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A Low Cost Tester of Glaze Melt Fluidity

Description

This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.

Article

There are many complex and expensive instruments designed to observe and measure the goings-on in firing kilns. Generally this type of equipment is expensive and measures absolute physical properties that can be quantified easily. However glaze melt flow is like clay plasticity, it is more subjective and not so easy to quantify. It is best measured comparatively, that is, one specimen directly compared with another. That being said, each glaze does have a unique melt flow patterns over a range of temperatures, these can be like a finger print. Melt fluidity testing can be done using inexpensive methods and devices.

I would like to submit a general purpose testing method for many glaze melt properties that is both inexpensive and easy to use: The GLFL test. So many factors related to the melting, solidification and physical properties and defects of fired glaze surfaces are related to melt viscosity. Thus a test that provides information about this has the potential of being very valuable.

Before going on, I will give credit where credit is due. This is not an original idea. I have seen this device described in industry literature to compare melt properties of nepheline syenite and feldspar. Also, I was sent a very nice dual-flow mold by Hugh Nile at Sterling China (it had the initials IMC embossed on it). I am aware that other industries also use similar devices. However I want to take it to the next level by clearing documenting its advantages and a procedure to use it.

I have made a rubber master mold of the one described herein and can making working molds for others. If you would like one please see the bottom of this article.

Testers that do not work well

Small or steep angle testers: Although I have messed with smaller sizes in the past I have now seen the light. They just do not work as well. You need a large enough reservoir, and long enough flow ramp at a shallow enough angle to get repeatable and sensitive tests.

Inclined tile testers: Some companies prepare a lump of the glaze to be tested and glue it to one end of a tile using a slurry made from the same material. While this will often work it is problematic with compounds that shrink a lot or those lacking dry hardness. The former could crack off and the latter may crumble off. I'll leave it to your imagination what might happen if pieces or the whole sample rolls into contact with a kiln element.

The Dual-Flow Large Tester

This is shown in the picture. It is 13.5cm high while standing (5.5 inches). The long runway is at less than a 45 degree angle for extra sensitivity (there are actually two orientations for two different angles). One of the big advantages of the dual tester is that it can be employed for side-by-side testing of two specimens (e.g. one alongside a benchmark). It is amazing how close you can match the melt fluidity of two materials using this method.

This device is cast in a plaster mold using a mix refractory enough to resist warping if walls are cast thin (in production situations flow testers should be made from the same clay that ware is made from but if such is too vitreous you can reduce the feldspar content somewhat. See below for more information on the slip recipe. I usually bisque fire these testers for extra strength. The reservoir accepts a 10-12 gram ball of material that you can just drop right in. These balls are easy to make by dewatering the glaze or material slurry on a plaster surface to the right working consistency and then rolling the ball in your hands, drying it and shaving material off to achieve the right weight. (thus the glaze does need to have enough plastic ingredients to enable this workability or you need to add some bentonite to impart it).

I have defined a procedure for this test in the testing area of this site. As noted in the procedure there, for repeatable results it is important that your testers be the same thickness, made from the same clay, fired at the same rate of rise and to the same temperature, and the ball sample must be the same dry weight each time.

In case you are not yet clear on how this tester is used: Two glazes are compared by dropping dried balls of each into the reservoirs at the top and the whole thing is fired to the desired temperature (with a tile below to catch any glaze that runs right off the end of the runway). During the firing, the glazes flow down the runway according to melt development, melt surface tension (and other factors like bubble development).

What this tester can show you about glazes:

• If glaze ingredients shift in particle size or chemistry and thus change the melt, it will be immediately evident either by the flow reaching further down the runway or by a change in the character of the flow. This information is valuable in quality situations since it is so hard to guage glaze fluidity by simple observation of a normal thin layer on glazed ware or a test tile.
• Information from a flow tester is valuable when adjusting the recipe of a glaze for other things like thermal expansion, color, material substitution for no-longer-available materials, etc. while trying to maintain the same fluidity.
• This test helps to rationalize discrepancies between between what the chemistry of a glaze indicates should happen and what actually does happen in the melt. Often glazes of very similar chemistry will have different flow properties due to factors related to mineralogy and physical properties of materials. For example, the tester shown here is two glazes with the same chemistry, one sources CaO from calcium carbonate, the other from wollastonite.
• Often there is a need to maximize the amount of SiO₂ in a glaze. For example, this test can demonstrate if changes made in the chemistry of a glossy glaze to reduce its thermal expansion also produce a more fluid melt. If so, the SiO₂ can be increased, producing an even lower expansion. Likewise, this test will demonstrate if more silica can be tolerated if adjustments to produce more hardness also yield more fluidity.
• A flow test does not just show the degree to which the glass is melting, it also reveals the surface tension. Melts of high surface tension meet the tester are 90 degree angles whereas those of low surface tension spread out. The better the melting of a high surface tension glass the narrower the flow will be. In transparent glazes the flow will be white because it is not releasing the entrained bubbles. Very low surface tension melts meander down the runway in a thin layer. Knowing about this can really help analyze the cause of crawling, blistering and clouding problems.
• Ball milling time: By extracting samples from your mill at regular intervals, firing, and comparing the degree of flow you will be able to assess the mill's effect on glaze maturity and melt development.
• Because the glaze is so thick in a flow tester, bubbles resulting from products of decomposition within the glaze will be evident by the character of the thick flow and in the broken cross section (bubbles can even disrupt the melt flow).
• The glazes ability to wet the surface of the clay is evident by the angle at which the leading edge and sides of the flow meet the runway surface.
• These testers are great educational tools. This one, for example, shows the impact that a simple addition of opacifier has on glaze flow.
• Changes in properties like opacity or tendency to crawl, blister, pinhole, crystallize, craze or shiver, develop entrained bubbles or boron-blue clouding are often amplified by this test. The flow provides for an opportunity to see a very thick layer of your glaze and this can reveal differences not noted in thinner layers. Glazes which do not necessarily run on ware may run very badly on a flow tester, this indicates a lack of SiO₂ and Al₂O₃ and is a warning for susceptibility to cutlery marking and leaching. Since so many glaze defects are either related to melt viscosity or revealed by a thick flow, monitoring this property is important.
• This device can even be used to help determine the optimal firing temperature, by experience you will know the fluidity of glazes that perform well. In addition firing a glaze in a flow tester at a range of temperatures may reveal that fluidity begins to increase more quickly through a narrow temperature range. After all, what is more significant to determine the freeze-point than flow of the glaze melt?
• This test is ideal for the development projects of under and overglaze colors. These need to be less fluid than glazes. Different stains require differing amounts of flux in the host to get the same degree of flow, using this type of test a line of products can be made that all have the same melt flow properties.
• The character of the melt flow, especially when compared over a range of temperatures, is unique to each frit. It is possible to identify them and even spot equivalent products from other manufacturers by using this test.

Raw Materials Testing

Most companies can readily test clay materials for use in bodies and glazes using physical testing methods that require a minimum of equipment. But it is not so obvious how to compare and test fluxing materials like feldspar for consistency. One can just trust the particle size and chemistry information provided by the manufacturer for each shipment and compare numbers. But what is the actual relationship between these numbers and the consistency of product on a production line? Can you trust the numbers anyway? The tester is an elegant simple alternative. It accurately shows melting power, color and impurities, you need to see two feldspars side-by-side to see how sensitive it is (see pictures at bottom for an example).

Product Development

Many ceramic products are tuned to melt to a certain extent to achieve their function. For example, an engobe needs to have a stiffer melt than a glaze, but much more maturity than the underlying body. Likewise, a ceramic printing ink must have a specific degree of melt fluidity, enough to adhere or melt to a smooth hard surface, but not so much as to bleed into the covering or underlying glaze. Melts used for bonding purposes likewise need to develop enough glass to bond, but not so much that fired geometry cannot be maintained. A standard and a test can be evaluated side-by-side using this tester. If the melt is not fluid enough, then it can be fired higher, or a percentage of frit can be added.

Taking Photos

Since these fired testers are quite large, storing them for future reference can be a problem. Taking a picture of them and scanning it onto the computer for archival purposes makes more sense. Make them at least twice as large as the ones shown here and they should still take less than 100kb of memory. You may find that making the testers from an off-white, grey or even tan body might be better to prevent washed-out results when taking photos. Also, have plenty of side lighting so that gloss is highlighted.

Slip Recipe

A good starting recipe is #L2540, it is 50% ball clay, 25% feldspar and 25% silica. This does not cast quickly but the pieces have good green strength and the clay will vitrify around cone 10-11. For a more refractory mix replace some of the feldspar with kyanite, calcined alumina or some other non-plastic high temperature material. You will need to know how to mix and deflocculate a clay slip, search in this library for the word "deflocculation" for an excellent article on understanding the casting slip mixing process.

Getting a Tester

We have provided detail pictures of our mold so you can make your own. We are planning to add 3D geometry to enable printing plastic shell molds for rubber masters (from which you can pour working plastic molds). The GBMF test is an alternative to this one, although not as accurate.

Related Information

Form a ceramic glaze into balls? Why would you want to do that?

The vast majority of glazes are plastic (but less than clay bodies). They can be dewatered on a plaster surface and formed. Why do this? To make 9-10 gram balls and fire them on flat tiles (or inclined flow testers) to see their melting characteristics. It is surprising how much this can tell you about the glaze. To make the ball, mix the slurry well and pour a little on the plaster. It should dewater in less than 30 seconds. As soon as the water sheen is gone, scrape it up with a rubber rib, hand-knead it and flatten it back down to dry a little more if needed (leave it only for five or ten seconds and rework it. Repeat until it is stiff enough to form balls of about 12 grams. Stamp them with ID numbers and dry them.

Flow tester original plaster model angle view

This is one of multiple views of the solid plastic original model of a glaze melt fluidity tester.

G1215U vs. G1215W glaze flow test

These recipes have the same chemistry but the 1215U uses frit to source the MgO and CaO. This demonstrates that it is not just chemistry that determines melt flow. Raw materials are crystalline and have different melting patterns than frits (which have already been melted and reground).

Does Zircon only whiten and opacify a clear glaze? No.

This GLFL test for melt flow demonstrates how zircon opacifies but also stiffens a glaze melt at cone 6. Zircon also hardens many glazes, even if used in smaller amounts than will opacify.

Glaze flow tester mold (three pieces). Piece on the left goes on top.

Glaze melt flow tester after casting

Severely 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.

How runs of Alberta Slip are compared in production testing

These are two runs of Alberta slip (plus 20% frit 3134) in a GLFL test to compare melt flow at cone 6.

Melt flow tester used to compare feldspars

Fired to cone 10 oxidation. Although feldspar is a key melter in high and medium temperature glazes, by itself it does not melt as much as one might expect in this GLFL test. The Montana materials on the right are not commercially available, they were being evaluated for viability.

Custer feldspar vs. G200 feldspar

This is a melt fluidity test fired to cone 10. By themselves, feldspar melts surprisingly less than you might think at cone 10.

Feldspars, the primary high temperature flux, melt less than you think.

A cone 8 comparative flow tests of Custer, G-200 and i-minerals high soda and high potassium feldspars. Notice how little the pure materials are moving (bottom), even though they are fired to cone 11. In addition, the sodium feldspars move better than the potassium ones. But feldspars do their real fluxing work when they can interact with other materials. Notice how well they flow with only 10% frit added (top), even though they are being fired three cones lower.

L3617 Cornwall Stone substitute vs. real Cornwall Stone

These flow tests demonstrate how similar the substitute recipe (left) is to the real material (right). 20% Frit 3134 has been added to each to enable better melting at cone 5 (they do not flow even at cone 11 without the frit). Links below provide the recipe for the substitute and outline the method of how it was derived using Digitalfire Insight software. This substitute is chemically equivalent to what we feel is the best average for the chemistry of Cornwall Stone.

Flow tester master model showing dimensions

This is one of multiple views of the solid plastic original model of a glaze melt fluidity tester.

Checking new glazes using a melt fluidity test

This is an example of how useful a flow tester can be to check new glaze recipes before putting them on ware and into your kiln. This was fired to only cone 4, yet that fritted glaze on the left is completely over-melted. The other one is not doing anything at all. These balls are easy to make, you only need weigh out a 50 gram batch of glaze, screen it, then pour it on a plaster bat until it is dewatered enough to be plastic enough to roll these 10 gram balls.

Comparing the brands of calcium carbonate

A cone 6 GLFL test for melt flow to compare two calcium carbonates (they make up 27% of this glaze recipe that was designed to maximize their percentage). Notice the amount of bubbles (due to the high loss on ignition of the material). Different brand-names of the material obviously have slightly different chemistries so they exhibit different flow properties during firing.

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.

Fusion 360 drawing for glaze melt flow tester is available

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.

Flow tester tells me if I have overfluxed the glaze

This is important because I am searching for a balance between the degree of melt fluidity of my original crazing glaze but having a thermal expansion to fit my porcelain (this is G3806E and F). With each adjustment to the chemistry to drop the thermal expansion I do a firing to compare the melt fluidity with the previous iteration. On the right I have too much boron, it’s melting more than I want. And bubbling.

Reverse-engineer a commercial transparent glaze to get the recipe

The commercial cone 04 clear brushing glaze on the left works really well on our bodies so I sent it away to be analyzed (about $130). From that information and using my account at insight-live.com I was able to create a recipe, having the same chemistry, employing two Fusion frits (which amazingly supplied all of the fluxing oxides). In this cone 04 melt fluidity comparison they are almost identical (mine, G3879, has a little more surface tension). The Al₂O₃ and SiO₂ levels would make, even a cone 6 glaze, jealous! So it should be very durable. And it has low thermal expansion (no crazing). With the bucket of dipping-slurry I made I can glaze a piece perfectly evenly in seconds rather than the normal 10 minutes of fiddling with a brush and three coats! I have used it on dozens of pieces, it's amazing. One caution: It is possible to duplicate a transparent glaze like this but not a coloured one (a lab could not analyze the complexities of the color, stain colors are about more than chemistry (firing method, particle physics). For colored glaze you have to do trial-and-error testing with stain additions to this base.

Flow tester measurements 2

Flow tester measurements front side

This is one of multiple views of the solid plastic original model of a glaze melt fluidity tester.

Melt flow tester mold - two main pieces

This is one of multiple views of the mold of a glaze melt fluidity tester.

Assembled melt flow tester mold

This is one of multiple views of the mold of a glaze melt fluidity tester.

Melt fluidity of Albany Slip vs. Alberta Slip at cone 10R

Albany Slip was a pure mined material, Alberta Slip is a recipe of mined materials and refined minerals designed to have the same chemistry, firing behavior and raw physical appearance.

Improving a clear by substituting frit for Gerstley Borate

Melt fluidity test showing Perkins Studio clear recipe original (left) and a reformulated version that sources the boron from Ferro Frit 3134 instead of Gerstley Borate (right). The later is less amber in color (indicating less iron) and it melts to very close to the same degree.

How can you test if a different brand of tin oxide will work?

This is a melt fluidity test comparing two different tin oxides in a cone 6 transparent glaze (Perkins Clear 2). The length, character and color of the flow provide an excellent indication of how similar they are.

This cone 6 transparent looked good, but I still improved it alot

The green boxes show cone 6 Perkins Studio Clear (left) beside an adjustment to it that I am working on (right). I am logged in to my account at insight-live.com. In the recipe on the right, code-numbered G2926A, I am using the calculation tools it provides to substitute Frit 3134 for Gerstley Borate (while maintaining the oxide chemistry). A melt-flow GLFL test comparison of the two (bottom left) shows that the GB version has an amber coloration (from its iron) and that it flows a little more (it has already dripped off). The flow test on the upper left shows G2926A flowing beside PGF1 transparent (a tableware glaze used in industry). Its extra flow indicates that it is too fluid, it can accept some silica. This is very good news because the more silica any glaze can accept the harder, more stable and lower expansion it will be. You might be surprised how much it took, yet still melts to a crystal clear. See the article to find out.

Melt flow comparison between Nepheline Syenite 270 and 400 mesh

This Nepheline Syenite GLFL test for melt flow did not demonstrate much of a difference in melting at cone 9 between 270 and 400 mesh materials.

Links

Media	How I Improved a Popular Cone 6 Clear Glaze Using Insight-Live How I found a ceramic glaze recipe on Facebook, substituted a frit for the Gerstley Borate, added the extra SiO2 it needed and got a fabulous more durable cone 6 clear.
Articles	A Textbook Cone 6 Matte Glaze With Problems Glazes must be completely melted to be functional, hard and strong. Many are not. This compares two glazes to make the difference clear.
Articles	Low Budget Testing of the Raw and Fired Properties of a Glaze There is more to glazes than their visual character, they have other physical properties like hardness, thermal expansion, leachability, chemistry and they exhibit many defects. Here are some simple tests.
Articles	Reducing the Firing Temperature of a Glaze From Cone 10 to 6 Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.
Articles	Limit Formulas and Target Formulas Glaze chemistries for each type of glaze have a typical look to them that enables us to spot ones that are non-typical. Limit and target formulas are useful to us if we keep in perspective their proper use.
URLs	https://digitalfire.com/uploads/GlazeMeltTester5v1.f3d Melt flow test 5v1 Fusion 360 3D file
Tests	Glaze Melt Fluidity - Ball Test
Tests	Glaze Melt Flow - Runway Test
Glossary	Surface Tension In ceramics, glazes melt to produce a liquid glass. That glass exhibits surface tension and it is important to understand the consequences of that.
Glossary	Ceramic Stain Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.
Glossary	Viscosity In ceramic slurries (especially casting slips, but also glazes) the degree of fluidity of the suspension is important to its performance.
Glossary	Melt Fluidity Ceramic glazes melt and flow according to their chemistry and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.
Projects	Tests
Projects	Temperatures
Projects	Frits
Projects	Troubles
Materials	Feldspar

By Tony Hansen

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