|Monthly Tech-Tip |
These are cross-cuts from slugs of a production run of clay that was improperly pugged (inadequate vacuum). The problem is not often evident at time of extrusion but exhibits after weeks or months of storage. This clay body does not contain significant fine-grained material, but 2% talc is added to aid vitrification. The production crew claims that this talc makes it doubly important to monitor vacuum at all times (or laminations will result). These are not actually a problem if you wedge the clay well. But if you do not (e.g. inserting them into a hand-extruder, pressing them in a mold) then they "build in" failure points that will initiate drying and firing cracks later. Even if they survive the drying and firing processes, weaknesses will persist makings pieces more prone to failure-on-impact or stress. This being said, does that mean you do not need to wedge plastic clay bodies if they are not laminated? No. All clays laminate to some extent, even if not visible. Because clays have such incredible particle surface area, even the best pugmills cannot wet the surfaces of all of them.
Laminations because of improper pugging of a clay body will cause separations and drying cracks in the ware.