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CMC Gum does not work well in a glaze if an important ingredient is missing

Bentonite and CMC working together

On the left, the brush-strokes of gummed glaze, which I batched myself, have been freshly painted onto an already-fired glaze. Notice the brown brush stroke holds its character. It has a high specific gravity (SG): 1.6. And contains 1.5% CMC gum. The white one to its left, whose brush stroke has flattened and it is running downward, has the same gum content but an SG of 1.5. Is it running because of its lower SG? No. Commercial glazes with an SG below 1.3 still hold in place well. How? Because they also have a gelling agent (e.g. Veegum - it has an unfortunate name, it is not a gum). That reveals a secret: Gums and gelling agents need each other. CMC Gum needs particle surface area to work its magic and bentonite, the gelling agent here, supplies that. And, the gelling agent needs the gum to slow down drying and enable a hard and crack-free dried surface. The dried strokes on the right demonstrate that - 2% bentonite has been added to the drippy one on the far left. They held in place because of the bentonite and hardened without cracking because of the CMC gum.

Context: CMC Gum, Bentonite, Veegum, Brushing Glaze

Monday 15th July 2024

Cone 10 Reduction with Alberta and Ravenscrag Slips

Outside is the GR6-A recipe (made using 50:50 roasted and raw Alberta Slip), there are no other additions. The inside is pure Ravenscrag Slip GR10-A (also a 50:50 mix of roast:raw), with no additions. The clay is Plainsman H550. The firing schedule is C10RPL.

Friday 5th July 2024

Variegating effect of sprayed-on layer of titanium dioxide

Pure titanium as a varieagant

The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. However, on the outside the dried-on glaze was over-sprayed with a very thin layer of titanium and water (VeeGum can be used to help gel the sprayable titanium slurry and suspend it). The dramatic effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide. It is a more consistent source of TiO2 than the often-troublesome rutile. Another advantage is that the variegation can be selectively applied in specific areas or as a design. This effect should work on most glossy glazes having adequate melt fluidity.

Context: Titanium Dioxide, GA6-D, Reactive Glazes, Crystallization, Glaze Layering, Variegation

Thursday 4th July 2024

A mug cracks before your eyes because of glaze compression

The glaze on this highly vitreous, thin-walled mug is normally perfect, it is under enough thermal compression to really increase ware strength. But, since this mug is glazed inside only, the compression is too great. While it looks OK, the glaze is constantly pressing outward, looking for relief. Watch as a tap with a spoon is enough to trigger a sudden crack. And it opens under the pressure, clearly revealing the piece was spring loaded. This is a simple test. A typical mug of this clay would survive hundreds impacts of this nature. Further, this did not happen just because it was not glazed on the outside. A mug with glaze under compression on the inside and under tension on the outside would fail this test even more dramatically.

Context: Glaze Compression

Monday 1st July 2024

The same glaze fires very differently depending on kiln cooling rate

Both mugs use the same cone 6 oxidation high-iron (9%), high-boron, fluid melt glaze. Iron silicate crystals have completely invaded the surface of the one on the left, turning the gloss surface into a yellowy matte. Why? Multiple factors. This glaze does not contain enough iron to guarantee crystallization on cooling. When cooled quickly it fires the ultragloss near-black on the right. As cooling is slowed at some point the iron will begin to precipitate as small scattered golden crystals (sometimes called Teadust or Sparkles). As cooling slows further the number and size of these increases. Their maximum saturation is achieved on the discovery, usually by accident, of the likely narrow temperature range they form at (normally hundreds of degrees below the firing cone). Potters seek this type of glaze but industry avoids it because of difficulties with consistency.

Context: B2O3, Fe2O3, Ravenscrag floating blue color.., The matteness of this.., Alberta Slip GA6-A glaze.., Cooling rate drastically affects.., Same glaze fires hyper-matte.., The appearance of this.., Melt Fluidity, Glaze Chemistry, Crystallization

Monday 1st July 2024

Is inside-only glazing a good idea? We say it is not.

Glaze compression fractures porcelain mug

This mug is made from 325 mesh MNP, the strongest porcelain I have. Since the walls are of even thickness with no abrupt corners or contour changes and the glaze is thinly and evenly applied I thought I could follow a social-media-driven trend and glaze only on the inside. But I got glaze compression time-bombs waiting for hot coffee triggering! Three other mugs failed this same way! But four with this same glaze inside and out were fine. Why? The outside glaze counters the inside one pushing outward. And it closes crack initiation points.

I got lots of pushback on social media saying glaze compression problems are overblown. But I also got stories and pictures much worse than this (especially with thick and drippy glazes). But, some still feel that outside-only glazing can work by carefully tuning the thermal expansion fit between body and glaze. Or even by accident. Either way, there is still an elephant in the room: Glaze fit has to be just right - too much and pieces break, too little and the glaze crazes. That is a problem because it brings intolerance of even slight changes in body, glaze or firing.

Context: Drip glazing and bare.., Example of COE curves.., Glaze Compression

Thursday 27th June 2024

A down side of high feldspar glazes: Crazing!

A runny, amber, crazed glaze

This reduction celadon is crazing. Why? High feldspar. Feldspar supplies the oxides K2O and Na2O, they contribute the brilliant gloss and great color but the price is very high thermal expansion. Scores of recipes being traded online are high-feldspar, some more than 50%! There are ways to tolerate the high expansion of KNaO, but the vast majority are crazing on all but high quartz bodies. Crazing is a plague for potters. Ware strength suffers dramatically, pieces leak, the glaze can harbor bacteria and customers return pieces. The simplest fix is to transplant the color and opacity mechanism into a better transparent, one that fits your ware (in this glaze, for example, the mechanism is simply an iron addition). Fixing the recipe may also be practical. A 2:1 mix of silica:kaolin has the same Si:Al ratio as most glossy glazes, this glaze could possibly tolerate 10% of that. That would reduce running, improve fit and increase durability. Failing that, the next step is to substitute some of the high-expansion KNaO, the flux, for the low-expansion MgO, that requires doing some glaze chemistry.

Context: Feldspar, Na2O, ChatGPT is completely wrong.., Cone 1 mug is.., Celadon Glaze, Calculated Thermal Expansion, Limit Recipe, Glaze Crazing

Wednesday 26th June 2024

The Prusa MK4 3D printer is worth the upgrade from MK3

MK4 3D printer for ceramics

It is 2024 and I just got this. This time I paid extra and bought it fully assembled. It arrived in Canada quickly and was packed very well. I have already made hundreds of things on it, it is the reason for current projects (I use this to make molds for ceramics).

This upgrade was really worth it. The biggest benefit of the MK4 is improved bed levelling and prints stick much better. The second biggest benefit is precision. It can print molds too large for the bed in two parts and they mate perfectly. 1/10 mm difference in size is the difference between parts fitting precisely for being too lose or too tight. The MK4 is also capable of printing quite a bit faster. And it has a much better controller board and control panel. Filament loading is easier and the controller is smarter about fault conditions. Like the MK3 this arrived with fantastic documentation, I was up and running in no time.

I don't know how I would live without this amazing machine now! This is an example of a recent print. This was printed in two halves (the top half was done upside down), and they fit together incredibly well. The walls are only 0.8mm thick. Using 3D printed side rails I will be able to fill this with plaster to make a block mold (having embeds into which 3D printed natches will mount).

Context: It is 2 18.., Prusa 3D Printers made.., Prusa 3D printers at.., Beer Bottle Master Mold.., Medalta Ball Pitcher Slip.., Coffee Mug Slip Casting.., 3D Printer

Tuesday 25th June 2024

Here is some motivation to make your own underglazes

Underglazes melting to different degrees

Commercial hobby underglazes are high in stain and very expensive. But does expensive mean suitable? To help answer we have over-fired these commercial products in a melt fluidity tester (to cone 8). They are recommended for use from cone 06-6 (some can go higher e.g. the green). Underglazes need to melt enough to bond with the underlying body, but not so much that opacity is lost (any melting loses opacity). Excessive melt can also cause design edges to bleed. To work well at greater thicknesses, underglazes need to have a firing shrinkage similar to the body (an ill-fitted underglaze and body forced into marriage are eventually going to divorce, in the form of flaking or cracking at their interface). Thus, while a regular glaze would melt enough to go well down the runway on this tester, an underglaze should not flow at all. At this temperature, none of these have achieved the right degree of maturity (the green is too refractory, the others over-melt to varying degrees). The only one that has a chance of suitability at cone 6, two cones lower than this, is the blue. Clearly, the base recipe and stain percentage in each underglaze recipe color needs attention, if that can be achieved all of these would mature to the same degree.

Context: Is your underglaze forming.., When making your own.., Here is why you.., Underglaze, Silk screen printing, Bleeding colors

Monday 24th June 2024

Example of COE curves considered a good fit for body and glaze

COE charts for sanitaryware glaze, body

These are from a sanitaryware plant in India. Long-term glaze fit is essential for their products. The glaze thus needs to be under some compression. That means the body must have a slightly higher coefficient of thermal expansion (COE) than the glaze. These two charts were created on the same dilatometer by the same person using well-defined procedures (the glaze and clay each have their own procedures). A history of measurements and associated knowledge of how the data relates to the quality of the fired products provides a context to interpret these reports. In other words, technicians have learned that the difference shown here is what is required to achieve optimal glaze fit for this specific body/glaze combination. Of course, some sort of database system (e.g. lab notebook, an account at insight-live.com) is needed to record the history of testing to be able to effectively compare the past with the present.

Context: Is inside-only glazing a.., Co-efficient of Thermal Expansion.., Sanitary ware, Glaze Compression

Wednesday 19th June 2024

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What people have said about Digitalfire

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  • Tony, the Boraq II substition for gerstley borate in the formula I sent you a while ago was succesful. Congratulations for figuring out boraq! I've tested Murray's, Gillespie, Laguna, IMCO, and also raw materials such as Cadycal, Ulexite, Colemanite, and the substitutes you recommended in your articles made of several components blended with Cadycal (I referred to these in my previous letter) all with varying degrees of success but only the boraq II produced results that were indistinguishable from GB. This glaze is very sensitive so I consider the test a success. It had to mess with the plasticity with additions of hectorite and ball clay.
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