I have a profile drawing I want to match (upper left). On the left is my first attempt to tool a model. Even though I attempted to jigger the plaster to match, the profile is way off. I lost the enthusiasm to even get it smooth. For the one on the right I 3D-printed a shell, poured the plaster in ... more I have a profile drawing I want to match (upper left). On the left is my first attempt to tool a model. Even though I attempted to jigger the plaster to match, the profile is way off. I lost the enthusiasm to even get it smooth. For the one on the right I 3D-printed a shell, poured the plaster in and then smoothed off a bit on the wheel after the set. It matches the template and it is perfectly round (because I have a good 3D printer, a Prusa MK3S). This is revolutionary! That drawing: I hired someone on upwork.com to draw it for me using Fusion 360. He draws things in such a way that I can fine tune them.
Getting a 31 inch porcelain plate through drying and firing without cracks
What does it take? Three months! Porcelains are fine grained and, for heavy pieces, they will not tolerate uneven drying at any stage. These cone 10 plates are made by Peter Flanagan at Okanagan Pottery in Nelson, B.C. Firing is also a real challenge. Pottery porcelains are high in quartz, getting a ... more What does it take? Three months! Porcelains are fine grained and, for heavy pieces, they will not tolerate uneven drying at any stage. These cone 10 plates are made by Peter Flanagan at Okanagan Pottery in Nelson, B.C. Firing is also a real challenge. Pottery porcelains are high in quartz, getting a piece like this down through quartz inversion (~1200-900F) without dunting is only possible if done very slowly. The fact that ancient Chinese potters made very large porcelain pieces means they knew about slow cooling also (and it was a natural consequence of the heavy kilns they used). But our modern kilns cool quickly so the drop must be slowed. Peter adds an extra level of "super humanness" by actually lustre firing these pieces, that means 2 more trips through the hazardous quartz inversion territory! If you do this be prepared to fire super slow (e.g. 25 degrees per hour) through this range.
For even coverage white majolica glazes must be applied by dipping
The mug on the left has three coats of Spectrum Majolica base, painted on by brush. Drying was required after doing the inside coats, so the total glazing time was several hours. The glaze layer is way too thin and it is not even at all! The one on the right was dipped in a 5 gallon bucket-full of ... more The mug on the left has three coats of Spectrum Majolica base, painted on by brush. Drying was required after doing the inside coats, so the total glazing time was several hours. The glaze layer is way too thin and it is not even at all! The one on the right was dipped in a 5 gallon bucket-full of Arbuckle white (that was weighed out according to a recipe and slurried at 1.62 specific gravity). It took seconds to dip-apply, the thickness coverage are good. As is obvious, it makes sense to make your own base white. Then, you can decorate using the overglaze colors (e.g. the Spectrum Majolica series). Another advantage of making your own white is that you can splurge on the amount of opacifier (in this case 9% zircon and 4% tin oxide), to achieve maximum whiteness and opacity. And, you can proportion a mix of two frits (having higher and lower thermal expansion) to fine-tune the fit with the body (a big issue at low fire).
Look at how translucent this is! I can fire one of these in three hours, cold-to-cold. I am casting them with molds made using the 3D-printing process. Anyone could do this. Incredibly exciting. And with the G3879 glaze it looks awesome, just like bone china. Notwithstanding this, the Zero3 recipe ... more Look at how translucent this is! I can fire one of these in three hours, cold-to-cold. I am casting them with molds made using the 3D-printing process. Anyone could do this. Incredibly exciting. And with the G3879 glaze it looks awesome, just like bone china. Notwithstanding this, the Zero3 recipe has to be altered for casting. Initially I have reduced the VeeGum to 1% but it is still casting too slow. And it is not shrinking enough to pull away from the mold well. I am considering strategies on how to increase drying shrinkage and am going to add more frit to take it down to cone 04.
This mug has waterlogged because it is not vitrified
Notice the water has wicked up to about 1 cm from the rim (the piece sat in water overnight). The glaze fits so there are no cracks for the water to seep through. However, being fired at cone 04, the body is quite porous. The piece has a unglazed base. Notice the water even travelled up the handle. ... more Notice the water has wicked up to about 1 cm from the rim (the piece sat in water overnight). The glaze fits so there are no cracks for the water to seep through. However, being fired at cone 04, the body is quite porous. The piece has a unglazed base. Notice the water even travelled up the handle. Less exposed bare clay on the base would improve the situation somewhat, however it would be much better to choose a body that vitrifies sufficiently dense so that it does not absorb water (or fire to a higher temperature). There is a not-so-obvious issue here also: Although this piece did not explode in the microwave, it got incredibly hot. Amazingly, through all of this, the glaze has not crazed. It is G3879.
Two batches of EPK tested quite differently in 2018
The EP kaolin has been mixed 70:30 with nepheline syenite. This creates a body that matures below cone 6. And it enables comparing the degree to which the two kaolins influence vitrification (and their contribution to fired color in a vitreous body). The darker one is more plastic, has higher drying ... more The EP kaolin has been mixed 70:30 with nepheline syenite. This creates a body that matures below cone 6. And it enables comparing the degree to which the two kaolins influence vitrification (and their contribution to fired color in a vitreous body). The darker one is more plastic, has higher drying shrinkage more soluble salts. These dry test bars were fired at cone 4-8 and had similar fired shrinkages and porosities. However the one on the right fired whiter at all temperatures. These differences would impact the plasticity and drying shrinkage of bodies containing a significant percentage of the material. They would also influence the ability of this kaolin to suspend slurries.
By the magic of delflocculation, this powder will mix into that water and still fit in the container
This is 1100cc of water and 3000 grams of M370-2 casting. Amazingly, it is possible to get all that powder into that little bit of water. And still fit in the container (2250cc) and still produce a very fluid slurry for casting. How is this possible? That water has 11 grams of Darvan 7 deflocculant ... more This is 1100cc of water and 3000 grams of M370-2 casting. Amazingly, it is possible to get all that powder into that little bit of water. And still fit in the container (2250cc) and still produce a very fluid slurry for casting. How is this possible? That water has 11 grams of Darvan 7 deflocculant in it, it causes the clay particles to repel each other such that you can make a liquid with only little more water than is in a throwing clay! This is a test mix of M370-2 casting (it uses a large-particle kaolin), my pieces cast in 7 minutes (less than a third of the normal time). Using a good propeller mixer (in a bigger container of course) the slurry can be mixed silky smooth in a couple of minutes.
Does a grog addition improve thermal shock resistance?
Pyrax (Pyrophillite) is a mineral having a very low thermal expansion. It stands to reason that if we can maximize its percentage in a body and not fire the body to a point that changes the crystal structure, it will be resistant to thermal-shock-resistant cracking. To that end I mixed it with only ... more Pyrax (Pyrophillite) is a mineral having a very low thermal expansion. It stands to reason that if we can maximize its percentage in a body and not fire the body to a point that changes the crystal structure, it will be resistant to thermal-shock-resistant cracking. To that end I mixed it with only kaolin (ball clay would add some quartz that would increase thermal expansion) and made slip-cast pieces. I fired them to cone 2 (after finding that by cone 4 shock-resistant properties begin to decline). As you can see from the video, the addition of grog actually harms the performance! The higher the Pyrax, the better. Will this work for kiln shelves? Yes!
What does it take to opacify a low temperature terra cotta glaze
Tin oxide is a powerful opacifier, but the 5% in the glaze on the left is clearly not enough. 10% more zircon had to be added to produce the one on the right.
Frits vs. raw materials in glazes: It is not just about the chemistry
I have done alot of work recently comparing the sourcing of fluxing oxides from frits vs. raw materials. The differences are often incredible. The oxides MgO and CaO normally come from materials that melt high. But common frits that source them melt low. The chemistry in the two cone 6 glazes ... more I have done alot of work recently comparing the sourcing of fluxing oxides from frits vs. raw materials. The differences are often incredible. The oxides MgO and CaO normally come from materials that melt high. But common frits that source them melt low. The chemistry in the two cone 6 glazes (comparing on this melt flow tester) are the same. But Y4 sources the KNaO from Ferro Frit 3110 instead of feldspar and alot of the MgO from Ferro Frit 3249 instead of talc. Y4 would flow even more but also has 10% calcined alumina! Sourcing Al2O3 from such a refractory material should make it melt less than sourcing it from kaolin, but the frits overcome even that to create this amazing fluidity in a matte glaze. The lesson: All glazes have a chemistry, but that cannot be taken in isolation from the materials that source it.
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