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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). It's color is whiter than paper! Using my 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). It appears to work well when applied to the bisque (because I added CMC gum), but during firing, it shrinks 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 body shrinks 4%, that would put it under compression, looking for opportunities to flake off at edges (e.g. rims of mugs). How to optimize this? The engobe needs about 3% Ferro Frit 3110 to raise its firing shrinkage by 2%. And, to be applied to leather-hard ware.
Context: Match the firing shrinkage..
Monday 13th October 2025
Transparent over-glaze for cone 6 stoneware
Is a good one even possible on brush decorated ware?
The mug on the right is terra cotta slipware fired at cone 04 using underglazes and a leaded transparent over-glaze (lead glazes are still commonly used in many parts of the world and considered safe there). Mug on the left: This potter wants to use the same technique on cone 6 stoneware. This is a typical transparent glaze (fluxed using a frit or Gerstley Borate). The result is micro-bubble clouding, boron blue, washed-out colors and surface defects. Because it is a dipping glaze it went on too thick and didn't cover well over the colored brushstrokes. However, achieving better warm browns is possible. A more refractory underglaze (made with stains, not iron oxide) that does not bleed. A more fluid melt transparent glaze that is better able to shed bubbles. A drop-and-hold firing would reduce surface defects. Finally, careful control of the glaze thickness and quality of laydown. To achieve the latter, it might be worth preparing the transparent as a brushing glaze, at least for application on the outsides (enabling a dense and even laydown over the whole surface).
Context: Why dipping transparent glazes.., Underglazes require a fluid.., The right transparent glaze.., Engobe, Underglaze
Tuesday 23rd September 2025
Stains are better in black glazes
5% stain vs 15% metal oxides
Consider the hazards and hassles before choosing a black matte or gloss recipe that has high individual or combined percentages of manganese dioxide, cobalt or nickel.
Gloss blacks: These are super popular right now as the base for layering of reactive glazes. DIY dipping versions thus make a lot of sense. They make even more sense when they don’t turn to jelly in the bucket because of the high percentage of red iron oxide in all blacks made using metal oxide colorants. And when the total percentage of pigment is as high, or higher than 15%. And when the pigments cause crystallization (especially when overloaded).
Matte blacks: The human eye can detect even slight differences in the degree of matteness (which is very difficult to keep consistent). Raw metal oxides affect the matteness, especially when overloaded with pigment. They are prone to cutlery marking if too matte. By using stains manufacturers, and even potters, have learned to tune recipes (lower left) and firing schedules to achieve consistency and functionality (even tourist souvenirs (lower right) feature them now). With stains, only one material is producing the color, its percentage (which can be as low as 4%) can be tuned.
Context: Ceramic Stain Toxicity Label.., Control matteness by glaze.., Heres evidence that using.., Ceramic Stain, Toxicity
Thursday 18th September 2025
Thickly applied slips must fit the body and each other
Fit? It has to stick well. And stay stuck during drying (and shrinking). And the bond has to survive shrinkage that happens during firing. This potter is doing thick applications of each slip (actually that makes them engobes). She uses stains, that's wise, metal oxides bring baggage when used to color slips (e.g. their decomposition can affect the bond, they can gel the slurry, flux the fired product thereby increasing the firing shrinkage of the slip). Stains are better because they affect slurry and fired properties less. But there are still enough issues that each colored slip deserves testing. This potter first slaked B-mix as a slip (it is highly plastic), using it at a runny yogurt consistency. But it bubbled when fired hotter than a cool cone 6. A switch to porcelain slip (which is non-plastic) is shown here. It flaked off as it dried (even in a damp box for 24 hours), also after bisquing to cone 08, and sometimes even after firing to cone 6. This signalled a drying mismatch between body and slip, the bond that managed to survive drying was weakened enough to fail on firing.
The solution was an engobe recipe that is super plastic and sticky. The popular Fish Sauce recipe is an example, it contains 10% bentonite and is unbelievably sticky. We formulated L3954B with this in mind. It adheres well to leather-hard clay and doesn't flake off (misfit is instead evident by surface cracking if its shrinks more than the body). And it is not highly vitreous, keeping its fired shrinkage low enough to match stoneware bodies. Mixing your own recipe also enables compensating the amount of feldspar if the stain affects the slip's degree of vitrification, and therefore fired shrinkage (e.g. blues, oranges, yellows).
Context: L3954B, This pottery glaze is..
Monday 15th September 2025
A giant cookie-cutter for slab built mugs
View and print it now using the Downloads page link
Available on the Downloads page
3D print four of these and glue them together to make a large cookie cutter for producing slab-built mugs. 3D print the cup, fill it with plaster and remove the PLA using a heat gun. Roll out a thin slab of clay, press the cutter into it using a round wooden batt, make sure it is not sticking to the board and flip it over onto the plaster form. Handles can even be attached while it is on the form. If clay is plastic it can be used quite stiff. Experiment, adjust sizing and dimensions and reprint to fine-tune.
This has been drawn "parametrically" with OnShape. I only had to draw half of one of the quarters (I mirrored that, extruded and then did a circular pattern of 4). The downloads page (link above) has a free link to view this drawing at OnShape.com. To 3D print, right-click the part (from the list on the lower left, "Quarter 1" and "Cup"). Choose the Export option and select 3MF as the file type (it should go to your downloads folder). Open it with your slicer and print (turn the quarter over and make four). You can even export in formats that other CAD software can open. Better yet, import it into your OnShape account to see the design history and change multiple aspects of the geometry in the variables panel (the drawing will adjust automatically). Other dimensions (e.g. cut depth) can be edited manually. Parametric design is revolutionary and it is now accessible even to hobbyists; it fits my try-it, adjust-it and try-it-again way of working.
Context: Large cookie-cutter 3D-printed in.., 3D Design, Pie-Crust Mug-Making Method, Cookie Cutting clay with..
Monday 15th September 2025
Recognize these universal oxidation glazes?
Almost every potter needs a Albany brown and rutile blue.
These are made by Barbara Childs Pottery (I saw them on sale in a tourist shop in Alaska). To keep costs down, I first assumed they use dipping glazes they mix themselves. Potter's Choice PC-32 Albany Slip Brown and PC-20 Rutile Blue hobby glazes emulate these long time pottery glaze recipes. However, a reader noted that Barabara Childs uses Clay Art Center’s Stellar Rust and Floating Blue (with guest appearances by Blue Green). But Amaco and Clay Art don't just use the traditional recipes; they adapt and improve them. Consider the rutile blue. Neither is using the traditional G2826R floating blue recipe, there are new and better ways using recipes like GA6-C and GR6-M. Likewise, with the brown, they are not using the traditional G2415E Albany Brown recipe. Rather, they improve it (e.g. like we did with G3933G1). High on their list of improvements would have been a way to reduce or remove the lithium to cut costs. Maybe you are a hobbyist and don’t feel you need to DIY your costs down. But do your customers feel the same way? Not buying just ten small jars of hobby brushing glaze will pay for a mixer and much of the ingredients to make gallons of each of these as dipping glazes. It will also set you on the road to gradually improving the glazes you use. And even reducing your prices. What about buying premixed powders? Yes, that is much less expensive. But if you are mixing the glaze from one manufacturer with the clay body from another, crazing is an ever-present issue. Mixing your own enables an adjustment to fix the problem.
Context: Adding 6 lithium carbonate.., PC-2 floating blue with..
Tuesday 9th September 2025
A draining issue with a slip cast bottle
It is turning inside out!
Why did this happen? There is a perfect storm of factors. Draining, during slip casting, creates suction and slip is heavy (1.8 times heavier than water). And this mold is tall with a narrow neck. So that creates a lot of suction. A slip having inadequate fluidity complicates draining. This round shape, even with printing artifacts, also releases well. How can this issue be avoided?
-Draining the mold carefully, holding it near horizontal for much of the drain.
-Use a well-deflocculated slip.
-Add bentonite to the slip, perhaps 0.5%, to make it stickier and slow down release time (which also slows down the casting time).
-At times, this will happen despite all efforts. In that case, if might be necessary to use a tube (e.g. 1/2 or 5/16”) to pump most of the liquid slip out of the bottle before inverting it. Adapt a 3D printed pour spout to keep the tube centered, at least near the mouth of the bottle.
Context: Slip Casting, Beer Bottle Master Mold.., Casting Slip Problems
Saturday 6th September 2025
OnShape CAD is Free for Hobby Makers:
Is it as good as Fusion 360?
It is very hard to let Fusion 360 CAD go. But the approaching $750 renewal is powerful motivation! OnShape is amazing. There is nothing to install, it runs in a browser tab like Google docs (see picture below). Sure, it won’t run offline, but I am almost never offline. It functions very similar to Fusion 360 for my basic requirements of making molds for slip casting. Recent experience with the complexity and slowness of Solidworks for Makers, which is total overkill for what I need, really makes OnShape look good.
My OnShape drawings are stored in my cloud account and are public. That sounded bad at first, but it also means that they are shareable with others (another person, whom I choose, can actually work on a drawing at the same time as me). The full OnShape is working in Firefox on my 2014 Mac Mini Ubuntu Linux machine. This is beyond exciting to me, traditional CAD has always required expensive hardware that is far beyond a hobbyist (of course, OnShape will also work in Safari on Mac and Chrome on Windows). A real bonus: I can edit drawings on iPad in what appears to be full power mode (although a mouse and keyboard are needed for serious work).
Besides the above, here are some of the features and advantages I am seeing:
-It opens and exports many professional CAD file types (a major drawback in SolidWorks for Makers).
-It is really fast, login is quick and a drawing can be open in seconds, this is way better than xDesign for Makers (from Solidworks).
-Documents are always saved, close one by simply clicking the home icon on the upper left.
-The timeline (called the "Feature Tree") can be reordered, turned back and has folders like Fusion 360.
-To 3D print just select part of your drawing, right-click and choose to export it in 3MF or STL format (it goes into the downloads folder).
-All tools are in one long, monochrome ribbon of tiny icons at the top but there is a tool searcher.
-Like Fusion 360, sketching constraints are inferred as sketches are created and applying them works in a similar fashion (but more aggressively). Their tiny symbols display in groups and associate to the point or line by a light grey line. Automatically applied constraints can make sketches behave in strange ways until you learn to find and remove the offending ones.
-Constraints and dimensions are movable so drawings can be uncluttered for printing.
-Section analysis is in the "Camera and Render Options" pop-up under the view cube.
-The spline and bezier sketching tools are not as interactive (a downside of running in a browser).
-It is not as good for making cookie cutters because it doesn’t do text nearly as well).
-Parameters, called variables, are more in your face; they are even shown in the timeline.
-Panning, rotating and the viewcube work a little differently. The iPad version of OnShape beats Fusion easily in this respect.
-OnShape does not appear to support text along a path like Fusion.
The secret weapon of learning OnShape: An AI chatbot. Just ask any question about how to do something. One helpful migration from Fusion 360 is to print the sketch(es) (with constraints and dimensions) and work from that to create the equivalent in OnShape. An advantage of OnShape is that if you get stuck (e.g. drawing goes red), you can share a link with a more knowledgeable friend to tell you what is wrong. Most often the issue is conflicting constrains.
Context: 3D mechanical design software.., 3D Printed Pour-spout Forms.., My Breakup with Fusion.., OnShape parametric cloud-native CAD..
Saturday 6th September 2025
Classic Medalta Potteries Beer Bottle
Make this mold using OnShape and Fusion 360
Danny Downsized: He's Being Outsourced.
He should have seen this coming!
Management now thinks they can outsource his technical work. Danny was good, but he didn’t build a centralized, searchable record of material testing, shipment histories, specs, production problems, and solutions. Instead, his knowledge is buried in thousands of Excel and Word docs and PDFs. He should have used Insight-Live.com to organize, interlink, and preserve this critical data. Sadly, the company does not even realize what they face without Danny:
- Slower problem-solving and loss of institutional memory.
- Greater dependency on outsiders who lack long-term investment.
- Inability to verify product claims.
- Quality drift leading to recalls, rework, and higher warranty costs.
- Innovation slowdown.
- Eroding customer trust and ballooning consultant costs.
Had Danny been not just a watchdog but a technical innovator, suppliers and consultants would have been secured to support—not replace—his in-house expertise.
Context: Protect your reputation as.., 2 Skids of Material.., Tommy Turnback Ignored AI..
Sunday 31st August 2025
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What people have said about Digitalfire- Thanks again, and again, as always, for your ever-so-helpful web pages.
- Firstly, I want to thank you about this very good site in the net which helps very much in the development of the ceramics industry.
- Thank u for your helping researchers .. Thank u for your efforts.
- Actually my brother asked to download the books. He is running a ceramic company in India. He has learned so many things in your free download, later he is impressed on your simple and clear explanation of ceramic technology.
- I just want to thank you for this generous, helpful and very educational website.
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- I really enjoy the information you have on your website ... it is the best I have seen on the internet ... I appreciated your help on using and creating slips and adjusting base glazes.
- First of all, I want you to know what an incredible resource your work has been for me the past several years.
- Your resources are truly amazing and as an ex electronic engineer (now a potter), I really am impressed with your analytical approaches. Your site is almost a complete college level course on pottery (less the throwing & handbuilding). Thank you for your wonderful contributions.
- 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.
What people have said about Insight-Live- I am a computer/tech guy and was drawn to your info because it is systematic, detailed and comprehensive. I started pottery and wheel throwing January 2018 on my 59th bday. I am ecstatic!
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- Sure do like Insight and I've switched to using the live version. It's excellent!
- I love your software and I really appreciate you being there when I need help. Keep on being amazing.
I am still enjoying insight every day. You were right - having it in the cloud is a huge advantage.
- I am eternally indebted to you for all that you have done to advance the technical abilities of non-industrial pottery! I love using Insight.
- 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.
- I am a ceramic manufacturing manager, and before this position, I had no training on ceramics. I was taught our recipes, but none of it was explained. The dragon ran amok. After reading through your articles and descriptions, I feel I have learned so much about my own products, and I have a foundation for changing, improving, and troubleshooting my materials issues that I wouldn't have had otherwise. They taught us in engineering school that we all stand on the shoulders of giants, because we all get to use knowledge and tools at a moment's notice that took others years to develop. Thank you for sharing all that you have, so the rest of us can stand on your shoulders to do our work that much better.
- I'm a professional potter from Western Australia with a 30 year career - self taught. I came across Digitalfire when researching raw materials - an incredible resource you've put together!
- Dear tony: Nice work keep it up!
- We love your work with Digital Fire. You've single handedly breathed life into many, many ceramics companies through your generous approach to sharing information. I can't tell you how valuable we have found your content to be. Thank you.
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