Slip-Cast Vessel Experimentation Kyle Johns

Appears in the June/July/August 2020 issue of Ceramics Monthly.

Inspired by the untreated honesty representative of Brutalist design, my current work starts from the notion of dissection and reassembly. My work most often revolves around the vessel, which I use to rethink molds and generate new ideas for forms. Using keyless molds, I can rearrange each piece in an infinite number of ways, creating one-off forms from one cast to the next. The shapes chosen as models are often ones with curves and angles, which evoke feelings of strength and structure.

Prototyping and Mold Making

The prototypes or models are formed from clay, which then get cast into plaster and are further altered by lathing or turning the plaster on the wheel. Forming plaster prototypes on the wheel is a time-sensitive technique, because the plaster must be stiff enough to hold its shape, yet soft enough to cut. Once carved to shape, the prototypes are then inspected for imperfections, which can be lightly sanded if necessary. After each plaster prototype has cured, it is placed in a dry box overnight or for several days to dry completely. When possible, I recast the initial prototypes as one-part mother molds, from which numerous copies can be generated (1). After the pieces are recast, they are inspected for imperfections, sanded again, and set aside to dry.

Once several plaster molds have been made from the prototype, I decide how to cut the molds into sections (2). Distinct cut lines are drawn on each mold with a fine-point marker in a number of ways: perpendicular, parallel, L shapes, T shapes, and diagonal cuts, to name a few. Using these various angles and directions prevents all the horizontal and vertical seams from ending up at the same point, which would make the form symmetrical. The plaster molds are then cut following the marker lines using a band saw, table saw, or hand saw (3). Each saw leaves unique marks on the plaster, which are left to reveal part of the deconstruction process. The molds I design are intended for slip casting, knowing any subtle nuance on the mold will be present in the cast clay. One of casting slip’s defining characteristics is its ability to pick up the finest details from a mold’s surface, including the cut marks made by the saw. After the mold pieces are cut, they are then washed to get rid of any loose plaster and set aside to dry.

The molds I make are fully modular, meaning there are no keys cast or cut into the plaster pieces, which would restrict how the mold blocks could be used. Unlike the traditional way molds are used in industry to produce the same form over and over, working this way allows for variation and change from casting to casting.

Drafting

Constructing a one-off form using plaster molds requires thinking ahead to how the negative interior space will translate into the positive exterior of a vessel. The forms start as a roughly stacked 3D sketch that is adjusted through minor changes of flipping and rearranging the molds, which can create subtle or dramatic alterations in the form. The pieces are assembled at least once prior to casting in order to troubleshoot any possible problem areas susceptible to a heavy leakage of the liner slip or to make any final revisions to the form. Image 4 shows a cutaway view of half of a stacked assembly of mold sections.

Once everything has been assessed, the mold can be disassembled and laid out in its construction pattern on the work surface (5). The organizational and numbering system for the mold pieces varies based on the form. Most often pieces on the top of the form are placed farthest from the center of the table working inward as the mold blocks are disassembled. Numbers that correspond to the percentage of stain in any given container are then placed next to each mold block, preventing confusion about which stain is being applied to each section. When working on vessels with a lot of mold parts, it can be helpful to label the colors that will be painted on each mold piece with a Sharpie marker and some masking tape (6). The slips often appear close to the same tone in their wet state, so it is important to have a system for keeping track of the colors. The slip-painted panels can peel if you do not work quickly and efficiently, so labeling the molds will help prevent having to repaint panels. The organizational method is a combination of both the slip color and the orientation of parts for assembly.

After all the mold blocks have been placed down, they then can be dunked in water or wiped down with a wet sponge to activate the mold.

Tinting Casting Slip

I use a porcelain casting slip because it results in more brilliant stained colors. I choose to use bright, fun colors more closely associated with contemporary design than a traditional ceramics palette. I borrow heavily from automotive and furniture design for my color inspiration. Vivid blues, greens, and oranges contrast against gentle pastel purples, blues, and pinks, much like the way an interior upholstery color complements an exterior paint color of a new car.

When mixing casting slip, add the measured dry colorant into the water and deflocculant mixture first to get a better blend. The clay is then added to the water. The casting slip is sieved, then allowed to rest for two days before use to give the particles time to fully homogenize. After the allotted time, the slip is sieved and tested by pouring a trial cast for consistency, before being used on complicated forms.

For each exterior color, I make several small containers of slip, ranging in percentage of added stain (0.25–10%) to allow for the ability to create gradient patterns on the work. The small containers of slip are not sieved, in order to encourage more surface variation. These small containers hold the same casting slip as the liner, but with a nearly pudding-like consistency for brushing onto individual mold pieces. Having a more concentrated slip means fewer layers need to be brushed on to reach the desired thickness.

Slip Application

Using a wide brush, I apply the slip in several coats onto the desired plaster areas (7), often coating more than one side. After applying the preferred thickness of slip to all the mold parts, an X-Acto knife can be used to cut a clean edge along all the sides of the painted slip sections (8). Following the cutting of the sections, the remaining runoff of unwanted slip can be scraped away with a soft rib to avoid scratching into the plaster (9). Once all the excess slip is removed, the bare areas can be cleaned with a wet sponge, making sure to also wipe the edges of the slipped sections down (10). For a smooth, clean layer of slip, be sure to load up your brush well with slip. Other surface designs can be created by dry brushing, dripping, or splattering one or more colors on a mold segment—this is a great phase of the process for experimentation. I don’t worry about different thicknesses on the painted panels because the liner slip will build up more thickness in the low areas due to mold saturation, compensating for any uneven thickness in application.

Assembly

Once all the molds have been painted and cleaned, I reassemble the mold, strapping down sections as they are stacked. The base mold section is raised up on blocks, allowing room for straps that go underneath the piece to tighten it down properly (11). The working time for each mold is dependent on the mold form’s size, dampness of the plaster, thickness of the slipped panels, etc. Typically, you have 15 to 30 minutes to work once the mold sections have been painted and the assembly process has begun.

Since the molds are keyless, they can be offset by shifting them in and out for final adjustments. To increase the thickness and span of the seam lines, clay or plaster pieces can be placed between mold parts, making the seams wider or more prominent, while exposing the cut marks. Once the form has been strapped together both horizontally and vertically, all the gaps where slip might seep through can be packed with clay on the exterior (12). The clay will typically resist the pressure from the weight of the slip long enough to allow the slip to stiffen onto the mold walls, sealing the slip into the mold, and locking the mold parts together.

Lining

The liner slip is poured carefully into the center of the base mold (13) with a consistent flow until reaching the top of the form, avoiding splashing and dripping down the interior mold sections (which will leave marks). If the slip is not poured quickly enough, it will create a stratified appearance, which may or may not be desirable.

Due to the variability of how these pieces are constructed, the tops are always shaped differently and rarely do the molds meet up to make a flat rim. Allowing the slip to flow over the lowest mold parts on the top can create a visual thickness change on the top of the piece that is not found in the majority of slip-cast work. Also, since I top off the slip as it is absorbed into the mold instead of using a reservoir, rings of slip develop as the thickness of the liner slip builds up (14).

Draining

The mold is drained with a ball-valve system assembled out of plastic PVC pipes (15), like what you find in a garden spigot, for controlling the rate of flow. My forms are cast thicker than most slip-cast forms so the width of pipe is crucial to prevent the casting slip from sealing the drain hole shut. The system I use keeps the pipes vertical to prevent the loss of pressure during drainage, as gravity is what makes the slip drain from the mold. Due to the scale of the work, there is inherently a lot of pressure from the slip, so the valve allows the rate of drainage to be controlled. Holes are created in both the mold base when it is constructed (16), and the casting table to run the plastic pipe assembly down to a drainage container. The drain leaves a hole in the bottom of the form that can be filled or left open. A tight fit between the holes and the drainage assembly are a must to prevent leaks, as is making sure that the base hole and table hole are lined up to properly set up the valve assembly when first setting up the mold. The valve assembly should be installed before the final construction of the mold pieces, strapping, and casting. When the casting reaches the desired thickness, the valve is opened and the clay drains into a bucket below the table (17).

Demolding

Timing of demolding is more crucial with this process than most casting processes. Since the mold sections are not beveled, the flat faces interfere with shrinking slip and can cause large cracks to appear in the form before and after firing. Smaller cracks that can later be repaired can happen from the combination of straight-sided pieces, aggressive removal of the mold pieces, and less saturated panels. The time needed to demold will vary from casting to casting, depending on the thickness of the cast wall and the dampness of the mold parts. Forms may need to be demolded quicker in some areas and slower in others, in order to prevent the piece from tearing apart or collapsing because of moisture content (18). If unwanted cracks do result from waiting too long to demold the form, they can be fixed with a damp brush (19).

The mold pieces get cleaned as they are removed, first scraping off heavy pieces of clay with a soft rib, then wiping down the whole piece with a wet sponge. The time consuming nature of the process necessitates that the mold blocks only get cast once per day.

Cleaning up the Cast Form

Cleaning the slip-cast work is often the quickest part of the process, as the forms are intentionally left unrefined. The bottoms are cleaned and wiped smooth with a soft rib, beveling the edge to prevent the vessel from scratching any surfaces (20). The flashing (or live edges) are inspected and broken off if they are too loose, large, or warped, then the piece is set aside to dry.

After the bisque firing, the bottom of the work is wet sanded with a fine-grade 400-grit sandpaper. Next, the inside of the work is lined with a clear glaze. On some works I treat the exterior surface with a residual transparent glaze by rubbing it into the textured surface. Each piece is fired to cone 7. After the high firing, the object is inspected for any major cracks or glaze flaws, then the bottom is wet sanded again with 800-grit sand paper.

the author Kyle Johns earned his undergraduate degree from Southern Illinois University Edwardsville (SIUE) in Edwardsville, Illinois, and received his MFA from Ohio University in Athens, Ohio. Kyle completed residencies at Red Lodge Clay Center in Red Lodge, Montana, and the Archie Bray Foundation for the Ceramic Arts, in Helena, Montana, and is now a resident of the Ceramics Program, Office for the Arts at Harvard in Allston, Massachusetts. To learn more, visit http://kylejohnsceramics.comor Instagram: @kylejohnsceramics.