Stoneware Limit Study
It has been two years since I began this research project. Perhaps it is time to make my final observations.
Stoneware is often referred to as dirty porcelain: some truth to that seeing as all clay is Aluminosilicate. With the exception of titanium: kaolin in general has less than 0.30% of iron and magnesium. Fire clay, ball clay, and other sesquioxides can have percentages of elemental metalloids over 8%. While both kaolin and stoneware clays has various levels of calcium, sodium, potassium ( rare cases lithia ) these add no added benefits other than supplying fluxes. The molar levels of iron, titanium, and magnesium determine the fired whiteness: anything under pure white then becomes " dirty." Above this, the real differences between stoneware and porcelain lies in platelet structure resulting in how kaolin and ball clay holds water.
Stoneware originated from basic brick recipes. High iron, large particle native clay were purposely fired to produce carbon coring; which in turn melted the iron in the
1800F ( 1000C ) temperature range. Ball clay was added to produce plasticity, making brick extrusion viable. Grog was added for structural purposes, and to keep oxygen available until the peak. These same brick recipe clays were rearranged to produce the early stoneware recipes. Orton ( the cone guy) heavily researched brick recipes, and subsequent stoneware recipes; with emphasis on remediation of carbon coring. The " slow" cycle on Orton Cone Schedule (108F an hour) came from this research; and specifically designed to overcome coring and bloating.
The basic stoneware recipe is: 80% clay(s), 10% silica and 10% feldspar. The only real dividing line in stoneware formulation is: functional or non-functional use. If functional: then absorption should not exceed 2%. The term " vitrification" is now applied universally to both stoneware and porcelain; however stoneware is actually measured by "densification." Densification being defined as the maximum particle distribution (PSD) obtainable to achieve the lowest possible absorption. Vitrification implies high glass content, densification implies maximum particle density. Given the large particles of fire clay, much lower spar levels: zero absorption is unobtainable.
Stoneware has a formulation deficiency not found in porcelain: cristobalite formation. Yes porcelain can develop small amounts, stoneware however can develop amounts sufficient enough to render the piece useless. This formulation issue is why all stoneware bodies require a minimum of ten percent feldspar addition. Enough KNaO must be present to incorporate the ejected silica from spinel development into the melt. Excess ejected free silica is the primary cause of cristabolite formation. Stoneware typically has 1/3 less molar flux levels than porcelain: another notable distinction between the two bodies.
In the UK during the 1990's, Limoge and Potclay began doing research on alternate flux addition in both porcelain and stoneware. It was found by Potclays that adding 1-2% calcium carbonate helped incorporate free silica into the melt: above these small additions created other chemistry issues. Both bodies produce different levels of mullite, glass, and free silica: although porcelain has little free silica because typical bodies have a minimum of 25% feldspar additions. Free silica in stoneware was problematic because during the spinel phase it was ejected. In addition to calcium, magnesium levels were also increased to counteract this problem.
Ougland and Brindley did a qualitative study for the British Ceramic society in the 1950's using X-ray de fraction to measure the amount of glass, free silica, and mullite in a typical recipe:
1200C. Glass 62. Free silica 21. Mullite 19
1300C. Glass 66. Free silica 16. Mullite 21
While additional heat work does add some vitrification properties to a fired piece, it is more reliant upon the molar percentages of flux, and supporting fluxes in general. Typically a cone six firing is the lowest possible peak required to produce functional ware, although some success at cone five is obtainable if flux levels and firing cycles are strictly observed. Regardless, the typical glass to mullite ratio is 3:1. In order for glass to develop: X amount of silica is required: and X amount of alumina is required for mullite to form. The combination of these two variables is where the modern formulation standard of 4:1 SiAl ratio comes from. Again noted this standard relates to functional use.
The molecular structure of kaolin and ball clay is the third determinate distinction between porcelain and stoneware. Kaolin is a 1:1 clay particle with no inner platelet: looks very much like a saltine cracker at 25,000X magnification. Ball clay is a 2:1 particle, with some smecite varieties classed as a 2:1:1 particle. Ball clay looks like a sponge magnified: the inner platelet capable of holding a little or a lot of water. Bentonite can hold up to 15X its weight in water. Typical ball clays can hold 1/4th to 2 times it's weight in water.
The chemistry is one thing; throwing either on a wheel is another. Kaolin only holds water on its particle surface: which is why it dries so much faster. In addition, this water film acts as a lubricant between particles: making porcelain soft and sensitive to pressure when throwing. Stoneware clays ( except fire clay) absorbs a fair amount of water creating density. Because moisture is absorbed into the platelets: the water film between particles is appreciably less. From this single difference in particle structure: stoneware takes longer to dry and has a much higher mechanical strength when throwing.
So my final conclusion: NO, stoneware is not dirty porcelain. There are other unique chemistry distinctions, but I will let them be. Maybe in another two years I will revisit this thread.