Slip Casting

Slip casting is a process where slip, liquid clay, is poured into a plaster mold.  The mold when you look at it looks internally like the inverse/reverse of the piece that it will cast.  Molds that cast complex pieces, such as figurines, pitchers, or Christmas trees, generally have several sections that fit together like a puzzle and can be split apart so that the ceramic pieces that are cast can be removed.  Molds allow identical pieces to be cast either by having identical molds or by using the same mold over and over to cast several pieces.

In the image below there is a mold for a fancy pitcher.  If you look carefully at the mold you can see that it defines the external surface of the pitcher.

                                                 

The mold is put together with banding straps that tighten to hold the mold pieces securely.  The image below shows the mold with the banding straps in place.

                                                 

Once the slip is poured in, the mold is allowed to set for somewhere between 30 minutes to 2 hours depending on the wall thickness desired on the finished piece.  The plaster absorbs water from the slip as it sits.  This causes the outer layer of slip that is in contact with the plaster to solidify.  After some time passes this outer solidified layer becomes thick enough to form a 1/8 inch to 3/8 inch shell on the inside of the mold.  This shell eventually becomes the cast piece.  Once the shell of solidified slip is thick enough the slip inside of the mold is poured out leaving the solidified shell within the mold.  In the image below you can see the slip being poured into the mold.

                                                  

The image below shows the slip being poured back out of the mold after 45 minutes have passed.  This leaves the outer shell of hardened slip still in the mold and the center liquid slip is poured back out.

                                                 

The mold is then allowed to dry for anywhere from one hour to several hours.  The drying time is dependent upon the size of the piece being cast.  Larger pieces are allowed to dry longer after the slip is removed.  As drying occurs the shell of clay within the mold gets harder and harder.  Eventually it is hard enough that the mold can be split apart and the piece removed.  In the three images below you can see the piece in the mold, the untrimmed piece, and the trimmed piece.

   

Slip casting is a useful way to make ceramic pieces commercially because of the ability to cast identical items.  Most items sold in stores, for example figurines, are slip cast.  Another useful characteristic of slip casting is that pieces cast separately can be attached while the parts are not completely dry by putting slip on the pieces much like glue and holding them together until the slip/glue is partially dry.  The piece then acts as though it is one item.

Slip cast items are first poured, then they are trimmed, for example with an Exacto knife or a Felting knife (a knife used to blend the seams where the mold splits).  After trimming they are allowed to dry and are then Bisque fired (a first firing without a glaze yet applied, makes the piece easy to handle) to about 1900 degrees Fahrenheit.  Once it is bisqued the piece is coated with glazes and then re-fired to a temperature slightly below the bisque firing temperature.  This second firing causes the glaze to become glass-like sealing the piece and giving it strength.

Once cooled from the glaze firing the piece is ready for use.  In the image below you can see the piece that came from the mold.  It has been trimmed once out of the mold, bisque fired, glazed with a transparent glaze, and glaze fired.