3D-printing artifacts under an amber glaze on light and dark bodies

The dark one is M370C with 10% added raw umber. The other is M370C. Both are glazed using GA6-B Alberta Slip amber transparent - the color difference is entirely a product of the difference in color of the fired clay body. The wood-grain texture is the artifact of 3D-printing the case mold flat rather than upright.

Related Pictures

Making a mini bottle prototype mold

Prototype plaster mold using 3D printing

PLA is the most common filament used in consumer 3D printers. Because this is just a quick trial slip casting mold it is practical to print two PLA case molds (lower right) with vertical sides (it is impossible to remove the plaster mold from these without significant damage to the corners. But this is not a problem, PLA filament has a very low melting point so even hot water can soften it - I use a propane torch to heat them (very carefully of course) and they peel off easily. Making these confirms how accurate 3D printers are nowadays - the plaster mold halves mate precisely. Notice there are no natches, they are not needed for prototyping (lining up the outer edges perfectly positions the pieces). And there is no spare, we use a pour spout instead.

This GA6-B glaze is better than beer bottle glass

Ceramic glazes are actually just glass. But they are not like bottle glass. The latter is formulated to work well in forming machines (harden quickly), melt and stiffen quickly, have low melt viscosity and resist milkiness and crystallization on solidification. The chemistries to accomplish this have adequate resistance to leaching and adequate durability for a single or few uses. A stoneware glaze melt needs to be much more viscous (to stay put on vertical surfaces). And, it must have a much lower thermal expansion (to match common clay bodies). And, it must resist crystallization more much (since it cools slowly). Fortunately, meeting these needs brings along big benefits: Greater durability, hardness and resistance to leaching. Common target formulas express typical oxide formulas of glazes. Stoneware glazes and bottle glass share a common trait: They have about the same amount of SiO₂. But the similarity ends there, stoneware glazes have:

-High Al₂O₃. Three to five times more! It is the key oxide to producing durable glass. And it stiffens the melt (that disqualifies high levels from bottle glass).
-The same fluxes (CaO, MgO, K₂O, Na₂O). But they distribute very differently (half the CaO, half to one third the KNaO, much more MgO). Other fluxes like SrO, Li₂O are also common.
-Low KNaO (which they call R2O). In glazes it produces crazing, 5% is a typical maximum. But bottle glass can have double or triple that (the high thermal expansion is not an issue and its cheap source materials supply lots of melting power).
-B₂O₃ melter. It is expensive but can be justified because the glaze is just a thin layer. Glazes at the low end of the stoneware range have 5% or more boron.

The ceramic bottles shown here are made from a dark burning stoneware, the glaze is GA6-B. On the left is the same glaze on a porcelain mug. For the above reasons this glaze is more durable and leach resistant that regular bottle glass.

Links

Recipes	GA6-B - Alberta Slip Cone 6 transparent honey glaze An amber-colored glaze that produces a clean, micro-bubble-free transparent glass at cone 5-6. Works well on brown and red burning stonewares.
Glossary	Slip Casting A method of forming ceramics. A deflocculated (low water content) slurry is poured into absorbent plaster molds. As it sits in the mold, usually 10+ minutes, a layer builds against the mold walls. When thick enough the mold is drained.

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