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Hi all. Am having trouble with pinholes. I am using some commercial glazes and one glaze I mixed up from a recipe I found online. 

I clean my pieces very well before I glaze. I allow to dry before glazing, and leave them covered until glazing. 

I believe maybe it could be my one glaze recipe, or perhaps it could be the firing.

I rent a space and the owner does the firings. He will not let anyone else fire and he blames all defects on you or your pots.


Any help is so much appreciated. I am attaching a photo of a bad example of a glaze with a pinhole I got this firing. 


I saw where I could add flux, or remove zinc or rutile? 

Here is my recipe for floating blue cone 6 I used:

Neph Sy - 45.2

Gerstley Borate - 24.5

Silica - 18.9

EPK Kaolin - 5.7

Whiting - 3.8

Talc - 1.9

Bentonite - 2

Rutile Flour - 4

RIO - 2 

Cobalt Carb - 1




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1 hour ago, WarEagle2334 said:

He will not let anyone else fire and he blames all defects on you or your pots.

If it's his kiln I can understand why he wouldn't want others firing it but that's just silly thinking the firing programs have nothing to do with pinholes and blisters (amongst other things). I would ask him for his firing schedule for both bisque and cone 6 glaze firings and ask he let you put some witness cones in the kiln next to one of your pots with this glaze on it. Is it just this one glaze that is having the pinholes and blister issue or others as well? Is this a white / light claybody?

Floating Blue is a really finicky glaze to get right, you picked a tricky one for your first attempt at your own glazes. Thickness, firing schedule and especially the rutile in the recipe can make a big difference in how it turns out. The version of it you posted looks like one of the earlier versions of it that has been slightly tweaked. Just from looking at the recipe I would be looking at lowering the amount this recipe is off gassing to start with. This could help reduce the pinholes but I'm thinking the firing schedule is part of the issue. Gerstley Borate has approx a 30% loss on ignition (LOI), whiting has a LOI of approx 34%, just be reducing those two things alone can help reduce the likelihood of pinholes. Are you okay with how tan the glaze is? If you want less of the tan colour then I would be looking at your rutile. Rutile really can vary in its composition. Look for a ceramic grade or it might be called calcined or light where you are. If that's what you are already using and you want to decrease the tan colouration then try a test batch with 2 rutile and 1.5 titanium dioxide in place of the 4 rutile. Regarding the cobalt, I'm thinking it might have been 1 % cobalt oxide not carbonate in the original recipe. Cobalt oxide is stronger than the carbonate, if using the carbonate I'ld bump it up to 1.5%

Your recipe (on the left) and what I would change it to (on the right). It's going to be less thick in the bucket since I really cut back the gerstley borate (which tends to gel up glazes in large amounts). I kept the formula the same but cut the LOI in about half. I kept the silica:alumina ratio the same (so same amount of gloss). Flux ratio is the same too however since I've used a frit in my altered version this glaze could melt more than the original. If you do try my altered version below then please don't mix up a bucket of it before testing with a small test batch first, what looks okay on paper doesn't always translate to what works on a pot. It's hard to tell in your image but it looks like the glaze is fairly thick in places, when you do some test tiles dip them so there is one dip over most of the tile, 2 dips over the top half of the tile then dip one top corner of the tile again to make 3 layers so you can see the difference glaze thickness makes. 

Cone 6 

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