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Bloating on a range of bodies at cone 6: Why is this happening?

Four pottery pieces with bubbles and bloating

The problem occurred with standard Plainsman M340, M390, M350, M370 and P300. The stonewares have porosities of 2.5%, the M370 1% and the P300 0.5%. Thus, all of these have comfortable margins for over firing. The G2926B glaze, used on all of them, does seal the surface pretty early so it will contribute to over-fired ware bloating sooner than typical. The problem here is the cone-fire modes on kiln controllers. For this kiln, the cone 6 program goes to 2236F. That's cone 7. Or more. Most people do not want to manually program their controllers. This is unfortunate since they do not benefit from the drop-and-soak PLC6DS and slow-cool C6DHSC firing schedules. And they end up over-firing their ware. Here is what many do: They cone-fire to cone 5. That produces cone 6!

Context: How many degrees between these cone positions?, Manually programming a Bartlett V6-CF hobby kiln controller, Are you using your expensive kiln like a pop-up toaster?, Bloating, Body Bloating

Sunday 24th October 2021

Severe cutlery marking in a glaze lacking sufficient Al2O3

The glaze is cutlery marking (therefore lacking hardness). Why? Notice how severely it runs on a flow tester (even melting out holes in a firebrick). Yet it does not run on the cups when fired at the same temperature (cone 10)! Glazes run like this when they lack Al2O3 (and SiO2). The SiO2 is the glass builder and the Al2O3 gives the melt body and stability. More important, Al2O3 imparts hardness and durability to the fired glass. No wonder it is cutlery marking. Will it also leach? Very likely. That is why adequate silica is very important, it makes up more than 60% of most glazes. SiO2 is the key glass builder and it forms networks with all the other oxides.

Context: Al2O3, A Low Cost Tester of Glaze Melt Fluidity, SiO2, Cutlery Marking, Melt Fluidity

Saturday 23rd October 2021

Retro glaze chemistry calculation - 1980

A 1980 desktop Insight report

I found this report in a notebook today, it is from 1980! I did this batch-to-formula glaze chemistry calculation on a version of Desktop Insight that ran on the TRS-80 Model I that I used at home (shown here is my Model III at work, running Insight). The TRS-80's were the first popular consumer micro-computers for business. Notice the report only uses capital letters, that machine did not support lower case! The dot matrix printers of the time lasted forever on an ink ribbon. Fanfold paper fed from a box, I could tear off only as much as was needed for a report. Boot time was less than 5 seconds. Here is what is amazing: Today, Oct 23, 2021 I found this same recipe in my Insight-live account (the green screenshot)! The results are a little different, I had the chemistry of talc wrong in 1980. I have always maintained my records on computer and through the years, as I migrated from one system to another, I wrote code to carry all the information forward. That started prior when I was storing data on cassette tape on the Model I.

Saturday 23rd October 2021

Get ready for supply interruptions

Bottled glazes, weighing out your own

Material prices are sky rocketing. Prepared glaze manufacturers have complex international supply chains. It is only a matter of time until you are affected. We are happy to sell you bottled glazes, if we can get them! But now might be the time to start learning how to weigh out the ingredients to make your own. Armed with good base glazes that fit your clay body (without crazing or shivering) you will be more resilient to supply issues. Add stains, opacifiers and variegators to the bases to make anything you want. That being said, ingredients in those recipes may become unavailable! That underscores a need to go to the next step and "understand" glaze ingredients. And even improve and adjust recipes. It is not rocket science, it is just work accompanied by organized record-keeping and good labelling.

Context: G2926B, G2934, G1916Q, G3879, Substituting Materials in Glazes and Clay Bodies, Where do I start in understanding glazes?, Digitalfire Reference Library, Base Glaze

Saturday 23rd October 2021

An original container bag of ceramic rutile

The primary use of this material is obvious: For welding rods. However notice the bag bottom is marked "Ceramic Rutile" (with a batch number). This bag is very small, this material is very dense and heavy. Why would a product intended for making welding rods be useful in ceramics? The answer is very interested.

Context: Ceramic Rutile

Monday 18th October 2021

A frit softens over a wide temperature range

Melt flow tests showing the a frit melting from 1550-1750F, Gerstley Borate from 1600-1625F.

Raw materials often have a specific melting temperature (or they melt quickly over a narrow temperature range). We can use the GLFL test to demonstrate the development of melt fluidity between a frit and a raw material. On the left we see five flows of boron Ferro Frit 3195, across 200 degrees F. Its melting pattern is simple and non-volatile: It starts flowing at 1550F (although it began to turn to a glass at 1500F) and is falling off the bottom of the runway by 1750F. The Gerstley Borate (GB), on the other hand, goes from no melting at 1600F to flowing off the bottom by 1625F! But GB has a complex melting pattern, there is more to its story. Notice the flow at 1625F is not transparent, that is because the Ulexite mineral within GB has melted but its Colemanite has not. Later, at 1700F, the Colemanite melts and the glass becomes transparent.

Context: B2O3, Frit Fusibility Test, The Chemistry, Physics and Manufacturing of Glaze Frits, What Determines a Glaze's Firing Temperature?, Ferro Frit 3195, Gerstley Borate, Melting Temperature, Frit, Common frits begin melting

Sunday 17th October 2021

A limit or target glaze formula. What does this mean?

A glaze formula compared to a target formula

Recipes show us the materials in a the glaze powder (or slurry). Formulas enumerate the oxide molecules and their comparative quantities in the fired glass. Oxides construct the fired glass. The kiln de-constructs ceramic materials to get their oxides, discards the carbon, sulfur, etc. and builds the glass from the rest. There is a direct relationship between fired glaze properties (e.g. melting range, gloss, thermal expansion, hardness, durability, color response, etc) and its oxide formula. There are 8-10 oxides to know about (vs. hundreds of materials). From the formula-viewpoint materials are thus "sources-of-oxides". While there are other factors besides pure chemistry that determine how a glaze fires, none is as important. Insight-live can calculate and show the formula of a recipe, this enables comparing it side-by-side and with a target formula (or another recipe known to work as needed). Target formulas are opened using the advanced recipe search, choosing the limits batch and clicking/tapping the search button (search 'target recipe' in Insight-live help for more info).

Context: Creating Your Own Budget Glaze, Digitalfire Insight-Live, Limit Formula, Oxide Formula, Ceramic Oxide, Chemical Analysis

Friday 15th October 2021

Frit Melt Fluidity Comparison - 1850F

Frits melting side-by-side at 1850F

Fired at 350F/hr to 1800F and held for 15 minutes (I already did firings from 1300F-1800F in 50 degree increments, all of them are visible in the parent project). Frit 3110, 3134, 3195, F75 have all flowed all the way down for many previous temps. LA300 and 3124 were just starting at 1800F, look at them now! 524 and F38 have gone from half-way at 1800F to water-falling over the end. Frit 3249 is still not out-of-the-gate but F69 (the Fusion Frits equivalent) is half-way. Note how the melt surface tension is evident by the way in which the melts spread out or hold together. By contrast, Gerstley Borate, the only raw material here, suddenly melted and flowed right over the cliff between 1600 and1650!

Context: Fusion Frit F-524, Fusion Frit F-69, Fusion Frit F-15, Ferro Frit 3602, Fusion Frit F-75, Fusion Frit F-38, Fusion Frit FZ-16, Ferro Frit 3195, Ferro Frit 3134, Ferro Frit 3124, Frit B325, Frit LA-300, Ferro Frit 3249, Ferro Frit 3110, Frit, Comparing the Melt Fluidity of 16 Frits

Monday 11th October 2021

Global supply chain issues? Learn to mix and adjust your own bodies, glazes

Shipping containers piled high

Material prices are sky rocketing. And, the more complex your supplier's supply chain the more likely they won't be able to deliver. How can you adapt to coming disruption, even turn it into a benefit? Learn to create base recipes for your glazes and even clay bodies. Learn now how to substitute frits and other materials in glazes (get the chemistry of frits you use now so you are ready). Even better: Learn to see your glaze as an oxide formula. Then calculate formula-to-batch to use whatever materials you can get. Learn how to adjust glazes for thermal expansion, temperature, surface, color, etc. And your clay bodies? Develop an organized physical testing regimen now to accumulate data on their properties, learn to understand how each material in the recipe contributes to those properties. Armed with that data you will be able to adjust recipes to adapt to changing supplies.

Context: Where do I start in understanding glazes?, Glaze Recipes: Formulate Your Own Instead, The Physics of Clay Bodies, Formulating a Porcelain, Ceramic Tile Clay Body Formulation, Substituting Materials in Glazes and Clay Bodies, Digitalfire Insight-Live, Digitalfire Reference Database, Substitute Ferro Frit 3134 For Another Frit

Wednesday 6th October 2021

Fight the dragon with Insight-live

Fight the glaze dragon. Disorganized documentation of your testing? You are playing into his hands. Replace that notebook or binder with pictures, recipes, firing schedules, test results, material and more in your own or a group account at insight-live.com.

Context: What is the Glaze Dragon?, Digitalfire Insight-Live, Digitalfire Reference Database, Insight-Live

Tuesday 5th October 2021

Contact Me

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

Another Way to Support My Work

Subscribe to Insight-Live.com. It is about doing testing and development, not letting the information slip away. Starts at $15 for 6 months.


Test, Document, Learn, Repeat in your account at insight-live.com

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. Because it is on-line you can access your data from any connected device. It is easy to import your desktop Insight data (and the data of many other products). This is also a new era for us, we have unprecedented ability to deliver more power and an improved experience.

Teaching ceramics? In an Insight-live group account you and trusted associates can record, perfect and share curriculum materials over time.

Still using desktop Insight? Convert to the new system. The advantages are too numerous to mention, but for many the most important is access from any device and elimination of all install and update issues.

Group accounts for industry and education. Private accounts for potters. This is the future! Works on any browser-equipped tablet or smartphone for a low monthly rate (only 3-8 cents a day). Get started.

Conquer the Glaze Dragon With Digitalfire Reference info and software

Download for Mac, PC, Linux

Interactive glaze chemistry for the desktop. Free (but no longer in development).

Teaching ceramics? This is highly a focussed way to do it. Students can later upgrade to online Insight-live.com. Download here or in the Files panel within your Insight-live.com account.


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  • As many other users on the web, I want to congratulate you for your amazing digital fire website.
  • I just want to thank you for this most interesting and informative article. I just did a general search for the compatibility of a stoneware glaze on porcelain, and your page came up! Wow! what a mine of information and just what I needed as I was also looking for a slip recipe for my students.
  • I love insight BTW. I have been showing everyone at Alfred how easy it is to do UMF while comparing two recipes next to each other. The up and down toggles make it easy to understand cause and effect. They are also impressed how easily i can switch between my computer and smartphone and maintain all my work and recipes. BTW I just love insight. It has changed my experience and knowledge of glazes drastically.
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