Click here for information about DIGITALFIRE Corporation

The multitude of things iron oxide can do in reduction

Glazes range from celadon to heavy iron crystal surface

Iron oxide is an amazing glaze addition in reduction. Here, I have added it to the G1947U transparent base. It produces green celadons at low percentages. Still transparent where thin, 5% is producing an amber glass (and the iron is showing its fluxing power). 7% brings opacity and tiny crystals are developing. By 9% color is black where thick, at 11% where thin or thick - this is “tenmoku territory”. 13% has moved it to an iron crystal (what some would call Tenmoku Gold), 17% is almost metallic. Past that, iron crystals are growing atop others. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.

Context: Iron Oxide Red, Reduction Firing, Celadon Glaze, Tenmoku, Crystallization, Mechanism, Metallic Glazes

Wednesday 13th January 2021

4, 5, 6, 7 percent tin oxide in a clear glaze at cone 6

Tin is super expensive, so how much should you use in a clear glaze to get a white? It is a trade-off of cost and whiteness. Nothing else can make a glaze this white and opaque and these low percentages. Consider this: A tin-opacified glaze may only need to be half as thick as a zircon opacified one. Tin has other advantages over Zircopax. First, the percentage required could be half or one third. That takes us down to four or six times less tin being needed (Zircopax is five times less expensive at time of writing). These two factors mean thermal expansion mis-fits between body and glaze start off four or six times less likely to produce shivering or crazing! And tin affects the melt fluidity and thermal expansion half as much. On these samples, the higher percentage of tin seems to produce an even better glossy surface. Crawling is a classic issue with high-zircon glazes (because it impedes melt fluidity, that is what holds super thickly applied majolica glazes on the ware). Tin is the opposite; even though this recipe is high in strontium, and thus has a high surface tension, there is no indication of crawling with the tin addition. A final issue is cutlery marking, a common problem with zircon-opacified glazes. But not with tin oxide.

Context: Tin Oxide

Tuesday 12th January 2021

What can happen when a glaze slurry is not sieved before use

This glaze has just been applied to a bisqued tile. It contains wollastonite, which can agglomerate in storage. It was propeller-mixed at high speed, but that was not enough to break down the white lumps (agglomerates). But they can be broken down by sieving the slurry through 80 mesh or finer. Many other materials behave in a similar manner (e.g. barium carbonate, iron oxide, cobalt oxide, clays, tin oxide, zircon, titanium dioxide).

Context: Wollastonite, Sieve

Tuesday 12th January 2021

Four North American ball clays fired at high temperature

The top bars went to cone 10R, the others are cone 11 and 10 oxidation (downward from top). They are Gleason, Spinks Blend, OM#4 and NP Blend. It is amazing now similar different ball clays fire in the kiln. Clearly, soluble salts are an issue with all of them (the brownish scum). These bars are much cleaner on the backsides (since the solubles were left on the surface on the fronts during drying). The drying shrinkages, plasticities and fired maturities are also all remarkably similar.

Context: Ball Clay, Porcelaineous Stoneware, Ball Clay

Sunday 10th January 2021

Example of pinholes on three cone 6 clay bodies

Three pottery mugs, two having glaze pinholes

This was a fast firing. The glaze is G2934, a silky matte. But that does not mean it is pinhole-prone, it has good melt mobility. The clay on the right is Plainsman Coffee Clay. It contains 10% raw umber, that generates plenty of gases during firing. The centre one is Plainsman M390, not normally difficult to fire defect-free. The left one, M332, should be the worst, but is the best! What is needed to fire these without pinholes? The drop-and-hold and slow-cool C6DHSC firing schedule. It is extra effort to program your kiln controller, but well worth it. If you don't have a kiln controller then by a little experimentation you can develop a switching pattern to produce the same effect.

Context: Plainsman Cone 6 Slow Cool (Reactive glazes), Pinholing

Friday 8th January 2021

White stain. Does it work?

This is G2934Y matte glaze base with opacifiers added. It has been applied to a dark-burning body to demonstrate the comparative degrees of opacity. The stain is Mason 6700 white. While it does not opacify nearly as well as tin or zircon, it does produce a smoother surface.

Context: Stain 6700, Opacifier, Ceramic Stain

Monday 28th December 2020

The titanium/stain mechanism at cone 10R. Want to help us develop other colors?

An eye-popping red variegated cone 10R mug

These are porcelain and stoneware mugs. The glaze is G2571D (based on the fritted G2571B version of G2571A). This project started with calculations to source boron from a frit (instead of Gerstley Borate), MgO from talc and a frit (instead of dolomite). These moves enabled eliminating raw silica from the recipe. This produced a finer silky texture and better melt fluidity (for hosting colors). I started by adding rutile and zircopax (and got a great bamboo). But what about variegating using titanium instead of rutile (it contains no iron so colors should be brighter). Look what happens with this red stain! The titanium has done it's job even a little too well (creating a slightly more abrasive surface). Cobalt and titanium also worked. Imagine if other stains would produce dramatic results like this, each with the amount of titanium tuned to it! Would you like to participate? Click the link below to see the project I have shared. If you can try a color, please report your results to me at the bottom of this page. I am hoping this will lead to a detailed mechanism description (the base glaze, the titanium, the color) that people will be able to tune to their own needs.

Context: Titanium Dioxide, G2571A cone 10R silky matte recipe development project, The cone 10R Titanium/Stain Variegation Mechanism

Thursday 24th December 2020

Why this transparently-glazed terra cotta is better at cone 03 than 04

Two terra cotta mugs, the glaze on one is more glossy and the body color is richer

Terra cotta bodies are typically fired between cone 06 and 04. That being said, many, like this Plainsman L215, develop richer color at cone 03 and fire much stronger. Glazes, of course, melt better and micro-bubbles pass through easier at cone 03. But this happens only if the body has not begun to decompose (and therefore generate a lot more gases of decomposition). Notice that crazing is beginning on the one of the left. Apparently the improved body:glaze interface and the development of better vitrification reduces the problem. Cone 03 is somewhat of a sweet-spot for this specific body, however firing higher begins decomposition processes that generate gases that disrupt the surface. Needless to say, accurate firing is needed to fire at cone 03 for ongoing success (because cone 02 is too high for this body, glaze will blister). Do you know what terra cotta actually is, if not click the link to learn more (this is a big topic).

Context: A terra cotta clay fired from cone 06 (bottom) to 4, G1916Q transparent on terra cotta body at cone 06, 05, 03, Terra cotta, Vitrification

Monday 21st December 2020

An example of dunting on a low, flat casserole shape

A casserole dish with a dunting crack going down from the rim

The firing crack from the rim down has released the stresses produced by uneven thermal contraction during cool-down in the kiln. Any factor that contributes to a temperature gradient within a piece will contribute to the likelihood of dunting. Cooling too quickly through quartz inversion, for example, can cause this in almost any piece. Pieces that are thick and heavy, or have uneven cross section (with thick foot and thin walls, for example) will certainly suffer gradients, even in slow cooling. A wide, flat bottom (that is heat-sunk by the a heavy shelf) will also increase the temperature gradient between the outer walls and the inner foot. If that wide piece has vertical walls that get direct radiant heat, especially if one part is more exposed to the elements, it will start a gradient during the up-ramp in the firing. And, on the down-ramp, it will "come back to bite you" with a crack.

Context: Quartz Inversion, Dunting, Body Cracking and Dunting During Firing

Monday 21st December 2020

Is it possible to make a thin flat porcelain tile from a plastic pottery body?

Yes. The body is Plainsman M370 (~ 25 silica, 25 feldspar, 30 kaolin, 20 ball clay + talc to tune maturity), a plastic throwing clay with far too much drying shrinkage to be suitable for tile. It is 3.8 mm thick fired (vs. commercial tiles at 5-7mm). It was rolled and dried completely between sheets of plaster board. Bisque and glaze firing were on an alumina shelf in an electric pottery kiln (at 300F/hr up through quartz inversion on the glaze firing), a completely unsuitable method for firing tile evenly top and bottom. Cooling on both firings was free-fall in a fairly empty kiln. Yet, it is flat! And flexible enough that I could lay it on the cement floor and stand on it without it breaking! Of course, to produce these consistently special furniture that sinks minimal heat and a kiln that can evenly apply it front and back are needed. This is doable for custom applications. Of course, to compete in the commercial market, they need to be dust-pressed and there are lots of specifications to meet.

Context: Ceramic Tile, Making a high quality ceramic tile

Monday 21st December 2020

Contact Me

Use the contact form at the bottom on almost all the pages on this site.

How to Support My Work

Subscribe to my (see below). It is about doing testing and development and not letting the information generated slip away. Starts at $15 for 6 months.

Test, Document, Learn, Repeat in your account at

Login to your online account

Chemistry plus physics. The on-line successor to desktop Insight. Get an account for as little as $15. It does so much more.

Maintain your recipes, test results, firing schedules, pictures, materials, projects, etc. Organizing that data and learning from it will empower you. You will formulate and fix your own recipes and processes.

Conquer the Glaze Dragon With Digitalfire Reference info and software

Still available for Mac, PC, Linux

Interactive glaze chemistry calculations. Free and unrestricted (legacy, no longer in development).

What people have said about Digitalfire

  • This is a fantastic source of on-line information. Thank you for permitting me to access the contents.
  • 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.
  • Thank you so much. This is what a good business looks like; great product, immediate response from the owner no less, and over the top service.
  • Thanks again, and again, as always, for your ever-so-helpful web pages.
  • I enjoy and appreciate your work very much.
  • I have found your information extremely helpful - re firing and troubleshooting. Thank you!
  • Great customer service!!

What people have said about Insight-Live

  • Love the post (about leaching in glazes). I am the bad guy on Facebook ceramic because I always bring up this subject. So sad.
  • You are brilliant .. You have provided so much info that is great. I have been a full time potter for 44 years and am still learning. Thank you so much for your generosity in sending this very pertinent information to me. It certainly has me thinking I should sign on to Digital Fire. Thanks again.
  • Your site is a crucially important one, and I'm happy to help you, even in small ways.
  • First i'd love to thank you for all the info you provide on digitalfire. it is an absolutely amazing resource and the way that you explain glaze chemistry/reactions has really helped expand my practice...I crave understanding and wisdom.
  • This is really cool. Thanks for sending this to my email.
  • Anyway I thank you for your efforts in putting together this wealth of ceramic science in this accessible manner.
  • First of all I want to say that your website is just incredible. So many great information and tips!

DigitalFire Logo

Privacy Policy