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Monthly Tech-Tip from Tony Hansen

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The difference between Silica 90 and Silica 45 will affect the glaze melt

Quartz particles have a high melting point, they must enter the glaze melt by being dissolved by it (usually the last particles to do so). Obviously the silica should be as fine as possible to increase its surface area to be more readily dissolved. The more that dissolves the closer the physical properties of the fired glaze will be its theoretical (e.g. degree of melting, thermal expansion, transparency, durability). This brand of silica, #90 (likely 45 microns) classifies as 200 mesh even though 2.8% remains on the 200 mesh screen. Not surprisingly, their #45 grade retains 1.9% on the 325 mesh screen. However, the most significant aspect is how much of the #90 is on the 325 and 270 mesh screens: 26%. The #45 grade only has 2.6! This is a huge difference and shows the value of using the finer material. It would take a typical ball mill hours to make this difference.

Context: Silica, Melting Temperature, 200 mesh

Thursday 1st April 2021

Substitute Ferro Frit 3134 in three glaze types - using glaze chemistry

Insight-live screen shot, substituting frit 3134

Can't get frit 3134 in early 2021? You are not alone. Don't listen to people who say you can just replace frit 3134 with 3124 in glaze recipes. That is wrong. Frit 3124 has five times the amount of Al2O3 (the second most important oxide in glazes) and half the amount of boron (the main melter). The glaze chemistry approach is much better, and easier than you think. To be able to do it you need two other Ferro frits, 3110 (or Fusion F-75) and 3195 (Fusion F-2). As it turns out, Frit 3195 is more important than is 3124! A key goal in the way I do this was to end up with at least 15% kaolin (to suspend the slurry). I have chosen three types of recipes to demonstrate. Dealing with each requires a unique approach. Two of the calculations produce improved slurry properties and one a recipe of significantly lower cost. I made a video demonstrating substituting Frit 3134, see the link below. If you have a recipe that needs this, get an account, enter it there and I can help you do the calculation.

Context: Ferro Frit 3134, Ferro Frit 3195, Ferro Frit 3124, Is Ferro Frit 3124 a viable substitute for Frit 3134, Do you know the purpose of these common Ferro frits?, Substitute Ferro Frit 3134 For Another Frit

Monday 29th March 2021

Laminations: Will a pugmill solve the problem?

This company was plagued with drying cracks in their solid porcelain pieces. After some time they discovered that the deaired plastic material received from their suppliers had laminations (revealed in a cross section cut of the slug). Since they were not wedging, but simply inserting the clay into their hand extruders and presses, these laminations produced built-in weaknesses, the stresses of drying later exploited these. The obvious fix seemed to be to buy a vacuum pugmill to remix the clay. But that did not work. Why? Commercial pugmills commonly have multiple shafts, hundreds of blades, large powerful motors, separate mixing and vacuum chambers, shredders, high-compression heads, etc. Small studio pugmills have none of these features. They are still great for recycling and mixing clay that will later be wedged. But for the machine-forming purposes of this company, this pugmill actually made the laminations worse!

Context: Laminations, De-Airing Pugmill

Tuesday 23rd March 2021

Are manganese speckled clay bodies a toxicity hazard?

A mug made of manganese speckled clay

Before jumping to conclusions consider all the factors that relate. Such bodies contain only about 0.2% of 60-80 mesh manganese granular compared to many glazes that contain 5% powdered manganese (as a colorant). The vast majority of the metal particles are encapsulated within the clay matrix. The tiny percentage exposed at the body surface are under the glaze. What bleeds up through the glaze, to either near or at the surface, is not the particle itself. Rather, the particles dissolve partially and bleed out in all directions. The glaze stains in relation to the MnO concentration and its own melt fluidity. The transparency and thickness of the glaze either amplify or subdue the appearance of the specks. Thus, a food surface stained by manganese is not at all the same thing as raw melted manganese in contact with food. Consider also that total area of manganese-stained glass on a functional surface is extremely small. Notwithstanding all of this, there is a public perception of glaze toxicity that must be considered, some potters thus use engobes under the glaze on food surfaces, these limit the ability of the metal particles to diffuse.

Context: G2934Y glaze on Standard #112 body at cone 6

Tuesday 23rd March 2021

Crawling in G2934Y zircon white glaze: There are simple fixes

A matte glazed G2934Y mug where the glaze has crawled on the upper handle join

G2934Y is a fabulous base glaze but it is not without issues. It has significant clay content in the recipe and high levels of Al2O3 in the chemistry, these make it susceptible to crawling. While it is normally fine as is, when you add certain stains to color it (especially at significant percentages) or opacify it using zircon (this has 10%), it can become more susceptible to crawling. On this mug, the glaze layer thickens at the recess of the handle join, that produces crawling during firing. Crawling can also happen on the insides of mugs, where wall and foot meet at a sharp angle. This happens, both because the glaze cracked here during drying and because the zircon stiffens the melt, making it less mobile. Rounding such contours will help. Even better, adjust the glaze recipe so it shrinks a little less on drying (by trading 5% of the raw kaolin for calcined). Adding a little CMC gum (e.g. 0.1-0.2%) will make it adhere better.

Context: G2934Y, CMC Gum, G2934Y variations for fired hardness, COE adjustment, less crawling, etc

Saturday 20th March 2021

Blisters in a highly melt-fluid cone 6 sculpture glaze

A copper sculpture glaze is blistering

Why are these happening (on this piece by Paul Briggs)? It is not completely clear. The glaze has plenty of carbonates, including copper, enough for over 20% LOI. But these normally produce high populations of small blisters, this is the opposite. The melt appears to have enough surface tension that the bubbles survive and endure top-temperature-soaking. And they don't pop until the temperature has dropped so far that insufficient melt-mobility remains to heal them. The glaze has an unconventionally low SiO2 content, that makes it flow vigorously, well enough that the melt is moving and collecting in surface contours. The glaze recipe is quite unconventional, any effort to "improve" its adherence to limits would likely lose the visual aesthetic. A drop-and-hold firing schedule is likely the key to alleviating this.

Context: Blisters, Melt Fluidity, Cone 6 Drop-and-Soak Firing Schedule

Friday 19th March 2021

Cone 6 oil-spot glaze effect, what works and does not work?

Cone 6 glazed tiles of white-on-black oil spot glazes

Simulating a white-on-black oil-spot effect at cone 6 oxidation proved to be a matter of repeated testing (that got me past some misconceptions). Stopping to think about the results at each step and keeping a good audit trail with pictures, in my account at, really helped. I had three black glazes: G2934BL satin (G2934 with black stain), G2926BB super-gloss (G2926B with black stain) and G3914A Alberta Slip black. Going on a hunch, I mixed up a bucket of the G3914A first (with some gum to help it survive second-coating without lifting). Rather than just try any white, I created G3912A by substituting as much CaO and MgO as possible for SrO in the G2934Y base. I later learned this to be an error, SrO reduces the surface tension, I should have used MgO (the G2934Y is a high-MgO glaze so it would have been fine as-is)! As you can see on the far right, this white still worked (at cone 5, 6, 7, 8). Why? There is another factor even more important. The effect only works on the Alberta Slip black. But its LOI is not higher than the others. And it worked even after ball milling. So I need to continue to work on this to learn more about why this works.

Context: Creating a Cone 6 Oil-Spot Overglaze Effect, Oil-spot glaze

Friday 19th March 2021

Casting plates, is it practical?

A leather hard plate cracked in the plastic mold

No. Because you will face a whole array of problems. This is the first, poor mold release, or more correct, impossible mold release! Plates will be too thin walled. If you cast them longer wall thickness will be uneven. Edges will crack like this (because of poor plasticity). They will warp during drying. They will lack dry strength for handling. They will warp during firing. You won't be able to get a good rim. You won't be able to cast a foot ring without an indent showing on the inside. Note here that another issue is at play: The clay is either not plastic enough to cut cleanly at the rim, without tearing. Or, it is being cut too late or with a dull knife. These tears provide places for cracks to initiate. Plates are much better made using the jiggering, ram pressing or dust pressing processes. Or by throwing them on plaster batts.

Context: Jiggering, Casting-Jiggering, Slip Casting

Tuesday 16th March 2021

Drip glazing and bare outsides: Deceptively difficult.

thick glaze drips on ceramic vessels

Why? Glaze fit. These are available on Aliexpress (as Drip Pottery) and they are made by a manufacturer that has close control of body maturity (and thus strength) and a dilatometer to precisely match the thermal expansion of the glaze. The glaze has to fit better than normal because of the absence of an outside glaze. Too low an expansion and it's compression (outward pressure) will fracture body (because these are thin-walled pieces). Too high and it will craze. And that thick glaze? It will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. These pieces are a visual and technical achievement. If you are a potter you had best think twice before attempting the same.

Context: Glaze at 1.7 specific gravity on green-ware. Way too thick!, Why are these vessels cracking when hot water is poured in?, Deflocculation, Glaze fit, Glaze Compression, Glaze thickness

Monday 15th March 2021

How to make an incredibly white engobe for terra cotta

Two terra cotta slip-cast vessels with white inner surface

I found seven secrets with recipe, process, glaze and firing. 1. A lot of Zircopax, in this case 20% (for whiteness, opacity). 2. The whitest burning materials: New Zealand Halloysite as the kaolin and nepheline syenite as the feldspar. 3. 3% Veegum to gel the slurry (enabling low specific gravity for thin and even coating). 4. The recipe, L3685Z2, has 55% kaolin, that will certainly produce drying cracks. But 1% CMC gum stops that and makes it brushable. It even works on on bisque, I pour-applied it to the insides of these two slip-cast pieces, it drained to perfectly even coverage (in a very thin layer). 5. A terra cotta casting/throwing body to fit the engobe to (has the same fired shrinkage at a target temperature, e.g. cone 04): Initially I am using L4170B. 6. A clear glaze that fits and is transparent: Notice how much whiter the left one is, G3879. At the same thickness as the G1916Q on the right, it is more transparent, better transmitting the white of the engobe. 7. The right firing curve: The 04DSDH drop-and-hold schedule for defect free surfaces.

Context: Fitting an engobe: It is about the data!, Terra cotta casting slip, engobe and glaze recipes for cone 04

Thursday 11th March 2021

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What people have said about Digitalfire

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  • However, I must add that it would be well worth it--the Magic of Fire is one of the best-written books I've read! I must admit, it's the first technical manual to keep me up 'til 3 a.m. because I just couldn't put it down... In fact, it has inspired a bit of a revolution in my small corner of the world... you see, I am one of those dreaded hobby ceramicists (well, actually, I cast cute kittens and ducks and what-have-yous for a ceramics store in Dallas, Tx), and I often have customers ask advice regarding firing, pouring molds, etc., and since I've been reading your book, I've been able to provide MUCH better answers... and I've often referred them to the book directly when they seem so inclined. I am beginning to have a hope that more hobby amateurs will take an interest in the whys and wherefores instead of the hit-or-miss-and-hope-like-hell method so prevalent in this side of the ceramics industry.
  • Your site is one of the most unusual sites I have encountered since I began exploring ceramics on the web. I am a student in a 2 year pottery program, and would like nothing better than to understand glazing from the very beginning of my career as a potter...It is pretty overwhelming. To tell you the truth, you almost come off as a Southern Baptist Revival Preacher the way you rant and rave against the "Dragon." It is what got my attention, however, and I appreciate the quality of your work, but it is very overwhelming.
  • You have such a goldmine of information available on your Digitalfire website and I am asking your permission to, not only use some of it, but to direct the students to your website for more info than I could ever convey.
  • When my group of potters are pressed to improve our glazes they say if the old way is not broken yet why don't you buy yourself some kind of a kit to play with new glazes and then we can make it for everyone. You got to be kidding I say to myself. Anyway, your website will help me help the group out of some old and boring glazes if I can see forward enough. Thanks for all this important info, I can't believe this website is here!

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