, our clay-making extraordinaire with expertise in big, loud mixing machines.

Detail of the Blunger Mill interior, and Cameron’s special missing brick water fill line.

The red-brick interior of the Blunger Mill when it is empty.

Cameron then dons a respirator mask and works his way down the list of dry materials, dumping bag after bag into the water, all while the mixing paddles do their circular dance inside the chamber. By the time all of the ingredients have been added, he’s looking at what we call a slip - the name for a humble mixture of dirt and too much water - weighing in at 2200 pounds of dry material mixed in a 1:1 ratio with the wets. This mixing and filling process takes a full workday, and Cameron is very happy to remove his mask at the end of it.

Pewabic craftsperson, Cameron Hodge, working through the clay-mixing process.

The next day, Cameron starts the system back up, lets the Blunger Mill paddles turn, and mixes the slip for another eight hours. This constant agitation over a long period of time ensures that the clay particles hydrate and distribute evenly throughout the mixture. He then prepares the filter press - which we will meet again shortly - for its job of filtering out all of that extra water, turning the liquid slip into solid clay.

Once the slip is thoroughly mixed, it is emptied into the Agitator Tank in a PVC piping system with a large ball valve, through a metal sieve to catch any unwanted solids, beginning our second journey underground. This subterranean chamber is larger than the Blunger Mill, able to hold about three times the amount of slip in the upper chamber. It was designed to age the slip for days or weeks at a stretch, as well as provide a storage area to draw from in a pinch. It has stainless steel mixing paddles to keep things moving, rotated by another set of two gears - one of which cracked into pieces back in 2022, a fun story we’ll come back to later.

At this point, a pump located above ground is activated, drawing the liquid slip back up to the surface through a metal pipe that connects to a large flexible hose running across the room to the Round Rail Filter Press. These contraptions are also used in the wine and whiskey manufacturing process, as well as wastewater management, in addition to making clay. A manually driven compression screw seals each of the 36 separate chambers of the filter press, which are divided by a layer of nylon and a cast iron disc set on two runners so they can slide back and forth to access each chamber. Once the system is sealed, the pressure from the pump forces liquid to slip through an opening in the center. The middle section of 35 chambers (one remains empty) fills up pretty quickly, in about 30 minutes. 

As more material is drawn into the press, water is filtered out as it passes through the layers of nylon between chambers. The water drains along vertical channels in the cast iron, out the bottom into yet another chamber below ground, which empties into the sewer. After about three hours of hard work for the trusty pump, each chamber is filled with solid clay to its outer rim. Once the filter press is full, the pump is turned off, and the material is left to sit overnight to dry out and help make removing it from the chambers easier.

The underground tank has been back up and running for a few years at this point, but now the same deterioration is evident in the main gears above the Blunger Mill. The vertically positioned gear is offset slightly, causing friction with the lateral gear, tossing metal shavings onto the top of the chamber and leaving bits of cast iron in the sieve on its way underground. We can only assume that eventually the system will fail as the other set of gears did before, leaving us in another emergency, slowing down the production process and, more importantly, putting this historic equipment in danger of complete disrepair. These emergency fixes are not only stressful for all of us at Pewabic, but they are more expensive than planned repair work, and often implemented as quickly as possible - a recipe for further trouble down the road.

We are trying to learn from previous experience and get ahead of this issue by implementing the necessary repairs before they stop production in its tracks or, at the very least, make our jobs much more difficult than they need to be, resulting in lower quality and quantity of our beloved tile products.

The cast iron shaft is deteriorating significantly, and the friction between the gears above the Blunger Mill is dropping metal shavings all over the place. Not a good sign! 

During that first round of repairs, we contacted Patterson for any helpful information or assistance they could provide. They were absolutely floored by the fact that we were not only running the system as part of our historic tours, as they do at Greenfield Village, but that it was still the main source of our production material. To our knowledge, Pewabic is one of only a few ceramics manufacturers still using this system regularly.

The filter press and mixing chambers are regarded as the best way to produce consistently plastic, workable clay. We’ve heard from other small ceramics operations and artists over the years, and more so recently, inquiring about how our system works and how they may implement something similar. Patterson discontinued this production line as their services underwent a significant expansion during World War II.

Another detail shot of the Patterson system taken in the 1980s. The operation looks remarkably similar today, 113 years later, to the surprise of current Patterson staff. 

Now that you know the time, effort, and literal magic that goes into every batch of clay we make in-house here at Pewabic Pottery, we hope you’ll join the team, help us keep Mary’s dream alive and our staff busy, and keep on enriching that human spirit through the wonderful world of ceramics for another century. You can do all of this by contributing to our repair fund at Pewabic.org/donate.