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Monthly Tech-Tip from Tony Hansen

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Choose the right transparent glaze to cover your underglaze decoration

The background plate was decorated using Amaco Velvets and overglazed with Amaco Glaze HF-9 Zinc-Free clear. The front one was overglazed using Amaco Celadon clear. The hazing of the latter is most evident in the center area where it has been applied in a thicker layer. However this made no difference when using the HF-9 transparent. For complex designs like this it is often better to paint on the clear rather than dip, since the highly gummed Velvet underglazes impede the absorbency of the underlying body, and thus its ability to build up a layer during dip.

Context: Underglaze

Friday 30th July 2021

Fusion 360 drawing for glaze melt flow tester is available

3D rendering of Fusion 360 drawing of glaze melt fluidity tester

We have promoted this device for many years as a way to compare glaze melt fluidity, surface tension, bubble retention, crystal growth, transparency, melting range, etc. Download the Fusion 360 file using the link below.

Context: A Low Cost Tester of Glaze Melt Fluidity, Melt flow test 5v1 Fusion 360 3D file, Melt Flow Tester 3D Scan STL file

Thursday 29th July 2021

Lindsay Montgomery Majolica Plate: Lake of Fire (White Women Elected Trump), 2017

Made from Tucker PHB (tested on our code L4183). Glaze is G3890 Arbuckle majolica recipe. Imagery done using Spectrum Majolica colors (the white tested under our code G3883). Glaze firing temperature is cone 05 (bisque cone 04). "I see medieval symbols as powerful images that we can use to talk about environmental activism today, women's rights today, just all kinds of political ideas about speaking truth to power," she says.

Context: For even coverage white majolica glazes must be applied by dipping, Majolica

Thursday 22nd July 2021

Match the firing shrinkage of an engobe to a terra cotta

Test bars of a black engobe and their data plotted

These lines plot the firing shrinkages for three versions of L3685Z2 engobe. Notice the terra cotta body I want to match (red line) and the black engobe (green line) do not cross anywhere. That means there is no temperature at which they fit each other (the engobe always has 2% or more firing shrinkage). Notice the L4170B terra cotta fits the white version, Z2, at 2150F (red line crosses blue line, but the body is over-fired by that point). For a fit at my preferred 2000F (cone 02) I need the Z4 engobe to shrink 2% more (a 3% addition of frit 3110 will do that). What about the black Z4? That is the opposite situation, it already contains 5% frit, removing that will drop that green line about 3%, hitting the red line at 2000F (and following it all the way down past 1950 into the cone 04 range). I ignored all of this and used the Z2 white on L4170B, L210 and L215. It looked good on most pieces, but sure enough, it did crack around abrupt contours on some. Of course, this does not assume a thermal-expansion-match of body and engobe.

Context: Can engobes be applied to bisque?, Terra cotta

Wednesday 21st July 2021

Can engobes be applied to bisque?

Engobes can be incredibly opaque. This very thin layer of L3685Z2 completely covers these terra cottas (L210 and L215). Its color is whiter than paper! Using our G1916Q and G3879 clear overglazes, ware can appear as white as porcelain! But notice there are tiny cracks in the white on the edges of contours (most noticeable on the left sample). CMC gum was added to this engobe, enabling applying it to the bisque. It appeared to work well, but during firing the engobe shrank 2%, putting it under tension (the body had already shrunk during its bisque). If it were applied to the leather hard ware that would not fix the problem. Why? Because the engobe has a 2% lower firing shrinkage than these bodies. That would put it under compression, looking for opportunities to flake off at edges (e.g. rims of mugs). How to fix this? The engobe needs about 3% Ferro Frit 3110 to raise its firing shrinkage by 2%. And, to only be applied to leather hard ware.

Context: Match the firing shrinkage of an engobe to a terra cotta

Wednesday 21st July 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. This is M340S, it is fired at cone 6. That temperature is a "sweet spot" for this effect, high enough for the particles to bleed and low enough they do not bloat the body. Such bodies contain only about 0.2% of 60-80 mesh granular manganese (compare this to many glazes that employ 5% powdered manganese as a colorant). Further, the vast majority of the manganese particles are encapsulated within the clay matrix. The tiny percentage exposed at the body surface are under the glaze. It is not the manganese particles themselves that expose at the glaze surface. Rather particle surfaces that contact the underside of the glaze bleed out into it from below, doing so as a function the glaze thickness and melt fluidity. Thus, food contact with a glass surface having isolated manganese-pigmented regions is not at all the same thing as with raw manganese metal. Consider also that the total area of manganese-stained glass on a functional surface is extremely small for this effect.

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

Monday 19th July 2021

Never do ceramic body or glaze testing without code numbers

Two code-number ceramic mugs

That is how you know what it is. The recipe. The firing schedule. The pictures. The notes. The project it was in, information about what came before it and what developed from it. These mugs and that test bar are the same clay, I am doing a preliminary tests on a new material from our quarry, it is called "Battle Clay". This code number identifies all my records regarding this test mix in my account. In future I can identify the specimens from my records and my records from the code-numbered specimens.

Context: Is this your record keeping system?, How many simultaneous testing projects can you manage at once?, Digitalfire Insight-Live, Insight-Live Help for the Recipe Manager

Monday 19th July 2021

Metal leaching from ceramic glazes: Lab report example

Metal leaching from ceramic glazes: Lab report example

This lab is certified by the US Department of Environmental Protection (DEP) for drinking water and waste water analysis. They also provide pottery glaze leaching analyses (an acid solution is kept in contact with the glaze then analysed for trace levels of specific metals). Each suspected metal to be tested for entails a separate charge ($30-60 in this case, could be less for you). That means that testing one glaze for several metals could cost $200. How to make sense of these numbers? Google the term: "heavy metals drinking water standards", and click "Images" to find charts with lots of data. Searching pages for this term will find books having detailed sections on each of the metals. Typically you are only interested on one metal in a specific glaze (often cobalt or manganese). There are ways to sleep better (about the likelihood your glazes are leaching metals) if you cannot do this: Do a simple GLLE test. And avoid the online trafficking in hazardous recipes. Better to find a quality base glaze (matte and transparent) that works well on your clay body. Then add colorants, opacifiers and variegators; but doing so in a conservative manner.

Context: Are Your Glazes Food Safe or are They Leachable?, Attack on Glass: Corrosion Attack Mechanisms, Having Your Glaze Tested for Toxic Metal Release, Glaze Leaching Test, Concentrate on One Good Glaze, Want bright orange? Use a stain in your own base transparent recipe., Trafficked online recipes waiting for a victim to try them!, USA DEP water testing info page, Chemical contaminants in drinking water, Brandywine Scientific Lab Website, Liner Glaze, Leaching

Saturday 17th July 2021

Now that is a translucent porcelain! But much more.

Polar Ice porcelain mug with a light inside to demonstrate its translucency

These are two cone 6 transparent-glazed porcelain mugs. On the left is the porcelainous Plainsman M370 (Laguna B-Mix 6 would have similar opacity), it is semi-vitreous and has no translucency. Right is a highly vitreous, New Zealand kaolin based porcelain, Polar Ice. The secret to making this porcelain super-white is the NZ kaolin. The secret of its impossibly-high plasticity is the very expensive plasticizer, VeeGum T. What about the translucency? Nepheline syenite is used as the feldspar, but it alone cannot deliver this kind of translucency at cone 6. Amazingly the 4% Veegum acts as a translucency catalyst, it is the secret. Commercial manufacturers could never use a sticky and difficult-to-dry porcelain like this, but a potter can do incredible things with it (e.g. throw thinner, lighter, bigger than any other clay he/she has ever used!).

Context: Bone Ash, Formulating a Porcelain, New Zealand Halloysite, Plainsman Polar Ice cone 6 translucent porcelain, Bone China, A 3-minute Mug with Plainsman Polar Ice

Saturday 17th July 2021

How many degrees between these cone positions?

Two orton cones, one bent to 6 oclock, the other 4 oclock.

I was consistently getting the cone on the left when using a custom-programmed firing schedule to 2204F (for cone 6 with ten minute hold). However Orton recommends that the tip of the self supporting cone should be even with the top of the base (they consider the indicating part of the cone to be the part above the base). So I adjusted the program to finish at 2200F and got the cone on the right. But note: This applies to that kiln at that point in time (with that pyrometer and that firing schedule). Our other test kiln bends the cone to 5 o'clock at 2195F. Since kiln controllers fire cone 6 at 2230 (for the built-in one-button firings) your kiln is almost certainly over firing!

Context: Bloating on a range of bodies at cone 6: Why is this happening?, Cone, Firing Schedule, Plainsman Cone 10R Firing, Plainsman Cone 6 Electric Standard, Cone 03 Fast Fire, Low Temperature Drop-and-Hold, Cone 6 Drop-and-Soak Firing Schedule, Plainsman Cone 6 Slow Cool (Reactive glazes), Plainsman Typical Cone 04

Saturday 17th July 2021

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What people have said about Digitalfire

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  • 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!
  • Tony Hanson is like the person that is always waiting to answer my questions. He is very thoughtful and quick with response. I wish he was my neighbor.
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What people have said about Insight-Live

  • I work at a small, but rapidly growing custom slip-cast studio. The first few years were a learning curve gauntlet that, in retrospect, I am surprised we exited intact. This preface is to give a sense of how in debt I am to your research and hard work. I have had to learn as I go along, and the information you provide at has been indispensable, and, quite frankly, an excellent cover for my ignorance on countless occasions! It is also the only source of information about many ceramics topics that I feel I can trust, knowing that it has been backed up with careful consideration and testing. Thank you again.
  • Thanks Tony. You are offering a very valuable service to the potters.
  • Tony, you're a gentleman and a scholar. Thanks for the information. I'm a self-taught potter living in Hansville, WA. could not have done it without your scientific approach and papers.
  • Your site is great! It is so good to have your site as I venture into making my food-safe glazes. I live in São Paulo, Brazil, so the raw materials here naturally can be very different from the ones in most of the books and websites, as we don't have much of that available here. So your site is the only that I have found so far that is really helpful if you want to start from scratch, so thank you! Its like my dream as a potter to make beautiful, lasting and safe ware so your website and insight-live are invaluable sources of information for me. I am so glad to be able to work with this! I really admire your work and tell my students about your website. Here in Brazil, studio ceramics is not a very developed area. Many people don not even know/care about being precise and knowledgable about safety with glazes. So your website is like an oasis in the tropical desert.
  • Thanks again for your website. It has helped me no end.
  • I read your articles for years, and it really helped me to better understand ceramics.
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