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Technical Tips Blog
The Iron Red mechanism is working in one base but not the other
These two pieces were fired in the same kiln using the C6DHSC firing schedule. Fluid melts are an essential enabler of crystal growth during cooldown, both of these contain significant Li2O to help the B2O3 achieve that. Glaze #1, G3948A, has less iron than is typical and much more MgO/CaO. Glaze #2 has much more Na2O and it has both SrO and ZnO that #1 does not have. #2 is much higher in Al2O3 and has more than double the amount of SiO2. So which of all these factors is responsible for #2 having zero crystals? Very likely its low CaO/MgO levels. Fixing it does not appear practical because of the "major surgery" needed to exchange ZnO/SrO for MgO and the extra B2O3 needed to restore the melt. Since #1 has a low thermal expansion it should fit most bodies anyway.
Wednesday 3rd August 2022
Delflocculation is a "magic process", appreciate it
Casting slips require a minimum of water. Amazingly, it is possible to get this 3000g of M370 powder into only 1100g of water! And the fluid slurry produced, 2250cc, still fits in the container. How is this possible? That water has 11 grams of Darvan 7 deflocculant in it, it causes the clay particles to electrolytically repel each other! An awareness of “the magic” can help give you the determination to master deflocculation, the key enabler of the slip casting process. Determination? Yes, the process is fragile, must develop the ability to “discover” the right amount of Darvan for your clay mix and water supply. And the ability to recognize what is wrong with a slurry that is not working (too much or little water, too much or little deflocculant).
Saturday 30th July 2022
Drying shrinkage + firing shrinkage ≠ total shrinkage
Plainsman Clays, for example, publishes dry and fired shrinkage data for their clay bodies. The former is the shrinkage from wet-to-dry. The latter is the shrinkage from dry-to-fired. You cannot add the dry and fired numbers together to get the total because the two are based on different starting points. Consider this example: 6.25 dry shrinkage + 6.66 fired = 12.9 whereas the actual total shrinkage is 12.5%. Shown is the way to calculate the total shrinkage correctly if you only have drying and fired values (thanks to Tom Hittie for deriving this for us). Of course no one is going to bother actually doing this calculation! So just remember that the actual total is a little less than adding the two together.
Monday 25th July 2022
Liquify a pint of brushing glaze so well it does not need sieving
This photo shows a method we used to use for adding CMC gum solution to glazes to make them brushable, this made it possible to produce high specific gravity slurries. But we don't do it this way any more. The reason is that kitchen blender. It can mix a slurry containing gum powders quickly and thoroughly, even on low speed. There is another thing we also do now: Make low specific gravity glazes, they are more brushable. That can be accomplished by also adding Veegum (it also gels the slurry). Commercial brushing glazes are made this way, that is why they often require many coats to get adequate thickness. In this example of cone 6 black G3914A, we added 5g Veegum and 5g of CMC gum to 340g of ball milled glaze powder. After shaking it together in a plastic bag and adding it to 440g of water the blender turns it into a gelled slurry that is a delight to paint with. This process produced just the right amount to fill a 500ml jar.
Friday 22nd July 2022
Alberta Slip based black passed all four leaching tests
This is the G3914A recipe on Plainsman M340 test tiles. They were fired at cone 04 using the PLC6DS schedule. We tested them in four different caustic liquids (using the GLLE test), there is no sign of leaching on any of them. This recipe contains only 4% black stain, that is enough to stain the base GA6-B glaze to a jet black. The surface has a unique iridescence not found in any other glossy black we have used.
Context: Liquify a pint of brushing glaze so well it does not need sieving, Cone 6 oil-spot glaze effect, what works and does not work?, Two black cone 6 glazes recipes. One is much safer., Your supplier is out of stock on glossy black glaze? We made our own.
Sunday 17th July 2022
A practical dust collector you can make
An example of a custom-made dust collection hood in our repackaging and lab recipe mixing area. The slots along the front suck particles into the duct directly away from the operator's face. Suction comes from a centrifugal exhaust fan downstream where the pipe exits the building, it is driven by a 3/4hp motor (these fans are best at sucking, not blowing, so they need to be located at the exit). About 40 feet of 8 inch heating pipe connects from the hood to a fitting that expands to 12 inches going into the fan. The sliding damper above the hood enables stopping all airflow (to prevent heat loss during cold days). Notice it is located above the scale and heat sealer where most dust is generated during weighing and packaging.
Friday 15th July 2022
Gold decal on a gunmetal matte black glaze
This is a cone 6 porcelain mug and glaze. We get this satin matte effect in our test kilns using the PLC6DS schedule. Larger kilns cool slower so this glaze will turn out more matte, possibly too matte. When needed we deal with that by increasing the percentage of glossy base (this is a 15:85 blend of G2926B glossy and G2934 matte with 6% black stain). The decal is from Sanbao Studio. On the left, it has just been applied. But, other than the glossy finish revealing its location, no gold design is visible. But, after the decal firing, using the MDDCL schedule, we get the result on the right.
Context: Ceramic Decals
Saturday 9th July 2022
Transparent glaze quality on terra cotta is about two main things
Slurry preparation and application thickness. These two mugs are made from the same clay, Plainsman Snow, and Amaco V-303 Terra Cotta underglaze has been applied to each. The dipping glaze on the left was mixed from a recipe, slurried and then sieved at 80 mesh. However the sieve did not break up the agglomerates of Veegum and New Zealand kaolin. These are not visible on white bodies but destroy the glaze appearance over this red surface. This problem can be quickly resolved by mixing batches of the glaze in a kitchen blender at top speed. The glaze on the right is Spectrum 700, obviously it has been well mixed and sieved. The second issue is thickness: The mug on the left was dipped and held in too long, getting it on far too thick. The glaze was painted on to the one on the right, giving better control of thickness (three coats were applied). The problem with the left glaze could be resolved by adding CMC gum to slow uptake by the porous bisque.
Context: Terra Cotta
Tuesday 5th July 2022
Cast handle on thrown mug, pulled handle on cast mug
Which is safest and least likely to crack on drying? The slip cast on thrown. The overall drying shrinkage of the plastic throwing clay is the main issue, likely at least 2% more than the casting clay. A cast mug needs to be stiff enough for handling in order to apply a plastic handle, that will mean that it is well along in shrinkage, not much is left. The plastic handle, conversely, needs to be soft enough to still have flexibility. That means it still has a lot of shrinking to go. While this one has survived, it likely has built-in stresses and therefore is more easily broken off when bumped. The situation is more favourable when a cast handle is applied to a thrown mug. The mug is already stiff enough to handle, perhaps almost leather hard. The handle is removed from the mould as soon as possible because it needs to have a little flex, which means it likely has more water. These factors help to equalize the amount of remaining shrinkage in the two.
Tuesday 5th July 2022
Why this dolomite body bisque is splitting after sitting around
On the loss of talc earlier this year we had to reformulate a low fire white burning body to use dolomite instead, recipe L4410P (like talc it raises thermal expansion to ensure fit of commercial glazes). As its advantages and disadvantages become evident we have been documenting them on the Snow page. A recent revelation has been the matter of rehydration of the limestone (dolomite is ground limestone): Bisque firing dehydrates it. The dolomite particles are neutralized somewhat by being isolated and having reacted to some extent with neighbouring clay and feldspar particles. Further, during dehydration, they leave considerable porosity into which they should be able to reexpand later if needed. This photo demonstrates something we have not seen in our dry climate: These 3D-printed bisque pieces have spontaneously cracked after sitting around for some time in the much damper climate of southern Ontario. In some cases, swelling occurs around the cracks. Until we can further tune the recipe to chemically tie up the dolomite take some precautions when using this type of body. Glaze ware soon after bisquing. Dry it as quickly as possible after glazing. If any surface has not been glazed then render it impervious to water penetration by using a silicone sealer. Photo courtesy of Nilou Ghaemi, Sheridan College.
Friday 24th June 2022
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