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A Low Cost Tester of Glaze Melt Fluidity
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

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

Volcanic Ash


Joseph Herbert overviews the technical and practical aspects of this interesting group of materials


Joseph Herbert   

This is material I posted in 1996 in reference to a thread about Mt. St., Helen ash, with some additions about specific rock compositions. I apologize to veteran readers, as necessary.

Using the word "ash" for the material that is expelled during volcanic eruptions is misleading and not helpful when thinking about its composition or origin. I suppose the idea of ash, as in wood ash, is left from the times when volcanoes were thought of as chimneys from inside the earth. It was obvious to the ancients that there was definitely smoke there, likely fire there, and so the material that rained down, being a gray, powdery material, was ash. (insert appropriate Latin phrase)

The eruption of oceanic, basaltic volcanoes is relatively simple and benign - as these things go - and are relatively easy to study. The eruptions usually go on for a long time, have "predictable" phases, and rarely do astonishing things. The molten rock material involved in these eruptions is similar to basalt in composition. It is silica poor and very fluid. While all erupting volcanoes emit lots of gas, the presence of the gas dissolved in the liquid rock is not as important in these eruptions as in some others.

The molten rock material in an oceanic volcano often forms a lake inside a crater or cauldera and runs back down into the vent from time to time. Lots of mixing. Sometimes, the wall of the crater is breached and the contents of the lake flows down the side of the mountain in a spectacular river of liquid rock that is some 1700 degrees F or more in temperature. It may be that this material is in the process of crystallization with crystals of the higher temperature minerals forming while the material flows. There might be some crystals of olivine and high calcium plagioclase in the liquid rock as it goes toward the sea. Other times the side of the mountain fails in a fissure and the contents of the lake runs out through the crack. This kind of eruption is dangerous because the fissure may be far from the crest of the mountain and a large volume of liquid rock is released in an unexpected place very quickly.

In contrast, the eruption of volcanoes that produce other kinds of rock takes a different and more violent course. Specifically, volcanoes that erupt material that is relatively rich in silica behave very differently. The presence of large amounts of silica in molten rock makes the material very viscous - it does not flow out during an eruption. The molten material does move inside the earth where the temperatures are higher and, more importantly, the amount of dissolved gas is greater. The dissolved gas, most of which is water, is the key to the behavior of continental volcanoes like Mt. St. Helens and the volcanoes in the Andes mountains.

Before ending the story, I would like to try to amplify on the difference in viscosity of the two kinds of melted rocks. A non-oceanic volcano in South America erupted (over the course of several days) a spine of molten rock that extended straight up 1300 feet from the vent where the eruption originated. This spine glowed red, and was a very viscous liquid - when the exterior of it solidified, pieces broke off and fell. A spectacular sight.

When a volcano that erupts a high silica material starts to erupt a couple of things happen rather suddenly. The gas rich liquid material melts its way up into the area below the volcano s cone. At some point, the material encounters a zone of weakness and the state of the material in the ground begins to change. First gas is released into the zone of weakness. This is actually a foaming process because the gas is dissolved in the liquid rock and when the pressure is released, the liquid foams. This is exactly the process that happens when a bottle of warm soda is opened. Now, an interesting thing happens, the rock that was flowing because of the dissolved gas becomes stiffer because the gas has left it. So the liquid that flowed well enough to form a bubble, suddenly stiffens once the bubble is formed. The gas is still expanding and soon the bubble breaks. The pieces of the broken bubbles of the foaming liquid rock are carried out of the volcano with the gas. This is volcanic ash.

In the case of Mt. St. Helens, the landslide that removed part of the mountain s summit relieved the restraining pressure on a large mass of liquid rock and all of it foamed up and blew out at once. The chemical composition of the contents of the magma chamber depends on the original composition of the melted rock, the changes in composition as it melted previously erupted material, and what ever removal of material there might have been by loss of early forming crystals. Because this kind of rock is not very liquid when liquid, there could be some variation in composition between the top and bottom of the rock mass. However, as this mass of material was being broken up and spewed into the air, there was mixing. I would be surprised to find large differences in composition of fallen ash from place to place. I have not, however, done any research that might injure that particular prejudice.

As a practical matter, erupted volcanic ash from silica rich volcanoes is composed of glass shards of various sizes. It is bad to breath them and they do settle rapidly in water. The glass may not be a very good glass and will probably leach soluble materials rather easily, especially when you consider the surface area available in fine powders. When left in the ground long enough, the "ash" turns to Bentonite clay. It is possible that some of the Bentonite used in yesterdays glaze batch could be traced to a particular volcanic eruption a few million years ago. Perhaps the famous Mount Mizuma, now Crater Lake, contributed that to your glaze.

Here is an additional story about the composition of volcanic emissions. There was a supposed geologist who observed the eruption (small scale) of lava at an African volcano and rushed to collect a sample of the just solidified lava. The sample was sent off to whatever American university home was and waited there for a while. Some time later, the geologist decided to analyze his rock sample, starting with a good soak and wash. He never got to the wash because the entire rock dissolved during the soak. The rock that had been erupted as a liquid and collected as a solidified specimen was Sodium Carbonate, common washing soda. While I cannot document the story, this supposedly took place in the part of Africa where some deep lakes contain so much dissolved carbon dioxide that when they "turn over" people and animals are killed by the smothering cloud of carbon dioxide that flows away from the lakes. This indicates a large amount of carbon dioxide in the water and, by extension, in the subsurface in general. If the amount of carbon dioxide dissolved in the melted rock below ground, there really isn t a reason that sodium carbonate might not form as a melted material.

What this means to the a glaze discussion is that erupted material can have about any rock composition. Some are much rarer than others, but a wide range exists. The material that is available to any of us at the local feed, glaze, and video store depends on factors like transportation and marketing rather than composition. As we have all noted, shipping pottery raw materials, which are all heavy and mostly used in large quantities, is expensive. In days of old, before the petroleum genie sprang from the bottle, potters went where the materials were or made do with the materials at hand.

To speak more specifically about actual rock, and potential volcanic ash, compositions I have prepared a group of unity formulae for various rocks. Except for the granite, any of these could be an extrusive, that is volcanic, rock. If the table doesn't come out well, silica content increases by nearly a factor of three while the alumina content only increases 50 percent. The total iron is about equal to the fluxes in the basalt while it is 0.2 to 0.4 in the granite type rocks. Magnesia, which I did not include in the flux unity goes from half of unity to 0.03 from basalt to rhyolite. Potassium increases about 5 times, sodium about doubles, and calcium decreases by a factor of three.

Column 1 Olivine Basalt Avg. Unity
Column 2 Tholeiite Basalt Avg. Unity
column 3 Andesite Unity
Column 4 Trachyte, New South Wales Unity.
column 5 Dacite, Mt. Hood, Unity
column 6 Granite, Skye, Inner Hebrides, Unity
column 7 Avg. Rhyolite, Taupo volcano, Unity

       1    2    3    4    5    6    7
SiO2  3.13 3.45 3.98 4.61 5.79 8.02 8.24
Al2O3 1.01 1.19 1.32 1.18 1.37 1.46 1.47
Fe2O3 0.28 0.14 0.28 0.10 0.22 0.15 0.10
FeO   0.63 0.48 0.27 0.23 0.21 0.18 0.10
MgO   0.54 0.50 0.24 0.04 0.23 0.06 0.03
CaO   0.72 0.80 0.59 0.14 0.57 0.13 0.18
Na2O  0.21 0.19 0.27 0.45 0.35 0.32 0.47
K2O   0.07 0.01 0.14 0.41 0.08 0.55 0.35

What this all means is that a volcanic rock, which includes volcanic ash, can have almost any composition - within a very wide range. The rocks included in my table run across the range of commonly found volcanic rocks. So, if you are making a glaze recipe using volcanic ash, be sure of your source. You must know that the volcanic material for your next batch is from the same layer as was your last. The composition of material erupted from a single volcano can change over time. Since most of the mined products cut vertically across deposits that were made at successive times, the mining process mixes material of different composition in ways you may not know about. If you are spending time adjusting an ash recipe, you might want to collect the ash yourself and note the location very carefully - maps, photos, markers. Memory fades and vegetation can fool you.

Related Information


Materials Mt. St. Helens Ash
Materials Navajo Pumice
Materials Volcanic Ash
Powdered volcanic ash is used to make ceramic glazes, some varieties have a chemistry similar enough to glaze that high percentages can be employed.
By Tony Hansen
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