-Many have too much melt fluidity. Aside from obvious issues of them running off the ware and being very thickness-sensitive, fluid melt glazes are less durable (because they lack Al2O3 and SiO2). Fluid melters also generally craze.
-Some fire cloudy when thicker. This can be from clouds of micro-bubbles (from materials that gas excessively on firing) or crystallization (e.g. boron blue from excessive boron and calcia).
-Many have tiny surface dimples that appear when held up to the light. These are the remnants of the healing of bubbles that broke at the surface.
-Many do not host stains very well, either producing the wrong color and going matte or cloudy with in-color or over-color applications.
-Many do not suspend well in the slurry and they turn to jelly over time (often because they are fluxed with Gerstley Borate). Some lack sufficient clay in the the recipe which also produces a powdery surface.
-The leach resistance of many is questionable. These either contain significant percentages of materials questionable in themselves of ones that seed crystals which are more likely to be soluble (e.g. lithium carbonate, zinc).
-Many do not slurry and apply well to ware, it is difficult to get on evenly.
When you find a good transparent, it stands out from all the others. Achieving such is part of the basic science of producing quality ware.
When we found the Perkins Clear glaze recipe we saw problems, but also promise. It just needed a couple of changes to become a great clear.
*This was among many clear glaze recipes we found on the internet. It stood out because it was crystal clear.
But it also had two problems:
Gerstley Borate. 21% of this can easily turn a bucket of glaze slurry into a jelly that is difficult to work with, and, when thinned with water, cracks on drying. Gerstley Borate also contributes to high LOI which potentially can produce lots of bubbles when certain colorants or additives are incorporated.
Thermal Expansion: It crazes on some of our porcelains. The thermal expansion needs to be a little lower.
*This recipe makes the most obvious adjustment to Perkins Clear: Sourcing boron from a frit instead of the troublesome Gerstley Borate (it is a bubble source, gels glaze slurries, is inconsistent).
The chemistry of this is very close to the original Perkins Clear. However because the boron is being source by a frit rather than a raw material, we added a little more Al2O3 and SiO2 (both beneficial to drive down expansion and improve durability). However the melt is still quite fluid, that means the glaze can tolerate still more SiO2 and Al2O3.
Since we want this to be super-gloss it seems appropriate to add only SiO2 (to increase the SiO2:Al2O3 ratio).
G2916F Commercial Dinnerware Clear vs. G2926A (Cone 6)
G2916F is sold by Plainsman Clays as Stoneware Clear. Its melting pattern is more bubble-free because its LOI is even lower (the talc in the 2926A is likely contributing these bubbles). However when used on ware these bubbles do clear.
G2926 Perkins Clear vs. Fritted version G2926A
Notice that the frit melts the glaze more even though the amount of boron it contributes is the same. This is good because it enables us to add more SiO2 and Al2O3 (both beneficial).
There are lots of pages and pictures online about this recipe and stain-added versions (e.g. at digitalfire.com and plainsmanclays.com). The page at Plainsman Clays has mixing and usage information.
This recipe has proven to work very well as a general purpose cone 6 transparent. It works best if ball milled (for lowest possible thermal expansion, best surface quality, least surface defects, best clarity). Unlike many others, this has very high SiO2, that means the best possible durability. It also has significant Al2O3, further contributing to stability of the melt and glass.
Although EP Koalin is used here, we have produced even better results using Grolleg and New Zealand kaolins. Using all three it is possible to get a slurry with excellent suspension and applications properites if attention is paid to the thixotropy.
This fits most Plainsman Clays bodies (and those of other manufacturers). But it does not fit Plainsman P300, a cone 6 whiteware, unless ball milled or a 325mesh silica is used.
*This is an adjustment to G2926B to reduce its thermal expansion. However, before trying this, we recommend ball milling the G2926B or using 325 meshsilica in that recipe. Only if those do not work, then try this (use 325 silica in this one also, the fine particles assure all of it melts).
This recipe increases MgO at the expense of KNaO. Even though MgO levels are quite high, no loss of gloss has happened! The effect on lowering the thermal expansion is dramatic.
With 10% added zircopax this produces a white of even lower thermal expansion. We have subjected various Plainsman P300 pieces (a difficult-to-fit-glazes-to body) to 300F:IceWater thermal shocking and there is no crazing. However that much zircon can induce crawling issues, we have found that 3% tin and 5% zirocpax is better.
For use as a transparent consider adding 2% zircopax. That is insufficient to opacify but the zircon does act as a fining agent to remove microbubbles and make the glass more transparent.
Ferro Frit 3249 can be substituted for Fusion F-69 (although F-69 is more reliable for ceramics). Likely any kaolin can be used.
Although there is only 12.5% kaolin this will suspend well if the slurry is thixotropic. In our initial mix 5000g water and 6000g powder produced 1.47SG. That was very watery so we added about 10g of Epsom salts to gel it.
G2926S after 300F:IceWater test on P300
No crazing after the test.
The transparency and clarity of the glass is great, this looks very promising.
P300 after 300F:IceWater test
S is not crazing after two days. B crazing in the cold water.