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Two awesome low fire transparent highly fritted glazes for pottery

Melt fluidity comparison of two clear glazes

These work well from cone 04 up, they are better than any commercial transparent we have used. And far better than glazes made using raw sources of boron (like ulexite, colemanite, Gerstley Borate). These glazes have lower thermal expansion and do not craze on any body we have tried (yet are ultra gloss and ultra clear). They are G1916QL1 and G3879C. We developed them for use on the dolomite-based (rather than talc-based) L4410L low temperature art clay body. These are a good demonstration of the technical and economic sense it makes to use highly fritted glazes at low temperatures. Having a good base glaze is the key to adopting low temperatures for your production. You would likely agree that no stoneware glaze has melting patterns like these shown in this melt fluidity test! These recipes and all details about their development and adjustment are openly available.

Context: Transparent Glazes

Thursday 16th September 2021

How do you decide what temperature to fire this terra cotta at?

Test bars of a terra cotta clay fired from cone 06-8

Let's suppose you need strength and density for utilitarian ware.This is a terra cotta body, L4170B, it has a much wider firing range than most terra cottas. Still, it is not like white-burning bodies, its "practical firing window" is much narrower than what looking at these fired bars and graph suggest. On paper, cone 5 hits zero porosity. And, in-hand, the bar feels like a porcelain. But ware will warp during firing and transparent glazes will be completely clouded with bubbles (when pieces are glazed inside and out). What about cone 3? Its numbers put it in stoneware territory, water tight. But decomposition gases still bubble glazes! Cone 2? Much better, it has below 4% porosity (any fitted glaze will make it water-tight), below 6% fired shrinkage, still very strong. But there are still issues: Accidental overfiring drastically darkens the color. Low fire commercial glazes may not work at cone 2. How about cone 02? This is a sweet-spot. This body has only 6% porosity (compared to the 11% of cone 04). Most low fire cone 06-04 glazes are still fine at cone 02. And glaze bubble-clouding is minimal. What if you must fire this at cone 04? Pieces will be "sponges" with 11% porosity, shrinking only 2% (for low density, poor strength). There is another advantage of firing as high as possible: Glazes and engobes bond better. As an example of a low fire transparent base that works fine on this up to cone 2: G1916Q.

Context: Formulating Your Own Clay Body, Fitting an engobe: It is about the data!, Should you expect to vitrify terra cotta?, Terra cotta

Thursday 16th September 2021

What did I do to make this glaze stop crawling?

This opacified cone 6 glaze was crawling on this stoneware casting body. I thought it was because of

This is G2934Y white (with 10% Zircopax). I was blaming the zircon for the crawling. But, since the slurry had settled somewhat I was able to remove about 15% of the water and replace it with CMC gum solution. This gum addition was not enough to slow down the drying much (a reason that I avoid gum if possible). That fixed it! Meaning that adherence of the dried layer to the bisque was the issue. This being said, there were still a couple of small spots where it crawled. So I will replace another 5% of the water the next time I use it.

Context: CMC Gum, Crawling

Thursday 16th September 2021

Are you using your expensive kiln like a pop-up toaster?

A small controller-equipment kiln and views of programming it

Put the pots in, select a cone, press start. It is time to rethink that approach! Seriously. The Bartlett Genesis kiln controller is standard equipment on hobby and production electric kilns now. It is not meant to be run like a toaster! Good glazes are about much more than recipes, they are about firing schedules. None of the built-in "toaster schedules" have hold times on any segments, drop-and-hold sequences or controlled cools. Or even fire-to-cone accuracy. Yet such are a must for defect-free glazes, enhancing the effects of reactive glazes that must develop crystallization or variegation or firing accurately. It is easy to program: Tap the blue edit button to edit a program, tap a column of any segment to edit its value. Tap a segment number to delete or duplicate it. Google "bartlett genesis controller" for videos on creating and editing a schedule.

Context: When the cone does this I need to adjust the program, Manually programming a Bartlett V6-CF hobby kiln controller, Bloating on a range of bodies at cone 6: Why is this happening?, Be thankful for the hobby kiln controllers we have in North America, Bartlett Genesis Kiln Controller Programming, Firing Schedule, Kiln Controller

Thursday 16th September 2021

Fire organic ceramic shapes with this warping body for cone 6

Two warped small square planter shapes

Left is L4410K, right is L4410L. Very similar, but L has 5% more dolomite. And these cone 6 glazes work well, G2934Y left and G2936A right. These are slip-cast pieces, the walls are not super thin, but the straight-sided shape makes them more susceptible to warping and buckling. This effect can also be achieved using talc bodies, but they have the issue of being brittle, volatile, bloating and not fitting glazes. But both of these pieces have excellent fired strength at cone 6, they have a porcelain-like surface. Of course you will need to test shapes, adjust dolomite content and control firing temperature carefully to be able to do this with consistency. If you are near a Plainsman Clays distributor, they made a test run of L4410L (as the new L213), boxes are available for order. If not, you can mix your own.

Context: Warping

Sunday 12th September 2021

One reason why frits are such a good source of boron

A melt flow comparison between two glazes

These two glazes have very similar chemistries (except that G2931G has a much lower SiO2 content, that is why it is melting better). Left: 0.75 molar boron is being sourced by a high quality frit in G3879C, Fusion Frit F-524. Right: Ulexite is sourcing the same 0.75 boron. Ulexite is a natural material (it is mostly ulexite, but does have other contaminants). Unlike the frit, it contains volatiles that produce an LOI on firing, these gases are generated during the melting phase, producing obvious bubbling.

Saturday 11th September 2021

Three rutile blue glazes at cone 6

Three mugs with floating blue glazes

These are GA6-C Alberta Slip floating blue, AMACO Potter's Choice PC-20 Blue Rutile, GR6-M Ravenscrag floating blue. The clay is M390. The firing is cone 6, the schedule is C6DHSC (drop-and-hold, slow cool). The inside is GA6-B. The two on the left develop the blue color because of the slow-cool, the one on the right works on fast-cool because it contains cobalt (although it will fire somewhat more mottled). The centre one is a bottled commercial product, it was painted on in three coats. The result is quite compelling, this is a good place to start if you want the rutile-blue effect. Remember, these work best on dark-burning bodies.

Context: Ceramic Rutile, Ravenscrag cone 6 floating blue thinner and thicker applications, Ravenscrag Cone 6 Floating Blue on porcelain and a red stoneware, Rutile Glaze

Saturday 11th September 2021

An Alberta Slip based black passed all the leaching tests

Four black-glazed test tiles

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.

Saturday 11th September 2021

What is the simplest, most practical raku base crackle recipe?

A glazed tile showing the raku crackle effect

Many people suffer high-percentage Gerstley Borate "bucket-of-jelly" raku recipes they find on line. Don't do this. There is a common Ferro frit that is perfect for this application, frit 3110 (Fusion frit F-75). All it takes is 15% kaolin (e.g. EPK) to produce and easy-to-use recipe that is guaranteed to craze. We have assigned it a code number of L4264, a raku base transparent recipe. We have also catalogued some common recipes that people use and outlined the issues they have: L4264A, L4264B, L4264C, L4264D. Do you need a white? It is a simple matter of adding 10% Zircopax to this.

Context: Raku

Saturday 11th September 2021

Are frits partially soluble? Yes, many are.

These 1 mm-sized crystals were found precipitated in a couple of gallons of glaze containing 85% Ferro Frit 3195 (we have seen these with frit 3249 also). They are cubical, hard and insoluble. Why and how to do they form? Many frits are slightly soluble, the degree to which they are is related to the length of time the glaze is in storage, the temperature, the electrolytes and solubles in the water, interactions with other material particles present and the diligence of the manufacturer in mixing, correctly achieving the target chemistry and firing. The solutes interact or saturate to form insoluble species that crystallize and precipitate out as you see here. These crystals can be a wide range of shapes and sizes and come from leaded and unleaded frits. In industry this issue is not generally a problem because glazes are used soon after being made.

Context: Ferro Frit 3195, A glaze slurry precipitates flakes, G2925B glaze can precipitate crystals like this over time, Precipitated crystals from a glaze having 60% lead bisilicate frit, Ooids in Glazes, Precipitation

Friday 10th September 2021

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