Reactive glazes don't melt into a homogeneous melt and they don't freeze as a typical glass. The physical nature of the material powders (e.g. their particle size and the individual nature of how they respond to heat, soften, melt and interact with their own kind and others) create a melt that does not solidify into a homogeneous glass. These glazes are said to be dynamic. And unpredictable effects often occur during firing, like color variegation, speckles, streaks, mottled and flowing textures, crystallization, pooling, etc. The outcome is influenced by factors such as the materials chosen to source the needed oxides, firing schedule, kiln atmosphere, cooling or heating cycle, etc. These glazes are at their best when each piece has a unique, artistic character. But, this is also their worst feature, making them "tipping point glazes", ones whose visual character is a product of fragile and not well understood features of the materials and process. Small changes typically produce big changes in fired appearance (often to the chagrin of the potter).

Here it is fired to cone 8 where the melt obviously has much more melt fluidity! The photo does not do justice to the variegation and crystallization happening on this surface. Of course, it is running alot more, so caution will be needed.

Engobes were applied to the insides, and up over the rim, of these M340 pieces at the leather hard stage. They were then bisque fired, clear glazed and fired to cone 6. On the left is L3954B, no zircon was added. The one on the right is L3954S2, in has 20% Zircopax. These are part of a fitting effort to match the firing shrinkage of the body, M340, to a base engobe (and colored versions of it).

These fired bars are nepheline syenite (NS) fired to Orton cone 03 (~2000F). The top bar, L4526C, is 95% NS and 5% Veegum. Its porosity is 3%. The bottom bar, L4526B, is 90% NS and 10% raw bentonite. Its porosity is 19%! The top bar has much higher fired shrinkage, it looks and feels like a porcelain, that is clearly what is densifying it so much (the bottom one looks and feels like Plaster of Paris). Veegum is a plasticizer, not a flux. But it is acting as the latter here. Or perhaps its surface area enables acting as a catalyst to initiate the melting of the nepheline syenite. Imagine what Veegum can do in a porcelain-like Polar Ice.

If you do DIY pottery glazing you may have recipes scribbled onto cards like this. But the card is not the big issue here, it is that recipe! It really needs some work. Here is what could be done.
-Add this as a recipe in an account at insight-live.com (and assign it a code number) to start a testing project. Along the way document it with pictures, firing schedules, general notes, etc.
-With all that feldspar it is sure to craze, reducing the high thermal expansion K2O it contributes in favor of low expansion MgO (from talc) is the most likely fix.
-With all that clay (29 total) it is likely to crack while drying (and then crawl during firing), split it into part calcined kaolin and part raw kaolin (the ball clay is not needed).
-And those colorants: It is better to use cobalt oxide than carbonate. Perhaps the burnt umber could be increased to eliminate the need for both the manganese and iron (since it supplies both and has zero LOI). Better yet, remove all four and use a black stain (7% would likely be enough).

It took a lot to be able to throw this moderately bellied vessel because the clay is pure calcined kaolin. It has zero plasticity. Actually it is worse than zero. That is why 25% bentonite was needed to make it barely plastic enough. That 25% would have done much better with other non-clay materials like feldspar or silica! How can this be? In its natural state, kaolin’s plasticity comes from its layered crystal structure, the water both bonds the plate-like particles together and lubricates their lateral movement against each other. The chemically bound water in the natural kaolinite crystals, which are tiny water magnets, is the secret to their ability to create plasticity - calcining drives it off. This dehydroxylation also changes the crystal structure, converting kaolinite into non-crystalline metakaolin, a particle that is actually hostile to plasticity. Calcined kaolin is also subject to shear thickening, a thin slurry thickens when propeller mixed - the particles form structural resistance, the opposite of what raw kaolin does.

We find the 0.047 relief depth shown here is best (K152). Shallower ones will stamp a crisper design but K152 is better if pigment is used to highlight the recesses. For some things, it can be valuable to put a border around the outside of a design so that when the stamp is pressed hard into the clay, the edges do not smear outward. These do not need to be stuck to a piece of wood, just lay them face down on the clay and use a wooden block to press them down (because they are flexible it is easy to peel them out). When the clay is stiff enough no parting agent is needed. The cost: In 2024 the minimum charge is $37.5 for 50 square inches. They accept PDF and AI vector files and the shopping cart enables previewing. The cart might generate all four CMYK plates, remove the CMY ones and keep the K (black). The most common mistake is having too much detail or too small printing. Or, forgetting to make them reverse-reading. It is best to make your images using vector graphic software like Illustrator or Inkscape.

Let's make a low SG version of G2934BL, totally DIY. Weigh out a 340g batch of dipping glaze powder. Include 5g Veegum (to gel the slurry to enable more than normal water) and 5g CMC gum (to slow drying and impart brushing properties). Measure 440g of water initially (adjusting later if needed). Shake-mix all the powder in a plastic bag. Pour it into the water, which is blender mixing on low speed, and finish with 20 seconds on high speed. This just fills a 500ml jar. In subsequent batches, I adjust the Veegum for more or less gel, the CMC for slower or faster drying and the water amount for thicker or thinner painted layers. Later I also assess whether the CMC gum is being degraded by microbial attack - often evident if the slurry thins and loses its gel. Dipping glaze recipes can and do respond differently to the gums. Those having little clay content work well (e.g. reactive and crystalline glazes). If bentonite is present it is often best to leave it out. Recipes having high percentages of ball clay or kaolin might work best with less Veegum. Keeping good notes (with pictures) is essential to reach the objective here: Good brushing properties. We always use code-numbering (in our group account at Insight-live.com) and write those on the jars and test pieces. This is so worthwhile doing that I make quality custom labels for each jar!

CAD software and 3D printing are a potential revolution in vessel mold-making for ceramics (3D modelling is another topic). But there are two big problems: There is no way a potter, hobbyist or even small manufacturer can afford the typical software cost. While it is true most have free or low-cost trial or hobby versions, the strings attached are deal breakers. The second problem is the complexity of learning - that can be a bigger obstacle than cost.

Fusion 360 seems to offer a way to on-board the CAD world, using the free version and its great learning resources and best-in-class user interface. It is new and modern, a YouTube star. It is fully parametric supporting constraints and a timeline. True, it can choke on more complex drawings on consumer computers, but we don’t need to do those. But, for commercial use, it costs $680/yr. But that is cheap compared to some others! Upon discovery of the capability, the cost might be doable for you. If not, there is a second option: Move to Shapr 3D after learning. It costs $299/yr, also works on iPad (which Fusion 360 does not) and it is similar enough to make the transition easier. It is less powerful and lacks the training support, but the things it does do well are what is needed for mold making in ceramics. It uses the Parasolid engine like OnShape and SolidWorks (that royalty is probably what raises the cost).

Here are the ones you cannot afford (and maybe don't want):
-OnShape runs in your browser. It focuses on collaboration for teams. Free-version drawings are public but going private costs $1500/yr!
-Rhino is usable for CAD but targeted at modelling. It is not fully parametric and does not have a traditional timeline (however Rhino+Grasshopper is life-changing for geeks, both for CAD and modelling). $1000 to buy but upgrading is $500+.
-Solidworks is fully parametric with editable history. But it is old, the interface shows it. It is low cost for hobby use but for commercial use it is far out of reach for individuals ($2600/yr in 2025).
-FreeCAD is becoming more viable. It is parametric, has constraints and exports and imports popular formats (but with lots of issues). Its model tree is equivalent to the Fusion 360 timeline, but more clunky and depends on careful setting of constraints. The learning curve right now puts it out of practical reach of most. But a capital injection, like Blender got, is coming.

Stains can and do influence the degree of vitrification of a porcelain. Some stains will make a porcelain more refractory (decreasing fired shrinkage), others will make it more vitreous (increasing the firing shrinkage). Obviously, the greater the percentage of stain the greater the effect. Stained porcelains having differing fired shrinkages will stress at boundaries in accordance with the degree of difference in their fired shrinkages. In this piece, you can see how the boundary between the red (more vitreous) and green (less vitreous) porcelains is the point-of-failure. The only solution is to adjust the porcelain recipe to move the fired maturity in a direction that counterbalances the effect of the stain. For example, you could employ three recipes (regular, more vitreous, less vitreous) and use the indicated one for each stain added.