Hydraulic Press

Octagonal yunomi, 4 in. (10 cm) in diameter, stained porcelain with slip and glaze, fired to cone 8 in an electric kiln, 2018.

It wasn’t an easy decision to purchase a hydraulic press. There are a lot of factors involved and big things to consider, like the cost, the time, the space, the research and development, and the risk involved. 

Deciding Factors

A few years ago, I started making eight-sided cups. I would throw a thick cup, facet, shave, scrape, then sponge and rib to refine and smooth it. To remove some of the labor involved, I thought that I would slip cast these cups. But, I realized the detail I loved most was the round interior and octagonal exterior. If I was to slip cast that form, I would lose the round interior. After seeing a few RAM presses in action years ago, the process was exciting to me and always in the back of my mind.

Out of curiosity, I started familiarizing myself with the complexities of the RAM-press process. This mild obsession helped make the decision a realistic one. Buying a new press and getting it up and running can cost more than $25,000, and that was out of the question for me. I was fortunate to find a used one for sale in Montana. After weeks of communication and logistical planning, I took out a no-interest loan from a family member and purchased a 30-ton studio press for $6000 and paid another $1400 to have it shipped to New York. The deciding factor was the opportunity I had to purchase this machine at a price I could realistically pay back. Without the right set of financial circumstances and support, this would not have been possible. A few additional essential equipment costs included: metal die frames ($300–$600 per mold), a 60-gallon air compressor ($800–$1200) for purging/releasing molds, and a pugmill ($3000–$5000) to process clay.

A New Process

What I love most about this process are the parameters. I see working within a construct as exciting, comforting, and exhilarating. Some of the limiting factors are the types of objects you can make. The dies (molds) consist of two parts: one top and one bottom. One die is the interior shape and the other is the exterior. The model/prototype cannot have any undercuts and its design is critical to success. The pressed object must be an open form, with a well-designed taper of the clay wall. The more open the form is, with no angularity, the less stress/better flow of the pressed clay, and the longer the life of your die. The design and form of my faceted cups are counter to some of these guidelines. While pressing out angular pots, the dies can make roughly 250–500 clay objects, give or take a few. The angularity of facets creates areas of extreme pressure, which eventually result in die cracks. I see the cracking of a die as an opportunity to make a new one and not be anchored to any one design for too long. Removing some of the preciousness from my process opens up possibilities for play and discovery. I am not interested in factory-style production, or using master molds and making the same thing over and over. I am, however, interested in pushing up against the limitations and preconceptions of this tool and discovering how this piece of equipment can free me up. The pressing process is supplemental to my studio tasks, as it allows me to spend more time on my wheel-thrown work and surface treatment of my pots. Using the pressed pots as a canvas for a variety of surface explorations has spurred fresh approaches and exciting discoveries.

1 Part 1 of a die casting of the exterior of a square plate in progress. Clay is built up to remove undercuts before pouring Ceramical (a type of very hard, dense gypsum and concrete only used to make dies for RAM pressing).

2 Setting up the second die casting of the interior of a square plate. The first half of the die is visible within the stacked steel frame.

Previous to owning the press, I would throw about 20 faceted cups in a day and take about another day to get them to where they are when they come off the press. With the press, I can make about 60–80 cups in a day and then may spend a week or more trimming, adding handles, and applying surface treatments. 

Making the Molds

Once working dies are made and pots are being churned out, you begin to see the real potential in this process. The hard work is getting to that point. Some potters outsource the making of dies because of the strain and challenge it presents. I make all of my own dies. It is a tedious, labor-intensive process, involving a long list of technicalities and design issues, all of which are critical to success and leave little room for error.

To make a die, I begin with a bisque-fired plate that has been sealed with wax resist (1). I build up clay to remove undercuts before later pouring Ceramical, a type of very hard, dense gypsum and concrete used to make dies for RAM pressing. Dies are housed in die cases made of ½- to ¾-inch-thick steel (2), and require an internal network of mold duct tubing that allows compressed air to run through the die, resulting in a permeable mold that easily releases pressed clay forms. This process of injecting compressed air into the die through the duct tubing, called purging, occurs while the Ceramical cures as well as in use when releasing pressed forms.

Part 1, the die of the back of the plate, has already been cast and purged. It is essential to carefully install the mold-duct tubing, which is suspended 1 inch from the surface of the ware and about 1 inch apart from itself (3). For more volumetric forms like cups, hardware cloth (metal mesh) is formed to shape and used to suspend tubes an inch from the ware prototype (4). Tubes are secured to the metal mesh with zip ties. The 4 bolts shown coming through the die case will help hold the solid plaster in place when put to use. The precision and preparation that goes into setting up for a pour can take days. 

3 The mold-duct tubing must be assessed before being encased in Ceramical. The tubing is suspended 1 inch from the surface of the ware and about 1 inch apart from itself.

4 Detail of part 1 of a gang cup mold setup illustrating how the cotton mold duct gets connected to the air compressor fittings with clear tubing and zip ties.

5 Peltzman pouring Ceramical with the assistance of his two sons, Grayson and Leo. 3, 5 Photos: Pam Peltzman.

6 Peltzman pressing some of the first cups from his cup die. Photo: Pam Peltzman.

Once part two is prepared to be cast, the Ceramical needs be properly measured at 1 part water to 2½ parts Ceramical. It needs to soak for 5 minutes, then be mixed at around 1700 rpms for 10 minutes, then left to stand for 2–3 minutes, and finally poured through a screen to remove any air bubbles (5). Once leveled to the back of the die case, the Ceramical cures to 105° F (41° C), at which point the purging process begins.

With both parts of the die cast and ready, before pressing anything, the clay must be properly prepared. It needs to be quite a bit stiffer than clay you would throw with, so it can be safely removed from the press without creating warpage later from handling. But when stiff clay is used, another great benefit of forming pots under intense pressure is the strength and density of the clay, both before and after firing. Wedged clay is loaded into the mold and pressed (6). I have added stamping to my pressing process using a piece of hardware cloth to add a gridded texture to the surface of pots (7).

Dies should be designed with an area of negative space, called a gutter. The gutter (see the indented ring around the cup rim in the bottom half of the mold in image 7) is critical because it gives the excess clay space to escape the mold and also creates extreme back pressure, which in turn makes the lips and the pressed ware overall highly compressed. After releasing a form from the die, this excess cleanly peels off and is immediately thrown into my pugmill to be reprocessed. One of the main areas that needs attention after pots are pressed are the lips as this is where the flashing or excess scrap gets torn off, which is visible around the edges of the pressed plates in image 8.

7 A freshly pressed footed cup stamped using a piece of hardware cloth to create a grid texture.

8 Freshly pressed square plates. One of the main areas that need attention after pots are pressed are the lips. This is where the excess clay escapes from the two compressed molds and the clay from the ware and the gutter meet. The flashing or excess scrap gets torn off, leaving a lip that needs to be cleaned up.

9 Pressed mugs and handled cups. These are all about to be dipped in white porcelain slip.

I am currently working with 4 dies (2 plates molds, a bowl mold, and a gang mold with a footed cup and a mug). Another challenge inherent to this process is the weight of these dies. The heaviest is my cup die, which weighs around 150 pounds. Recently, I installed a winch/hoist system in my studio as simple addition that eliminates the heavy lifting needed to move molds around.

Worthwhile Investments

The investments I’ve made in studio equipment are essential to assisting with the efficiency, quality, and longevity of my studio practice. Supplementing my thrown work with RAM-pressed work has been invaluable. This is the first year in my eight years as a full-time studio potter that I’ve had inventory and bisque ware on my shelves (beyond what’s needed to fill show commitments). So, after two years, I am certainly beginning to see this investment paying off. Just like with any new method of working, RAM pressing takes time to develop and fit into your existing studio practice. Patience and diligence are key. Making pots full time is hard enough, and taking risks and spending money can be terrifying. I am interested mostly in making the best work that I can. Meeting this goal by investing in my studio practice has been something I have never regretted.   

Dodecagonal handled cups, 5 in (13 cm) in width, stained porcelain with slip and glaze, fired to cone 8 in an electric kiln, 2018.

the author Doug Peltzman earned his MFA from Penn State in 2010 and has been a full-time studio potter since graduating. In 2016, he established a pottery studio with his wife, Pam Peltzman, in Shokan, New York. He is a father to three young children, and a dedicated husband. Doug is a founding member of Objective Clay and creator/organizer of the Hudson Valley Pottery Tour. To learn more about Peltzman’s work, visit dougpeltzman.com.


Leave a Reply

You must be logged in to post a comment.

Enter Your Log In Credentials
This setting should only be used on your home or work computer.

Larger version of the image

Send this to a friend