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A One-speed Lab or Studio Slurry Mixer
A Textbook Cone 6 Matte Glaze With Problems
Adjusting Glaze Expansion by Calculation to Solve Shivering
Alberta Slip, 20 Years of Substitution for Albany Slip
An Overview of Ceramic Stains
Are You in Control of Your Production Process?
Are Your Glazes Food Safe or are They Leachable?
Attack on Glass: Corrosion Attack Mechanisms
Ball Milling Glazes, Bodies, Engobes
Binders for Ceramic Bodies
Bringing Out the Big Guns in Craze Control: MgO (G1215U)
Can We Help You Fix a Specific Problem?
Ceramic Glazes Today
Ceramic Material Nomenclature
Ceramic Tile Clay Body Formulation
Changing Our View of Glazes
Chemistry vs. Matrix Blending to Create Glazes from Native Materials
Concentrate on One Good Glaze
Copper Red Glazes
Crazing and Bacteria: Is There a Hazard?
Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
Creating a Non-Glaze Ceramic Slip or Engobe
Creating Your Own Budget Glaze
Crystal Glazes: Understanding the Process and Materials
Deflocculants: A Detailed Overview
Demonstrating Glaze Fit Issues to Students
Diagnosing a Casting Problem at a Sanitaryware Plant
Drying Ceramics Without Cracks
Duplicating Albany Slip
Duplicating AP Green Fireclay
Electric Hobby Kilns: What You Need to Know
Fighting the Glaze Dragon
Firing Clay Test Bars
Firing: What Happens to Ceramic Ware in a Firing Kiln
First You See It Then You Don't: Raku Glaze Stability
Fixing a glaze that does not stay in suspension
Formulating a body using clays native to your area
Formulating a Clear Glaze Compatible with Chrome-Tin Stains
Formulating a Porcelain
Formulating Ash and Native-Material Glazes
G1214M Cone 5-7 20x5 glossy transparent glaze
G1214W Cone 6 transparent glaze
G1214Z Cone 6 matte glaze
G1916M Cone 06-04 transparent glaze
Getting the Glaze Color You Want: Working With Stains
Glaze and Body Pigments and Stains in the Ceramic Tile Industry
Glaze Chemistry Basics - Formula, Analysis, Mole%, Unity
Glaze chemistry using a frit of approximate analysis
Glaze Recipes: Formulate and Make Your Own Instead
Glaze Types, Formulation and Application in the Tile Industry
Having Your Glaze Tested for Toxic Metal Release
High Gloss Glazes
Hire Us for a 3D Printing Project
How a Material Chemical Analysis is Done
How desktop INSIGHT Deals With Unity, LOI and Formula Weight
How to Find and Test Your Own Native Clays
I have always done it this way!
Inkjet Decoration of Ceramic Tiles
Is Your Fired Ware Safe?
Leaching Cone 6 Glaze Case Study
Limit Formulas and Target Formulas
Low Budget Testing of Ceramic Glazes
Make Your Own Ball Mill Stand
Making Glaze Testing Cones
Monoporosa or Single Fired Wall Tiles
Organic Matter in Clays: Detailed Overview
Outdoor Weather Resistant Ceramics
Painting Glazes Rather Than Dipping or Spraying
Particle Size Distribution of Ceramic Powders
Porcelain Tile, Vitrified Tile
Rationalizing Conflicting Opinions About Plasticity
Ravenscrag Slip is Born
Recylcing Scrap Clay
Reducing the Firing Temperature of a Glaze From Cone 10 to 6
Simple Physical Testing of Clays
Single Fire Glazing
Soluble Salts in Minerals: Detailed Overview
Some Keys to Dealing With Firing Cracks
Stoneware Casting Body Recipes
Substituting Cornwall Stone
Super-Refined Terra Sigillata
The Chemistry, Physics and Manufacturing of Glaze Frits
The Effect of Glaze Fit on Fired Ware Strength
The Four Levels on Which to View Ceramic Glazes
The Majolica Earthenware Process
The Potter's Prayer
The Right Chemistry for a Cone 6 MgO Matte
The Trials of Being the Only Technical Person in the Club
The Whining Stops Here: A Realistic Look at Clay Bodies
Those Unlabelled Bags and Buckets
Tiles and Mosaics for Potters
Toxicity of Firebricks Used in Ovens
Trafficking in Glaze Recipes
Understanding Ceramic Materials
Understanding Ceramic Oxides
Understanding Glaze Slurry Properties
Understanding the Deflocculation Process in Slip Casting
Understanding the Terra Cotta Slip Casting Recipes In North America
Understanding Thermal Expansion in Ceramic Glazes
Unwanted Crystallization in a Cone 6 Glaze
Volcanic Ash
What Determines a Glaze's Firing Temperature?
What is a Mole, Checking Out the Mole
What is the Glaze Dragon?
Where do I start in understanding glazes?
Why Textbook Glazes Are So Difficult
Working with children

A Low Cost Tester of Glaze Melt Fluidity


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


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.

Large and small glaze fluidity flow testers

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:

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).

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

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.

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.

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

The process of dewatering a glaze slurry and making balls

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 (although there are exceptions e.g. glaze with Gerstley Borate). 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.

A bad batch of frit - Glaze manufacturers must deal with this

A frit batch has tested bad

Just as Mother Nature is responsible for variations in natural minerals, commercial frit manufacturers can and do release material with variations. Frits are made of recipes that must be dosed, mixed, smelted, quenched and ground - leaving plenty of room for error in the process. Shown here is an example of a combination of improper mixing and inadequate smelting. Frits can be made from soluble and carbonate materials that off-gas during smelting, leaving a glass of zero LOI. But that did not happen here: This melt fluidity test comparing two batches of the same frit shows a good and faulty one. The volatiles (as exhibited by the bubbles) were not the only problem, a glaze containing 50% of this had severely inadequate melting. Interestingly, the two-pallet batch (according to the marking on the bags) showed the best results at the top of the first pallet and the worst at the bottom of the second. Another load of faulty frit received later showed similar bubbling but was alumina-short (turning our production matte glaze glossy).

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

Albany vs Alberta Slip melt

Albany Slip was a pure mined silty clay that, by itself, melted to a glossy dark brown glaze at cone 10R. By itself it was a tenmoku glaze. Alberta Slip is a recipe of mined clays and refined minerals designed to have the same chemistry, firing behavior and raw physical appearance (but not plasticity). This is a GBMF melt flow test showing them side-by-side.

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.

How to reverse-engineer a commercial transparent glaze

Melt flows of a commercial glaze and my duplicate

The commercial cone 04 clear brushing glaze (on the left) works really well on our clay bodies so I sent it away to be analyzed (about $130). That revealed high Al2O3/SiO2 levels, this explains its resistance to crazing on our clay bodies and, even better, indicates high durability. In my account at I was able source the same chemistry from two Fusion frits (plus a little kaolin and silica). The melt fluidities are almost identical (my G3879 has a little more surface tension). I needed to make a dipping glaze version and chose a method that would produce a thixotropic slurry. One caution: An assay lab cannot analyze the complexities of a colored glaze, instead focus on the base clear and add stains to that. The first two-gallon bucket made saved the development cost plus more! And knowing the recipe made it possible to adjust for even lower thermal expansion. Another plus: I can now make my own low SG or high SG brushing version.

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.

I improved a cone 6 transparent glaze by removing the Gerstley Borate

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 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 (see the G2926B recipe that adds it and switches it to 325 mesh). 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.

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.

Flow tester measurements front side

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

Flow tester master model showing dimensions

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

Flow tester measurements 2

Melt flow tester 3D CAD drawing

3D rendering of Fusion 360 drawing of glaze melt fluidity tester

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. If you would like this 3D file in Fusion 360 and STEP format, it is available in the Files manager in your account.

Assembled melt flow tester mold

This is one of multiple views of the mold 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.

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

L3617 Cornwall Stone substitute vs. real Cornwall Stone

This melt fluidity comparison demonstrates how similar the substitute L3617 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). This substitute is chemically equivalent to what we feel is the best average for the chemistry of Cornwall Stone.


Materials Feldspar
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt.
Tests Glaze Melt Flow - Runway Test
A method of comparing the melt fluidity of two ceramic materials or glazes by racing them down an inclined runway.
Tests Glaze Melt Fluidity - Ball Test
A test where a 10-gram ball of dried glaze is fired on a porcelain tile to study its melt flow, surface character, bubble retention and surface tension.
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.
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 Low Budget Testing of Ceramic Glazes
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 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.
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, particle size and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.
Glossary Ceramic Stain
Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.
Glossary Surface Tension
In ceramics, surface tension is discussed in two contexts: The glaze melt and the glaze suspension. In both, the quality of the glaze surface is impacted.
Projects Tests
Projects Temperatures
Projects Troubles
Projects Frits
Media Remove Gerstley Borate and Improve a Popular Cone 6 Clear Glaze
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.
By Tony Hansen
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