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Tony Hansen's Thousand-Post TimeLine

I am the creator of Digitalfire Insight, Digitalfire.com and Insight-live.com. I have made hundreds of posts like these on my Facebook page and personal timeline. My posts are like no others, they help you understand your glazes and clay bodies, take control. They are also part of the Digitalfire Reference Database (referenced from one or more articles, glossary entries, materials, oxides, test procedures, etc). Visit and Like my page to get a notification each time I post.

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What temperature do Orton cones actually go to in my kiln?

The blue line on this graph represents numbers from the Orton cone chart for 108F/hr. It is not as straight as what I expected. The red line is the temperature measurements that we have recorded after many test firings at each temperature. We use large cones in these firings and finish the firings manually to shut the kiln off just before the firing cone touches. These are now target temperatures that we use for automatically firing each temperature.

Tuesday 16th September 2014

Shivering on a transparent over an engobe

Example of a glaze (G1916J) shivering on the rim of a mug. But the situation is not as it might appear. This is a low quartz cone 03 vitreous red body having a lower-than-typical thermal expansion. The white slip (or engobe) has a moderate amount of quartz and is thus put under some compression by the body.But the compression is not enough to shiver off (e.g. at the rim) when by itself. However the covering glaze has an even lower expansion exerts added compression on the slip and therefore causes a failure at the slip-body interface.

Tuesday 29th July 2014

Going to screen a glaze? Use the brush, not the spatula

Do you need to rescreen a glaze slurry. Using a brush like this you will be able to get it through the screen much faster. This is because the rubber edge forces particles into the screen openings, plugging them. The brush is gentler, the oversize material just rolls around on top. If you are screening a glaze for the first time, however, the spatula is better if there are agglomerated particles that need to be broken up (e.g. wollastonite, cornwall stone). When rescreening, any oversize particles (e.g. precipitates) should be discarded.

Monday 19th January 2015

Cone 6 porcelain marbled and thrown

These bowls are made from a mixture of white and stained New-Zealand-kaolin-based porcelain (Plainsman Polar Ice) fired at cone 6. The body is not only white, but very translucent.The clear glaze is G2916F clear, it uses a Fusion strontium frit.

Monday 30th September 2013

Making your own hexagonal shelves from alumina

The home-made kiln shelf (left) was fired it at cone 10. It is half the weight (and thickness) of the cordierite one (but remember that it does not have the thermal shock resistance of cordierite). It is made from a body consisting of 96.5% calcined alumina and 3.5% Veegum. It rolls out nicely and dries perfectly flat over about three days (the Veegum does not give up its water easily). I cut it 1/4" larger than the other and it has fired to the same size; this body has incredibly low shrinkage.

Tuesday 3rd September 2013

This is when you should program a firing yourself

This is Polar Ice cone 6 porcelain that has been over fired. The electric kiln was set to do its standard cone 6 fast fire schedule, but a cone in the kiln demonstrates that it fired much higher (perhaps to cone 7 judging by the bend on the cone). This is a translucent frit-fluxed porcelain that demands accurate firing, the over fire has produced tiny bubbles and surface dimples in the glaze. The mug rim has also warped to oval shape. The lesson: If you are firing ware that is sensitive to schedule or temperature, use large cones and adjust if needed. If it fires too hot like this, then program to fire to cone 5 with a longer soak, or cone 5.5 (if possible). Or, program all the steps yourself; that is definitely our preference.

Tuesday 20th January 2015

Compare two glazes having different mechanisms for their matteness

These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio (you can find more about it by googling 1214Z). It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived and 300F to ice water plunge without cracking). This looks like it could be a great liner glaze.

Wednesday 19th February 2014

Precipitate can forms in firtted glazes, remember to screen it

Potters often store glazes for long periods so tiny spherical precipitate particles can form. These were found in a months-old bucket of G2926B (M370 clear) cone 6 clear glaze (about 2 gallons). These can appear over time, depending on factors like temperature, electrolytes in your water or solubility in the materials (likely, the frit is slightly soluble). The glaze slurry should be screened periodically (or immediately if you note the particles when glazing a piece). This is an 80 mesh screen. Note the brush, using one of these gets the glaze through the screen much quicker than using a rubber spatula.

Monday 19th January 2015

Pinholes often happen on trimmed surfaces

Pinholing is often related to the smoothness of the underlying body surface. The lower half of this vase was tooled during the leather hard stage, all the pinholes occur there. Even though this glaze contains 10% grog, the pinholes are not appearing on the upper half because the slip generated during throwing has left a smooth surface.

Tuesday 20th January 2015

Why you cannot paint pure stain powders over glaze

An example of why you should not just paint pure stain powders over glazes. Left is a blue stain, right is a green. Obviously the blue is melting in much better, even bleeding at its edges. On the other hand, the green just sits on the surface as a dry, unmelted layer. Stains need to be mixed into a glaze-like recipe of compatible chemistry (a melt medium). The blue is powerful, it would only need to comprise 5-10% of the total, the green 10%-15%. Overglaze recipe development projects involve following the guidelines of the stain manufacturer for chemistry compatibility and adjusting the melt to compensate for each stains melting behavior.

Tuesday 20th January 2015

Why is the glaze crawling on these tiles?

An example of crawling in a zircon opacified glaze on a tile. The immediate source of the problem is likely at the decoration stage. The water from the blue overglaze is rewetting the white under glaze, expanding and reshrinking it. This compromises the white glaze's bond with the body, resulting in cracking and lifting of the edges of the cracks. A number of things can be done to improve the situation: Adding a binder to the white glaze, reducing the clay content or using less plastic clays in its recipe, reducing the water content of the overglaze, heating the tiles before glazing and/or decorating so they dry faster and reducing the surface tension of the glaze melt.

Friday 16th January 2015

Fired warping can be due to shape also

The goblet on the left is bending, not just because the clay is somewhat unstable at the temperature being fired, but because this shape is also inherently unstable. Where extreme shapes are prone to warping, ware must be made from clays that do not vitrify (that introduces issues of strength and functionality). In this case, the clay recipe is based on a terra cotta material that matures at a very low temperature. The problem was dealt with by employing a recipe of 60:40 clay:200# kyanite.

Friday 25th July 2014

A little kyanite might improve your clay body

Plainsman M340 with 11% added 48 mesh kyanite. The kyanite was added to improve the plastic strength to stand up when throwing large shapes. It has done this. Its grainy texture (in an otherwise smooth body) is only slightly noticeable while throwing, but it lifts better. The kyanite was simply wedged into the clay using a slice-and-wedge technique, the stiffness was affected only slightly. An added benefit will be a reduction in the thermal expansion (and thus thermal utility) of the fired clay (of course there is a chance that glazes will need to be adjusted to deal with crazing).

Tuesday 13th January 2015

90:10 Albany:Frit and Alberta:Frit comparison

These are three runs of Alberta Slip being compared with the original Albany Slip. These are ten-gram balls fired on porcelain tiles at cone 6. This test shows how the material flows, how much gases of decomposition it generates and how well it allows them to escape. As you can see, they are very similar in melting behavior.

Friday 9th January 2015

Optimimal casting slurry properties impossible without good mixing

A video of the kind of agitation you need from a power mixer to get the best deflocculated slurry properties. This is Plainsman Polar Ice mixing in a 5 gallon pail using my mixer. Although it has a specific gravity of 1.76, it is very fluid and yet casts really well. These properties are a product of, not just the recipe, but the mixer and its ability to put energy into the slurry.

Saturday 10th January 2015

Another reason why clay should be wedged or kneaded

Left: A high-contrast photo of a cut across the cross section of an eight-month-old slug of Plainsman M370 pugged clay. Right: A cut of a just-produced material (which will exhibit the same pattern in eight more months). You can feel different stiffnesses as you drag your finger across this clay, these are a product of the aging process combined with the natural lamination that a pugmill produces. Clearly, the older material needs to be wedged before use in hand building or on the wheel.

Friday 9th January 2015

Oversize particles in a typical manufactured porcelain body

Example of the oversize particles from a 100 gram wet sieve analysis test of a powdered sample of a porcelain body made from North American refined materials. Although these materials are sold as 200 mesh, that designation does not mean that there are no particles coarser than 200 mesh. Here there are significant numbers of particles on the 100 and even 70 mesh screens. These contain some darker particles that could produce fired specks (if they are iron and not lignite); that goodness in this case they do not. Oversize particle is a fact of life in bodies made from refined materials and used by potters and hobbyists. Industrial manufacturers (e.g. tile, tableware, sanitaryware) commonly process the materials further, slurrying them and screening or ball milling; this is done to guarantee defect-free glazed surfaces.

Friday 9th January 2015

A home made dust collector you can make

Example of a custom made dust collection hood in the material repackaging area of a supplier. The slots along the front suck particles into the duct, the suction comes from an exhaust fan downstream where the pipe exits the building. It has a wall switch and a sliding damper (where the hood enters the pipe) to enable stopping all airflow (to prevent heat loss in the room during cold days). Notice it is located above the scale and heat sealer where most dust is generated during weighing and packaging. Working in front of a system like this enables you to work without breathing any dust at all.

Saturday 25th August 2012

Ravenscrag oatmeal layered over black at cone 6

This is GR6-H Ravenscrag oatmeal over G1214M black on porcelain at cone 6 oxidation to create an oil-spot effect. Both were dipped quickly. You can find more detail at ravenscrag.com.

Thursday 9th January 2014

Getting a start on testing Bentone

Bentone (AKA Macaloid) is a super plastic additive used to modify rheolgy in many consumer products. It is made by refining Hectorite. It is very difficult to mix pure Bentone with water, it is just so sticky and the water content is so high, it takes a week to dry a sample and it cracks into pieces during drying. I am studying five different grades for use as a plasticizer in premium porcelains. I am interested in how they stack up against the king: VeeGum T (both in price and performance). The first step was to fire square tiles of the powder on small porcelain tiles at cone 6 to compare the iron content. Three sintered into a solid mass, shrinking to about half the size. The CT grade is the natural, untreated Hectorite clay (accounting for its darker color), the processing to purify the material obviously increases its affinity for water, shrinkage and fired maturity.

Friday 9th January 2015

More carbon needs to burn out than you might think!

Hard to believe, but this carbon is on ten-gram balls of low fire glazes having 85% frit. Yes, this is an extreme test because glazes are applied in thin layers, but glazes sit atop bodies much higher in carbon bearing materials. And the carbon is sticking around at temperatures much higher than it is supposed to (not yet burned away at 1500F)! The lower row is G1916J, the upper is G1916Q. These balls were fired to determine the point at which the glazes densify enough that they will not pass gases being burned from the body below (around 1450F). Our firings of these glazes now soak at 1400F (on the way up). Not surpisingly, industrial manufacturers seek low carbon content materials.

Friday 9th January 2015

Matte glazes must be tuned to compromise functionality and aesthetics

How much do additions of gloss glaze shine up G2934 cone 6 matte base? You need to know this to tune it to a compromise between the most matteness possible while still not cutlery marking. These ten-gram balls fired to cone 6 on porcelain tiles demonstrate. Far left: Pure G2934. Top row proceeding right: G2926B added 10%, 20%, 30%, 40% (100% far right). Bottom row is G2916F in the same proportions. The effects are similar, but as you can see, the top glaze produces a more pebbly surface.

Thursday 8th January 2015

Aged clay really needs to be wedged before use

This is a cut through an eight-month-old slug of pugged clay. The cut was done near the surface. The patchy coloration is a by-product of the aging process. If a slice of this was fired in a kiln, an even and homogeneous white surface would emerge, with no hint of what you see here. A few moments of wedging will mix the matrix and ready it for wheel throwing or hand forming.

Friday 9th January 2015

How much does a glaze need to melt before it sticks to the body well?

The back flat side of balls of 1916J and Q low fire glaze that melted into a dome shape after being fired to 1550F. They have been turned over to see the back side (the front side is still stained by volatilizing carbon). Clearly they have reached zero porosity and are beginning to melt, yet they have not adhered to the vitreous porcelain tile! This demonstrates the degree to which an engobe must melt to secure itself well to the underlying body.

Friday 9th January 2015

Resurrecting and old kiln by replacing elements

Replacing the elements in a old test kiln turns it into a new kiln! Relays are also checked. Notice how the elements are bent and pushed well into the corners. If this is not done properly, they will pull out of the corners after it is fired a few times.

Friday 9th January 2015

Will these glazes crawl on firing? Yes!

An example of how a glaze that contains too much plastic has been applied too thick. It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem you need to tune a mix of raw and calcine material. Enough raw is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen.

Wednesday 25th July 2012

Sparkles on clay. Is that possible?

An example of how a small addition of mica affects the fired appearance of a terra cotta clay. The effect is still working at cone 03 (left) but is more commonly employed at cone 06 (right). Notice that it is still visible even under the glaze. This body is popular on the west coast, it was designed by D'Arcy Margesson. Standard grades of mica are too fine for the effect, this is likely Custer LCM Drilling Mud Mica.

Wednesday 20th August 2014

Melt fluidity: Cornwall Stone vs. Nepheline Syenite

Three Cornwall Stone shipments fired at cone 8 in melt flow testers and compared to Nepheline Syenite. Each contains 10% Ferro Frit 3134.

Friday 14th November 2014

Bleeding underglaze. Why?

This cobalt underglaze is bleeding into the transparent glaze that covers it. This is happening either because the underglaze is too highly fluxed, the over glaze has too high of a melt fluidity or the firing is being soaked too long. Engobes used under the glaze (underglazes) need to be formulated for the specific temperature and colorant they will host, cobalt is known for this problem so it needs to be hosted in a less vitreous engobe medium. When medium-colorant compounds melt too much they bleed, if too little they do not bond to the body well enough. Vigilance is needed to made sure the formulation is right.

Tuesday 1st January 2013

Melting range is mainly about boron content

Fired at 1850. Notice that Frit 3195 is melting earlier. By 1950F, they appear much more similar. Melting earlier can be a disadvantage, it means that gases still escaping as materials in the body and glaze decompose get trapped in the glass matrix. But if the glaze melts later, these have more time to burn away. Glazes that have a lower B2O3 content will melt later, frit 3195 has 23% while Frit 3124 only has 14%).

Tuesday 30th December 2014

Why is that transparent glaze firing cloudy?

G1916Q and J low fire ultra-clear glazes (contain Ferro Frit 3195, 3110 and EPK) fired across the range of 1650 to 2000F (these were 10 gram balls that melted and flattened as they fired). Notice how they soften over a wide range, starting below cone 010 (1700F)! At the early stages carbon material is still visible (even though the glaze has lost 2% of its weight to this point), it is likely the source of the micro-bubbles that completely opacify the matrix even at 1950F (cone 04). This is an 85% fritted glaze, yet it still has carbon; think of what a raw glaze might have! Of course, these specimens test a very thick layer, so the bubbles are expected. But they still can be an issue, even in a thin glaze layer on a piece of ware. So to get the most transparent possible result it is wise to fire tests to find the point where the glaze starts to soften (in this case 1450F), then soak the kiln just below that (on the way up) to fire away as much of the carbon as possible.

Tuesday 30th December 2014

Even highly fritted glazes have to liberate some carbon

Five most common North American Ferro Frits fired at 1850F on alumina tiles (each started as a 10 gram ball and flattened during the firing). At this temperature, the differences in the degree of melting are more evident that at 1950F. The degree of melting corresponds mainly to the percentage of B2O3 present. However Frit 3134 is the runaway leader because it contains no Al2O3 to stabilize the melt. Frit 3110 is an exception, it has low boron but very high sodium.

Thursday 16th October 2014

Cone 03 stoneware. Red and white body and slips. Clear glaze.

Cone 03 white stoneware with red terra cotta ball-milled slip and transparent overglaze. These are eye-popping stunning. They are test L3685U (Ferro frit 3110, #6 tile kaolin, Silica), near the final mix for a white low fire stoneware. The G1916J glaze is super clear. Why? Two reasons. These were fired in a schedule designed to burn off the gases from the bentonite in the body before the glaze fuses (it soaks the kiln for 2 hours at 1400F). Terra cotta clays generate alot of gases at cone cone 03 (producing glaze micro-bubbles), but here the terra cotta is only a thin slip over the much cleaner burning white body.

Saturday 22nd November 2014

Transform the yellow-white of cone 6 to blue-white of cone 10R

Adding a little blue stain to a medium temperature transparent glaze can give it a more pleasant tone. Some iron is present in all stoneware bodies (and even porcelains), so transparent glazes never fire pure white. At cone 10 reduction they generally exhibit a bluish color (left), whereas at cone 6 they tend toward straw yellow (right). Notice the glaze on the inside of the center mug, it has a 0.1% Mason 6336 blue stain addition; this transforms the appearance to look like a cone 10 glaze (actually, you might consider using a little less, perhaps 0.05%). Blue stain is a better choice than cobalt oxide, the latter will produce fired speckle.

Wednesday 28th November 2012

Ceramic Oxide Periodic Table in SVG Format

The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.

Monday 29th December 2014

These two frits have one difference in the chemistry. What?

These two boron frits (Ferro 3124 left, 3134 right) have almost the same chemistry. But there is one difference: The one on the right has no Al2O3, the one on the left has 10%. Alumina plays an important role (as an oxide that builds the glass) in stiffening the melt, giving it body and lowering its thermal expansion, you can see that in the way these flow when melting at 1800F. The frit on the right is invaluable where the glaze needs clay to suspend it (because the clay can supply the Al2O3). The frit on the left is better when the glaze already has plenty of clay, so it supplies the Al2O3. Of course, you need to be able to do the chemistry to figure out how to substitute these for each other because it involves changing the silica and kaolin amounts in the recipe also.

Thursday 7th August 2014

Do you know the purpose of these frits?

Know the difference between Frit 3134, 3124, 3110, 3249 and 3195? These are 10 gram frit balls fired at 1700F. Each contains boron (B2O3), that is the magic of why they melt this low (Gerstley Borate is a raw source of boron, but it has a very high LOI, gels glazes and is inconsistent). Frit 3124 and 3195 are base glazes, just add 15% kaolin and go. Frit 3110 raises thermal expansion (substitute some of it in if the glaze shivers). Frit 3249 lowers expansion (sub it in if the glaze crazes). Frit 3134? It is similar to 3124 but without any Al2O3, it is useful where you need more clay in the glaze (the clay can source the Al2O3 instead). It is no accident that these five boron glazes are the principle ones used in North America, they complement each other well.

Tuesday 29th July 2014

4 good reasons to consider making your own underglazes

Commercial underglaze colors fired at cone 8 in a flow tester (this is another good example of how valuable flow testers are). Underglazes need to melt enough to bond with the underlying body, but not so much that edges of designs bleed excessively into the overlying glaze. A regular glaze would melt enough to run well down the runway on this tester, but an underglaze should flow much less. The green one here is clearly not sufficiently developed. The black is too melted (and contains volatiles that are gasing). The pink is much further along than the blue. And cone 5, these samples all melt significantly less. Clearly, underglazes need to be targeted to melt at specific temperatures and each color needs specific formulation attention. Silk screening and inkjet printing are increasingly popular and these processes need ink that will fuse to the surface of the body.

Thursday 6th November 2014

Stains having varying fluxing effects on a host glaze

Plainsman M340 Transparent liner with various stains added (cone 6). These bubbles were fired on a bed of alumina powder, so they flattened more freely according to melt flow. You can see which stains flux the glaze more by which bubbles have flattened more. The deep blue and browns have flowed the most, the manganese alumina pink the least. This knowledge could be applied when mixing these glazes, compensating the degree of melt of the host accordingly.

Saturday 27th December 2014

Silk screening using a professionally made screen

Silk screening is a popular decorating method. It is difficult to get a better quality screen than having an aluminum framed one made at a shop that specializes in this process (you can buy those hinges from a screen supplier). This screen is 16x20 inches and I have multiple designs on it (I made them in Adobe Illustrator). I am about to screen lettering and a logo onto a tile using an ink I made (because I have found drastically different melt behaviors in commercial underglazes). I find that simply mixing the ink with water to a very thick consistency works best (it is very easy to plug up the screen if you employ hardening mediums, they are difficult to wash out).

Tuesday 2nd December 2014

Bentone: The whitest burning plasticizer we have seen

Bentone (A.K.A. Macaloid MA) is a very plastic highly refined hectorite clay. This specimen has been mixed as a slurry, then dewatered until plastic on a plaster slab (it is very resistant to giving up its water). The plastic material has a very high water content, is exceptionally sticky and took many days to dry from the plastic stage. It shrinks 30% or more from plastic to fired and burns pure white at cone 6 (it can withstand higher temperatures). It burns whiter than similar materials from other manufacturers.

Wednesday 12th November 2014

Ceramic glaze oxides periodic table

All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.

Monday 29th December 2014

Use a flap wheel to grind foot rings on ware bottoms

Don't even think about using a regular grinding stone in your bench grinder to finish the bottoms of fired ware. Use a flap wheel. They are made of pieces of sandpaper and fit onto your grinder like a regular stone. No more chipping or broken ware.

Monday 29th December 2014

I have always do it this way. Why is it not working now?

If we do something for a long time and it is working, then an issue arises, do we blame a change in materials? I have been using an 85% Ravenscrag, 15% frit glaze for 10 years with no crawling problems. But now it is crawling I am doing nothing different (see mug on right). I have tried mixes with new materials and the old ones, it is still crawling. The problem? What was I thinking? An 85% clay glaze is going to crawl so the question should be: How did I get away with it for so long? I actually do not know! But I am now calcining Ravenscrag as appropriate (have documented this at ravenscrag.com) and I love the control this gives me in balancing slurry properties with drying hardness.

Monday 22nd December 2014

What is that black stuff on these two glazes?

Two low temperature glazes (G1916J, G1916Q) containing only frit and kaolin fired to 1250F (they have been dewatered enough to form into balls). The carbon is part of the LOI of the kaolin (that hardens and suspends the glaze). Yet these glazes have much lower carbon content than ones made from raw materials.

Monday 17th November 2014

L3724E ball milled flocculated slip has been applied to L3685U low fire white stoneware. Notice how silky smooth it is. What is the secret of getting a perfectly even layer that does not drip: Thin the slip until it is fairly watery (stays in motion for 10 seconds or more after stirring) and then flocculating it using Epsom salts until it gels enough to stay in motion for less than 2 seconds.

Thursday 27th November 2014

All that powder with that little water for a casting slip?

This is 8.4L of water (in the bottom of that pail) and a 20kg bag of Polar Ice porcelain casting clay. Amazingly enough it is possible to get all that powder into that little bit of water and still have a very fluid slurry for casting. The volume will increase to only 2/3 of this 5 gallon pail. How is this possible? That water has 100 grams of Darvan 7 deflocculant in it, it causes the clay particles to repel each other such that you can make a liquid with only little more water than is in a throwing clay! All it takes is 15 minutes under a good power propeller mixer.

Friday 21st November 2014

How much porcelain flux is too much?

A porcelain mug has pulled slightly oval because of the weight of the handle. This happens in highly vitrified porcelains (e.g. translucent ones). The amount of feldspar or frit in the body determines the degree of maturity, the correct percentage is a balance between enough to get the maximum translucency and hardness but not so much that ware is deforming excessively during firing. This is Plainsman Polar Ice at cone 6, this degree of warp is acceptable and can be compensated for.

Monday 1st December 2014

Two bentonites that should not look this similar

The powders of HPM-20 bentonite (left) and National Standard 325 bentonite (right) fired to cone 6. Both have sintered into a solid mass. The HPM-20 is much more expensive because of the extra grinding done to make it micro-fine (for non-ceramic uses). However, its data sheet shows an Fe2O3 content double that of the National Standard material. That means the latter should be firing to alot lighter color. But they seem very similar.

Saturday 29th November 2014

What is sintering?

Bentonite fired to 1950F in a small crucible. It is sintering, the particles are bonding even though there is no glass development. The powdered mass is behaving as a unit, the cohesive forces holding it together are enough to shrink the entire mass away from the walls of the container. This sintering process continues slowly, beginning around 1650. Most raw bentonites, this is National Standard 200 mesh, have a fairly low melting point, this will begin to fuse soon.

Friday 22nd July 2011


These posts are actually pictures referenced on pages in The Digitalfire Reference Database, thousands of pages of explaining things you need to know to formulate, adjust and troubleshoot traditional ceramic bodies and glazes. It is organized as: Oxides, minerals, materials, recipes, articles, glossary, hazards, library, MDTs for INSIGHT, pictures, properties, firing schedules, significant temperatures, tests and troubleshooting. Level 2 desktop INSIGHT and Insight-Live both interact with it.

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