There was a suggestion that kaolin was at the source of the milkiness so I decide to run a test. I mixed up the recipe and left out the kaolin and then just added a bunch of different ones I had on hand. They are EPK, Ball Clay, McNamee, New Zealand, Grolleg, Peerless #3, Standard, Super Standard, Dixie, and D'Arvor kaolins. Each of these have various amounts of iron, titanium and other impurities and they vary in amounts of Al/Si so it is hard to isolate one material when testing.
I tested these on test tiles in cone 6 electric E1 firing cycle.
The claybodies are Frost, P5, Redstone, and 266. All this is listed in each image.
The colorants I used were the ones for the Campana Grey - 0.15% Cobalt carbonate, 0.6% Copper Carbonate, and 1.8% Manganese carbonate.
(Remember that these were quick tests and not definitive, and there were some problems with mixing as can be seen on D'Arvor tests with white specks and in color set with some streaking because of no sieving.)
What I learned:
- Most of the clear glazes had a yellow cast from the iron in the kaolins, even though it is only a small amount. But the CLEAREST one was with New Zealand Kaolin. It was the clearest glaze I have ever seen! This may translate to other recipes ...getting a clearer CLEAR glaze. Best on Frost clay. The Grolleg was the next clearest. [remember that this particular recipe has low iron so if you use Custer or other feldspar with iron the clay may not help as much.]
- The next most amazing thing was the OM-4 set (which is a ball clay but I wanted to see what it would do since it and EPK is often used in glazes) This set was the most milky and it is very visible in both the clear set and colorant set. Probably caused by impurities in ball clay. ( it has a lot of iron and titanium compared to kaolins and it is quite a variable material!)
- There was a great difference when I added two kaolins, Peerless #3 and Dixie which required me to add more water than the rest. They are made to have high surface area for rubber manufacturing. You can google the kaolin name and "digital fire" and a description will come up.(e.g. Dixie digital fire). This cause come application variation because it was thicker.
- The colorant test had several with big pinholes/bubbles which are presumable from copper carbonate or manganese dioxide gassing off. (I have observed this in other colorant tests.)
- Not sure if it is visible the the Frost clay in the two colorant tests were all very clear but the test on the P-5 were almost all milky with blue and white flecks. So the clay body (P-5 is a domestic porcelain and I don't know the recipe but presume it contains EPK and some ball clay) with the colorants is clearly causing this effect. You can see this in the thickest drippy spots toward the bottom. So the effect Scott was seeing is possibly related to his clay body and the colorants. (this is not really visible in the clear set on the same two bodies.)
Campana Clear /Grey with D'Arvor Kaolin and colorants (Detail showing milky blue streaking and white flecks. )