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Cracking (in Handbuilt projects)
Cracking (in Handbuilt projects)
Why: Several factors contribute to cause pieces to crack including:
Parts of a project dry unevenly.
Inconsistencies in the clay body.
Inadequate scratching and slipping when joining the pieces together. The process of scratching and slipping pieces together helps to moisten the two pieces at the point of connection so that slight differences in dryness stages between the pieces is less of an issue. However, if insufficient slip is added after scratching, the attached pieces will naturally pull apart from each other at the seam as they dry.
Undue stress is applied to the piece. Stress cracks can appear if:
-A student drops a project on the floor when it is in the leather hard stage.
-The piece is left to dry while using too rigid of an armature that it does not allow the parts to move freely during the shrinking process.
-The piece is not supported adequately during the drying stages.
Tips to Prevent Cracking:
Wedge or at least compress and knead clay before using.
When possible, store all parts of a project together in the same conditions so that the parts dry at a similar rate.
Store in-progress work in plastic to redistribute moisture.
Handle projects with care.
Dry pieces slowly &/or at least evenly.
Remove rigid armatures when drying clay.
When necessary, use flexible armatures that provide the form with some support while being flexible enough to allow the piece to shrink. Examples of flexible armatures include paper (NOT tightly packed) & sponges.
Tight Deadlines
Tight Deadlines
Even more important than drying projects slowly is drying them evenly. Sometimes, tight deadlines, such as an upcoming art show or the end of a marking period, require projects to have a less-than-ideal drying time (3 weeks). In those cases, candling projects is a great option (See Preheating/Candling). In my personal experience, I recall having to use a hair dryer to dry mugs at home the night before they would be fired in a huge kiln at a community college. While drying with the hair dryer, I was careful to rotate the pieces during the drying so that the parts could dry more evenly.
Repairing Greenware
Repairing Greenware
Repairing Cracked Sculpture with Scratching & Slip
Repairing Cracked Sculpture with Scratching & Slip
Making & Using Paper Clay to Repair Greenware &/or Bisque. Paper Clay can also be used in place of traditional clay to create ceramic pieces.
Vinegar & Spit for Cracks & Breaks: Sometimes, simply scratching and slipping is not sufficient to repair a crack or break. In those cases, after scratching both pieces, using vinegar or even spit (YES - a potter's secret!) in place of slip or as the base (in place of water) for a new slip-paste (ex. mixing vinegar with small dried pieces of clay), can sometimes do the trick! Also, if the crack is significant enough or continues to reappear in a piece, it is sometimes best to break the piece at the point of the crack so that you can more thoroughly repair the piece by scratching and applying the vinegar-clay paste to both pieces completely before rejoining them.
Slowing the Drying of a Repaired Join: I also recommend that the area around the repaired join be slightly moistened. The repaired join and area moistened around the join should be wrapped in plastic wrap. This all helps to slow the drying of the join, reduce the shock to the sculpture of having only a small part being re-moistened for repair, and, thus, help reduce the chance of a re-break.
Digging Out Cracks: Sometimes, if caught early and provided that the crack is not too extensive, it is possible to scratch or dig out the crack, such as with a ball point pen. This process works by compressing the clay around the crack. Afterwards, the dry clay that was removed can be reattached by scratching the area and using vinegar or spit to gently model the clay dust into the crack and left to dry.
Magic Water & Magic Clay
Magic Water & Magic Clay
Paper Clay, Magic Water, Magic Mud recipe, slip - Lakeside Pottery Ceramic Art Studio
Repairing Bisqueware
Repairing Bisqueware
Amaco Bisque Fix: Many ceramics teachers and potters recommend using Amaco Bisque Fix to repair bisqueware. After applying to bisqueware, be sure to wipe off the excess and to re-bisque fire the pieces before glazing. If glazed before re-bisque firing, the glaze has a tendency to slide off of the area with the Bisque Fix.
Making & Using Paper Clay to Repair Greenware, Bisque, & Glazeware. If used to repair bisqueware, be sure to bisque fire once more before glazing.
Glazing Over Cracks: Sometimes applying extra glaze over a crack can cover up the crack or at least strengthen the structural integrity of the piece. Glazing a cracked piece can reduce the ability of a crack to worsen or to continue throughout the piece.
Strategic Placement of Glazed Pieces in Kiln: If desiring to glaze fire a broken bisqued project, there are a few options to consider:
If the broken piece can balance on its own while in its proper placement, one can attempt to fire the piece in that fashion, being hopeful that the broken piece does not fall off during the firing. Tip: Having the shelves painted in kiln wash and leaving adequate room around this piece in the kiln, can help reduce the damage to other projects and the kiln in the event that the broken piece falls off.
If the broken piece cannot balance on the rest of the project in its proper placement on its own, it is sometimes possible to lay the project down or tilt it and use a variety of metal stilts to support the piece so that it rejoins via the glaze, during the firing.
If there is no way to figure out how to hold together two broken pieces of a broken bisqued project together during the glaze firing, one can simply glaze the piece and its broken fragment separately, leaving the points at which the pieces should be joined unglazed, the broken piece can be later glued on to the project with super glue (provided that the piece is not intended for food). Broken fragments (whether small or large) can almost always be successfully fired by balancing them on metal stilts.
Magic Water & Magic Clay
Magic Water & Magic Clay
Paper Clay, Magic Water, Magic Mud recipe, slip - Lakeside Pottery Ceramic Art Studio
Broken Glazeware
Broken Glazeware
Gorilla Super Glue (clear) can be used by the teacher to repair broken glaze ware. Over my teaching career, this glue has been wonderful in successfully repairing students' projects. I advise wearing plastic gloves while using the glue in a well ventilated area after school when students have already left to avoid having them breath in the fumes. I also recommend that you have small strips of masking tape ready to help hold the broken pieces together until the glue has matured in 24 hours. Do not return projects back to students until the glue has thoroughly dried.
Original Gorilla Glue is a great alternative to Gorilla Super Glue if the student is going to be handling the piece a lot rather than simply looking at it on a shelf. The Original Gorilla Glue does produce a much stronger bond then the clear Gorilla Super Glue. However, the glue expands significantly when dry, and even if you cannot see the glue seeping out of the join after application, it will often appear as a foamy yellow hard substance during the drying if you have used too much. A couple of dots go a long way with this glue! Similarly to when using the clear Gorilla Super Glue, I advise wearing plastic gloves while using the glue in a well ventilated area after school when students have already left to avoid having them breath in the fumes. I also recommend that you have small strips of masking tape ready to help hold the broken pieces together until the glue has matured in 24 hours. Do not return projects back to students until the glue has thoroughly dried.
Making & Using Paper Clay to Repair Greenware, Bisque, & Glazeware. If used to repair glazeware, be sure to bisque fire once more before touching up the glaze and re-glaze firing.
S-Cracks (in Wheel Thrown projects)
S-Cracks (in Wheel Thrown projects)
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