|Monthly Tech-Tip |
This project is hopefully going to teach me something about how frits melt over ranges of temperatures, how they compare in melting, how that relates to their chemistries and which ones melt in a way that makes them better candidates as body frits, balanced glazes on their own or as a special-purpose additives to glazes.
I mixed 100g of each of the frits with 3.25g of Veegum to make them plastic. I shook the powders in a bag and then I stirred it into 35 grams of water in a cup. When they got to dry mud consistency I hand-kneaded them to a smooth consistency. I divided the batches into 12 gram pieces and rolled each into a ball for drying. That produced dried balls about 9 grams each.
In the tests done so far (up until early January 2020) it is evident that most fruits have completely done their thing before the kiln even reaches bisque temperature.
I am going to fire them at about 10 temperatures starting at cone 022. These will be a great to determine whether they suddenly melt or slowly soften over a range of temperatures. I will be able to see how wide that range is and how they behave as they go beyond it. I also included Gerstley Borate, mother nature's nature boron source. They way it melts should clear contrast with the frits.
Nothing compares the fired properties of frits as well as a melt-flow comparison. Remarkably, each frit has such a unique melting behaviour that it is possible to identify them and determine if two are similar (without knowing the chemistry). But frits need a binder to make them plastic-formable. I mix 50g of frit with 1.5g of VeeGum (or 100g/3g) then stir the powder into 40g of water in a cup. That produces a consistency that is easy to stir smooth (using a teaspoon) but is still fluid enough to level out on a plaster bat (for dewatering to plastic consistency). The 3% VeeGum is not enough to affect the melting behaviour, but it makes them pretty workable (however each frit does exhibit different tactile characteristics). I roll them into 12g (wet) balls, then dry them. They fit nicely into the reservoir for my GBMF test.
Fired at 350F/hr to 1300F and held for 15 minutes. Some are still burning off carbon (which seems strange). There are two early leaders: Ferro frit 3110 and Fusion frit F75 are starting to deform (they have almost the same chemistry). Amazingly, these two frits have low boron, they rely on high soda as the flux.
Fired at 350F/hr to 1350F and held for 15 minutes. Some are still burning off carbon (which seems strange). The two FZ16s are starting to move. Frit 3134 is expanding. 3602 is also starting to melt.
Fired at 350F/hr to 1450F and held for 15 minutes. Frit 3134 is still expanding. 3602 is blasting out of the gate, taking the lead. F75 is starting to flow.
Fired at 350F/hr to 1500F and held for 15 minutes. Frit 3134 is still expanding. 3602 and FZ16 are really starting to move. 3195, F38 and F15 are softening.
Fired at 350F/hr to 1550F and held for 15 minutes. Frit 3134 is still expanding. 3602 and FZ16 are going to be off-ramp by next firing.
Fired at 350F/hr to 1600F and held for 15 minutes. Frit 3134 is still expanding. 3602 and FZ16 are off-ramp. F-75 is starting to flow. Gerstley Borate has suddenly shrunk. Frit 3110 is demonstrating its wide range, slowly softening more at each temperature.
Fired at 350F/hr to 1650F and held for 15 minutes. FZ16 has turned crystal clear and spread out across the runway (has low surface tension). Frit 3110 has so much surface tension that the flow can be lifted off the tester. Since 1600F Gerstley Borate has gone from unmelted to passing all the rest!
Fired at 350F/hr to 1700F and held for 15 minutes. 3110 is finally starting to move. 3134 also (being full of bubbles). Gerstley Borate has turned almost transparent (because the Colemanite portion of it is now melting). 3195 is looking very well behaved compared to most others, forming a bubble free glass of high surface tension (F15 and F524 are starting to do the same).
Fired at 350F/hr to 1800F and held for 15 minutes (I already did firings from 1300F-1750F in 50 degree increments, all of them are visible in the parent project). Frit 3110, 3134, 3195, F75 have run all the way down. All of the frits have softened and melted slowly over a range of temperatures (hundreds of degrees). By contrast, Gerstley Borate, the only raw material here, suddenly melted and flowed right over the cliff (between 1600 and1650)! But not before Frit 3602 and FZ16 had done so earlier. Frit 3249 is just starting to soften but F69 (the Fusion Frits equivalent) is a little ahead of it. LA300 and Frit 3124 are starting also. F524, F38, F15 will all be over the end by the next firing. The melt surface tension is evident by the way in which the melts spread out or hold together.
These melt flow tests were fired at 350F/hr to 1850F and held for 15 minutes (I did firings at 50-degree increments across a wide range). It is amazing how active some frits are, even well below normal bisque temperatures! Frit 3110, Frit 3134, Frit 3195, Frit F-75 have all flowed all the way down for many previous temperatures. LA300 and Frit 3124 were just starting at 1800F, look at them now! Frit F-524 and Frit F-38 have gone from half-way at 1800F to water-falling over the end. Frit 3249 is still not out-of-the-gate but Frit F-69 (the Fusion Frits equivalent of 3249) is half-way. Note how the melt surface tension is evident by the way in which the melts spread out or hold together. By contrast, Gerstley Borate (labelled "GB"), the only raw material here, suddenly melted and flowed right over-the-cliff between 1600 and 1650! The best melter of all of them is high-boron high-zinc Frit FZ-16.
This melt flow tester demonstrates the beautiful crystal-clear glass this zinc frit creates by 1700F. It fits this porcelain without crazing, even though very thick and high in sodium (the high zinc and boron are countering it to keep the thermal expansion down). It runs off the end of the runway around 1600F on this GLFL test, rivaling lead bisilicate. This is a more concentrated boron source than even Gerstley Borate. Everything about this material screams “ultra gloss”, what a material to build a fluid-melt reactive super-glaze on!
This demonstrates the amazing melt behaviour of lead-as-a-flux for ceramic glazes. Not only does it melt early, but it softens slowly over a 300F range of temperatures before it goes off the end of the runway on this GLFL test. Then, when fired 200F hotter than that, it remains a stable, clear and uncrazed glass. Beginning around 1750F, this becomes a transparent glaze, by itself.
Frits are used in ceramic glazes for a wide range of reasons. They are man-made glass powders of controlled chemistry with many advantages over raw materials.
Frits are made by melting mixes of raw materials, quenching the melt in water, grinding the pebbles into a powder. Frits have chemistries raw materials cannot.
|By Tony Hansen|
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