I have been testing porcelain bodies for nearly two years. Recently I purchased a 1600X lab scope to help me see what I had long suspected.

This first image was taken of the clay/glaze interface: the dark area is glaze (cobalt.) The body has a strong glassy matrix and is fully vitrified: so I had to look further.

In going back over my flux tests in clay bodies, I began to notice a common COE pattern in pieces that were under-fluxed, or under fired.

The above picture is from the unglazed section of that bar, and the picture below is from the glazed area of the same bar. I used a monochrome filter to highlight the stress crack: hard to see in picture without it.

I have since learned from extensive testing that the flux in the clay bodies moved from the bottom (shelf contact) to the top of the bars. I made a series of test cylinders in various thicknesses: 3/8 to 1" to follow that flux flow through a body. Then a series of hollowed out cylinders to replicate an ovoid piece cavity walls. In every test: flux moved from the bottom to the top, or from the center to the outside of ovoid forms. As this example shows:

                                            < top of cylinder    >>> bottom of test cylinder

Sodium and potassium are in a gaseous state from cones 5-10; so simple physics explains the flux pushing upward or outward through the clay wall. You will notice the bottom of the sample shown is mature, while the top is not fully mature (some glassy matrix) but many voids. However, when you look through that same body on a cross section, the voids become more evident.

^^ top of bar   ( bar is upside down in this view.

An under-fired clay body can result in some COE issues of its own: regardless of how much care was taken to match the COE of the clay and glaze. The linear expansion shown in the second picture which was glazed: is not visible to the naked eye. However, they are prevalent all over this sample: only one was shown. ( my microscope has a very limited area of magnification)
Friend asked me to post it.

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Comment by Tom Anderson on September 22, 2016 at 6:09pm


Probably one of the most informative research docs I have read. The metes and bounds of feldspars are hashed through very well IMO. I also like they made distinctions between how sodium and potassium work in a clay body.

The Properties of Feldspars - PDF file


Comment by Norm Stuart on September 21, 2016 at 2:41pm

Tom - Without adding the flux / glass formers boric acid and aluminum phosphate to a porcelain body I've only seen full vitrification in cone 6 or cone 2 porcelain when the pieces are thick.  Now I know why, the thickness must trap enough of the carbonates of sodium, potassium or lithium long enough to react with the aluminum silicates.

In addition to the study I linked showing direct sublimation of sodium carbonate without decomposition into carbon dioxide and sodium oxide, a number of answers by chemists online have been quite assertive that decomposition of these three Group 1 "alkaline metal" carbonates cannot occur in an oxidizing atmosphere, unlike Group 2 "alkaline earth metal" carbonates like calcium and magnesium.

Yahoo - Does sodium carbonate have a decomp temp?

Incidentally, aluminum phosphate is "isoelectric" with SiO2 silica, (the exterior charges and shape are the same, in spite of having a different interior molecular structure.) so can directly replace a silica molecule in ceramic structures.

Comment by Tom Anderson on September 21, 2016 at 8:31am

Sorry.. forgot the pic for the post below--- need lots more coffee.. lol

Comment by Tom Anderson on September 21, 2016 at 8:30am

Norm: another example of flux movement through a clay body.

The yellow line is the edge of a test bar: below it is the side of the bar, and above it is the face of the clay. The red area is pointing to an un-vitrified body: full of pin holes and gas pockets. Below it is the green area, indicating a vitrified area. It was taken with a monochrome filter to help highlight these issues. In every example: vitrification moved from the bottom of the bar to the top ( in maturity). The last 1/16th inch of so of this bar is littered with gas pin holes: so that sorta speaks to how much is gassing off.


Comment by Tom Anderson on September 21, 2016 at 8:19am

Norm: once again TY for the reply and info. These tests are all results of pin pointing the flux limits for a cone 6 porcelain body. They were raised incrementally from 20% (recipe), all the way up to 32% (recipe). I sent these findings to Ron, and he is digesting them as well. Actually his initial response was "very interesting." Which for Ron means he is more than curious about it.

For cone 6 porcelain bodies: I have found 4.35 to 4.50% total fluxes (molar) to produce the best vitrified body. I have long known that KnaO are both in a gaseous state in c6 firings. What I have problems explaining is the flux flow through the body. When molar levels hang around 4.0%, I see the mason/dixon line almost every time as shown in pic 4. There is a distinct line where vitrification ends, and un-vitrified begins.

I can see the gas vapors moving up my test cylinders when I look at them close up. I know there is relatively no pressure in an electric kiln: but there has to be a small amount inside the clay walls. However slight, I see evidence where it is being forced to the surface. I have yet to find accurate information about the total loss of KnaO from vaporization: although I know it is occurring. I have pictures of a fully vitrified body: and the clay/glaze interface. Just having a hard time explaining the mason/dixon line in pic 4: I have seen that alot in underfired and or under fluxed bodies. 

Clay/Glaze interface:

This is what a fully vitrified cone 6 piece is suppose to look like. The clay and glaze are interlocked at the face.


Comment by Norm Stuart on September 21, 2016 at 2:10am

This is an interesting topic. Sodium carbonate melts at 1,564 F and potassium carbonate at 1,636 F with both exhibiting significant levels of sublimation (turning from a solid into directly into vapor) just below these temperatures. As with the vaporization of lead oxide from leaded glazes, this loss is also a function of time. Lithium carbonate may be more temperature stable, melting and sublimating at 1,333 F but not boiling until 2,390 F.


Sodium, potassium and lithium carbonates vaporize before reaching a temperature where they lose the carbon dioxide and are reduced to oxides. These metal carbonates are very different from "high temperature fluxes" magnesium and calcium carbonates which reduce to carbon dioxide and the metal oxide when heated in a kiln. The calcium and magnesium oxides don't vaporize until well above kiln temperatures.

I suspect most people dealing with ceramics assume the calcium / magnesium model for these other metal carbonate fluxes.

Fortunately for Cone 6 firings, boron oxide does not sublimate or vaporize until 2,732 F. As a result I've found boron oxide an essential ingredient when creating a cone 6 porcelain body.

While phosphorous oxides sublimate at fairly low temperatures, calcium and iron phosphates do not, remaining a flux in porcelain and bone china.

The volatile carbonates of sodium, potassium and lithium reduce the temperature and calories of heat required to melt / bond silica and alumina, but they can only perform this function while they remain in the clay or glaze.

Although they can form compounds with alumina and silica, I don't know if they can act as a non-consumed catalyst in these reactions leaving a densified ceramic after the metal carbonates have boiled off. I would assume this is unlikely.

Comment by Tom Anderson on September 20, 2016 at 10:31pm

To further illustrate COE issues of under-fired clay. This body formula is the simple 50% kaolin, 25% silica, and 25% Nep Sy. A very common blend fired from cone 5 up to cone 10. This test bar was fired to cone 6:

Again using monochrome filters to highlight the stress crack caused by immature (under-fired) clay. This perticular example shows a U shaped stress crack due to the lack of flux. It begins as a large crack on the left, goes down the side of the test bar, and reappears as a smaller crack on the right. (follow the arrows). This piece would be a prime suspect for glaze shivering.

Note: 25% Nep Sy recipe weight.  3.78% alkali weight, 3.70% alkali molar.  This is a commonly used cone 6 formula: not nearly enough to produce a vitrified body. In the "flux Limits for porcelain" thread: for cone 6 I recommend a minimum of 5.0% total flux weight.



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