What are slump, hump and sprig molds you ask? 

They are plaster forms used to fashion shapes with clay. Plaster forms and tools can be successfully used to create imaginative and unique clay pieces. Because of the absorbent nature of plaster, clay can be depressed into highly detailed sprig molds and lifted from the mold without sticking. Minute details can successfully be transferred from plaster to clay. The absorbent plaster forms help to assure an even and timely draw of moisture from the clay body, limiting cracks and sticking. Convex plaster forms are known as "hump molds" while concave plaster forms are known as "slump molds."

To select between a hump and a slump mold of the same shape, the artist must decide whether access to the inside surface (available with a slump) or the outside surface (available with a hump) is most important. Hump molds give the advantage of adding feet and handles to the outer surface of the piece. One must be sure to remove drying clay from a hump mold precisely at the leather-hard stage due to potential cracks from shrinkage strain. Slump molds allow the clay piece to dry with a limited risk of cracking because the clay is able to shrink away from the plaster wall.

 

Using Slump and Hump Molds

Step 1: Using a wire, slice off an appropriate amount of clay
from a block of moist clay.

Step 2: Use the palms of your hands to slightly flatten the clay
onto a piece of canvas.

Step 3: Begin rolling the clay using a large rolling pin. Press
firmly and roll the entire length of clay.

Step 4: Roll twice and turn the clay over. Repeat until the clay
is rolled to desired thickness (1/4"-3/8") and size. Compare
clay dimensions to plaster form to assure proper coverage.

Step 5: Create any optional impressions, textures or patterns
you wish on the clay side that will be facing the plaster sur-
face of the mold.

Step 6: Lift clay from canvas and place onto plaster form.

Step 7: Use a sandbag to pounce the clay into/onto the
plaster form. Pounce until smooth.

Step 10: Use a moist sponge to smooth and clean the edges or other imperfections in the clay surface.

Step 11: Create any texture, patterns, impressions or add sprig designs to the clay surface exposed.

Note: You can also add a coil or "feet" to the bottoms of
hump items. Simply roll out balls of clay or a coil, score
both sides (scratch lines with a sharp tool), dab on a little
slip and press the parts together. Make sure the piece is
level by turning the clay (with the mold inside) over and
pressing firmly on a flat surface.

Step 12: When the object is leather hard, remove it from the
mold and clean the edges with a cleaning tool. Allow to dry
completely before firing

Using Sprig Molds

Step 1: Place a piece of flattened clay over the design in the sprig mold

Step 2: Use the bag of sand to pounce the clay into the form. 

Step 3 Lift the clay from the form.

Step 4 Use a cleaning tool to cut the shapes from the clay. Repeat steps 1-4 until you have gathered the desired number of sprig objects.

Step 5 Use a moistened sponge to lightly clean edges of your objects.

Step 6 After arranging the objects on your hump or slump item, use a clean- ing tool to lightly score the area where the sprig objects will attach.

Step 7 Use a cleaning tool to lightly score the back of the sprig objects.

Step 8 Apply a small amount of slip to the back of the sprig item. Step 9

Step 9 Depress the sprig onto the scored area on the piece.

Step 10 Create any additional adornments with clay and attach the same way as your sprig items.

Step 11 Gently clean the entire object removing any imperfections with a sponge and cleaning tool where necessary.

Step 12 When the object is leather hard, remove it from the mold and clean the edges with a cleaning tool. Allow to dry completely before firing.

Note: Add a small ball of clay to the backside of sprig objects and fire to create clay stamps for making impressions in wet clay.

 

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