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Glazes melt by composition. Glaze melt generally cannot be calculated and usually need to be fired to confirm where they melt and don’t. The one exception I can think of is approximating where we think the melt will occur using Boron for cone six glazes. Layering glazes takes experience and often folks who know of a glaze that runs (overmelts) will use it over others as a rim accent etc...  The other common highlight technique would be to play upon the texture layering gloss over matte or vice versa  for a specific look.

All glazes started as cone ten, that is where the geology melts. Cone six and lower was achieved commonly with Boron, less silica and alumina, high fluxes, zinc for Bristol glazes etc..... Boron is easy and became popular for its ease so there is some linear predictability with boron. Even so, testing is the final answer to confirm where a glaze starts to melt and what it’s limit is before it runs or overfires.

Here is what I can say for sure

  • layered glazes are not predictable with respect to durability so outside vessel accents are likely more common than inside
  • Glazes that run are generally overmelted for the temperature they are fired at. Dial back the cone and they stop running. It’s a composition thing not necessarily a single number thing.
  • glazes melt in a range of temperatures, not necessarily a single temperature

others here will likely have experienced opinions for you as well. Glaze science is science but it’s also an art.

I will add an example, the vessel below was achieved by applying a rutile glaze over a runny matte. The runny matte is a titanium dioxide colored glaze that fires matte at cone six; however more boron was added to try and make it melt more like cone 4 so it runs a bit at cone 6. After a lot of testing this  worked out where the crystals from the matte run down the base glaze and produce the look. Yes a designed look, but some luck and lots of  testing including firing a cone up and a cone down to see how it performs.

Did we predict this? Yeah, sort of I guess. Could we predict from a recipe only? Not really, experience and testing count a bunch. Is this bowl durable? No idea, likely but maybe not. I would need to test.


Edited by Bill Kielb
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3 hours ago, redbourn said:

Can you tell from a glaze recipe what it's melt temperature is?

Not sure I understand the question. Are you talking about when the fluxes within the glaze start melting? Or if by looking at a recipe you can see if it's low, mid or high fire?

3 hours ago, redbourn said:

I've read about putting a high melt over a low melt and vice versa for interesting effects

If you take 2 dissimilar glazes, ie one high in alumina and low in silica and the other low in alumina and high in silica, where the 2 glazes melt together the one low in one oxide will pull it from the other glaze and (hopefully) create visual texture. If you plunk the recipes into a glaze calculation program just look at the silica:alumina ratios of the 2 glazes for an idea of how the glazes will interact. This is why using a gloss either over or under a matte glaze is often successful and layering 2 glazes with the same base don't interact much if at all. Titania, either from titanium dioxide or rutile, is a great oxide to include to create visual texture in a glaze, hares fur glazes are good examples of this. 

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9 hours ago, Bill Kielb said:

Glaze science is science but it’s also an art.

Glad I am OK with taking my chances with more of the art side of things (plus these days I'm happy with an array of commercial glazes). The chemistry knowledge is long, long gone from my brain and I'm not retaining it with re-learning. (Lee is feeling her age today!!) 

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On 8/29/2019 at 9:19 AM, Bill Kielb said:

Glaze science is science but it’s also an art.

At my last market I had some soda fired pots for sale. Had a woman who was interested in the process so I explained it rather carefully, including the sodium interaction with the alumina and silica in the claybody forming a glaze. She said something like, "wow, it's almost like science".  Oh, if only she knew! ;)

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