Slip can be used for surface texture, for color, and for joining pieces together. In this project, I use it for all three, and repurpose a laser spirit level to align the handle vertically.
Mixing the Slip
I make my slip from reclaimed trimmings of the same clay I use to form the piece. I collect the dry trimmings in a 5-liter container before adding water and leaving the clay to fully rehydrate and soften into a slip. I mix it and pass it through a 40-mesh sieve to ensure there are no lumps, then look at the fluidity of the slip to decide whether to add or remove water. I want the slip thick enough to hold most of its shape when I pull the mixer out, but it should not form sharp peaks. If it’s forming sharp peaks, then I add more water and mix again, repeating until I’m happy with the thickness. To remove water, I leave the slip covered overnight and then decant some of the water that rises to the surface as the clay settles in the bucket.
I use oxides and stains in various amounts to color my slip. I find that adding 0.1–1% cobalt oxide to white clay slip gives a nice range of blues.
Preparing the Mug
Throw the forms on the wheel and cover them with plastic overnight. This allows the clay to firm up slightly to prevent it from warping or collapsing when slip is added. This stage is a compromise between slip fit (the wetter the piece, the more closely the slip shrinkage will match the shrinkage of the clay body) and the strength of the piece. I find that the softer side of leather hard is a good midpoint.
Leave the pieces on the bats without using a cut-off wire to release them after throwing. This allows you to return them to the wheel the next day and have the pieces centered and securely attached. Trim and burnish the outer wall and rim before adding slip as this is harder or impossible to do afterward.
Make the handles a few hours before adding the slip so they can firm up to the same level of dryness as the body of the mug. I extrude my handles, but this will work with any process that allows you to make the handles in advance (i.e. not pulling them from the mug body).
Next, you need to set up the laser guide. I use a 2-axis self-leveling laser that will give a horizontal and vertical laser line, although in this case you only need the vertical line. The line needs to pass directly over the center of the wheel head, and will project a vertical guide up the side of the mug to assist in handle placement. Once set up, the laser level stays in the same place throughout the process. Then, assess whether the handle is the correct size and scale for the piece (1), but do not attach it.
Applying the Slip
I add several colored slips to the same syringe, which marbles the colors slightly and adds another layer of color pattern to the finished piece. If you want more control over the final pattern, then each color can be applied with a separate syringe.
Set the wheel to medium/slow speed and apply the slip in bands (2, 3). Try to make sure the application thickness shifts slightly around each band to give variation in the slip movement. Once the slip is applied to a sufficient thickness, lift the bat off the wheel and lightly tap it on the splash pan or wheel head to make the slip flow down the piece.
Adding the Handle
When you’re happy with the slip movement, return the bat to the wheel head and attach the handle. There’s so much slip on the piece that there is no need to score the connection areas; applying pressure will firmly attach the handle. Apply slip to the ends of the handle (4), then attach the top by supporting the clay on the inside of the mug and pressing the handle into the slip (5). Then use the laser to check the vertical alignment of the handle before pressing the bottom joint into place (6), as it will be obvious if the position is changed after pressing it into the slip.
Use a cut-off wire to release the piece from the bat (7), and set it aside to let the piece firm up before removing it. Rest the handle against a small pot (or chunk of clay); as the slip rehydrates the clay walls, the weight of the handle can cause the piece to slump slightly.
I use a Giffin Grip to trim slip-decorated pieces when they are on the drier side to prevent making unintentional marks in the slip pattern that would be visible in the final piece. If you do a good job of trimming the walls before applying the slip, then there is minimal trimming needed, just a little clay removed from the base to form a foot ring.
Glaze these pieces with a clear glaze applied over the slip. Brush a watered-down wax resist (I find it applies better when thinned) onto the exposed clay body at the bottom of the piece, then dip it in a clear glaze.
Pour the interior glaze while the piece is set on a scale. You do not want to pour too much glaze into the piece and risk contaminating the clear external glaze when removing the excess. By using a scale to add precisely as much glaze as needed by weight, there is no excess to pour out and the internal application is consistent. The wax resist prevents the glaze from adhering to the exposed clay body, and the clear glaze gives the slip a glossy, bright finish.
Joe Thompson is a graphic designer turned full-time potter working in a small studio in Surrey, UK. He works with wheel-thrown stoneware to produce colorful, functional pieces. Find out more at www.oldforgecreations.co.uk or follow on Instagram www.instagram.com/oldforgecreations and YouTube www.youtube.com/c/OldForgeCreations.