Potters & Sculptors - Making Rock from Mud
The generally accepted method is to use the same clay as the piece you want to repair, and it should be in the same fired state as your piece (raw clay to repair greenware and pulverized, screened bisque clay to repair bisque ware).
Cracks must be opened up to get enough of your repair compound down into the crack enough to be in full contact with the two sides of the fracture. Moisten the piece with water, magic water, or vinegar water, and work the patch compound into the crack very quickly, as the bisque piece will be pulling moisture out of your patch compound just like it does with a glaze. After drying, sand off the excess repair compound to level the surface. Re-bisque the repaired piece, prior to glazing.
Products like Amaco Bisque-Fix are a combination of Sodium Silicate with grog (previously bisqued clay) of a variety of sizes ranging from very fine to very coarse, possibly with the addition of an adhesive.
At our studio we use either sodium silicate or a Ferro Frit like 3124, though almost any frit will do, mixed with very inexpensive man-made 100 mesh Kyanite or Mullite.
If you need to attach an exterior piece, also add either bentonite (or better Bentone-EW) or White Glue (Polyvinyl Acetate), but do not use the Ferro Frit which has Boron which will cross-link the white glue into a useless gummy wad. Applying white glue over a reattached piece helps hold it in place - it burns-off in the kilns.
Kyanite and to an extent Mullite, have two very important properties:
1.) they irreversibly expand when heated which locks this material tightly into place
2.) microscopically they are long thin crystals which interlock, which make them shock resistant, which is why kyanite is added to Raku clay and kyanite is used to make thin lab crucibles which can be heated over a direct flame.
But there's a problem - Sodium Silicate, or Amaco Bisque-Fix, bisques to a surface which is very non-porous and is thus very resistant to glaze application. When fired, glaze tends to crawl away from treated areas. You can solve this problem by applying a thin layer of slip made from the clay you used over the fixed crack, or just a thin slurry of kyanite with a hint of betonite.
I knew Norm would add to this topic. One point of about the repairing compound -- under a clear glaze, a repair with anything other than the original clay will show a color difference.
Thanks George. A thin layer of slip made of the clay you're on top of the Kyanite repair is a must if your clay body is not slightly off-white, the same color as fired Amaco Bisque-Fix.
The Kyanite and Mullite we purchase from Laguna/Axner is a dark gray (which is almost certainly carbon) which fires slightly off-white compared with porcelain. So I'd guess Amaco calcines-off the carbon in the Kyanite they use.
If you use sodium silicate as a flux and binder, you'll also want that thin layer of slip made from your clay just to allow glaze to adhere properly and not crawl away from the sodium silicate surface. This is partly due to sodium silicate on rebisque creating a fairly glassy surface, and partly because the free sodium adding additional flux to your glaze, making it more fluid, crawling off the surface with sodium silicate.
Adding 5% Ferro Frit 3124 to your slip can prevent it from cracking off, as your clay will shrink more than the bisqued piece you apply it to, but for some reason it does not create the glaze crawling problem that sodium silicate does. My guess is the fluxes in the re-bisqued Ferro Frit are less likely to migrate into the glaze you apply, being already melting into a glass and reground.
We have a fairly constant influx of studio members with limited ceramic experience, so fixing damaged bisque is a common occurrence at our studio. One other comment is Amaco Bisque-Fix needs to be recapped over a layer of Saran wrap, otherwise it will dry out to an unwettable compound due to the loose fit of the jar lid. It is initially covered with an aluminum foil and plastic composite seal.