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•The secret to know what to test is material and chemistry knowledge.
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LOI


Simplistically, LOI is the percentage of weight a material loses on firing. Assuming firing to a typical stoneware temperature of 1200C, the amount of weight loss can be surprising. Kaolins, for example, lose around 12% (mainly crystal-bound water). Ball clays lose about half of that (a combination of crystal water, organic carbon and sulfur). Many raw stoneware, earthenware and fireclays can have high sulfur content (high enough to create a strong odor during firing). Carbonates decompose and gas off CO2. Dolomite and calcium carbonate lose over 40% of their weight and copper carbonate is not far behind. Barium carbonate loses 20% plus and lithium carbonate a whopping 60%. Gerstley Borate and Colemanite are other high weight losers (20%+). Even feldspars lose some weight (usually less than 1%). LOI can also be other things (e.g. chlorine, oxygen, fluorine; the latter might break down quickly in one use but linger in a melting glaze or frit).

Each of these materials has its own thermal history, losing weight at one or more temperatures as it decomposes with increasing heat (some materials can actually qain weight at certain temperatures; non-oxide materials, for example, capture oxygen from the atmosphere or other materials). LOI is a big problem in firing of many types of ware, and must by minimized. Some of these materials can be calcined for some uses (e.g. some of the kaolin in glazes). Others cannot be calcined because their raw properties are needed (e.g. plasticity of clay) or they become unstable (e.g. carbonates). The oxides of others (needed for glazes) can be sourced from frits (e.g. boron). Lower carbon ball clays can be used in bodies. Stains can be employed instead of metallic carbonate colors. Calcia and magnesia for glazes can be sourced from wollastonite and frits instead of calcium carbonate or dolomite.

Notwithstanding all of this, the mechanisms that produce gases in many of these materials happen at fairly low temperatures and defect-free fired product can actually be done using bodies and glazes having significant LOIs. How? Those who fire kilns concern themselves with the temperatures at which weight loss events occur in the bodies and glazes. Engineers can use instruments (e.g. DTA devices) to create a weight-loss profile of the entire firing range of a body that contains multiple materials having an LOI. Or a simple study of the materials used can produce the same information (although less precise). In periodic kilns the firing curve can be flattened in the zones of high gas generation. Potters have often learned by experience when and where to fire faster or slower.

In many industries where tunnel kilns are used the firing curve is much less flexible. And firing must be done quickly (fast-fire is less than three hours cold-to-cold). Weight loss profile information would liley be used to choose a bisque fire curve and temperature tuned to burn away all volatiles (thus preventing all body gas related defects in the glaze). But in other industries (like tile, brick) ware must be single fired in tunnel kilns. Obviously, technicians have a challenge to produce quality ware, so knowledge of LOI information is doubly important.

Technicians doing chemistry have an entirely different perspective of LOI. In glaze chemistry, we think of LOI as being like the shells thrown away from a bag of nuts (if you need 1 kilo of peanuts that might mean needing to buy 2 kilos of shelled nuts). It is likewise with glaze materials. For example, 100 grams of generic kaolin going into a kiln sources only 87.5 grams of Al2O3 and SiO2 for glass-making. To get 100 grams of SiO2 ane Al2O3 we would need 100/0.875 or 114.3 grams of the raw kaolin powder. Digitalfire desktop Insight used to store materials in its MDT (materials database) as formulas. So generic kaolin, for example, had to compensate by recording a formula weight of 253.9 instead of 222.2 (Insight was able to calculate the 12.5% LOI from that differenxe). Insight-live.com now stores material chemistry as analyses, the LOI is specified as a percentage along with the other oxides.

When the LOI for each material in a recipe is known, it is easy to calculate the LOI of the raw recipe as a whole. Insight-live does that automatially when it displays the chemistry of recipes. Because of this it is possible to use simple procedures (within an Insight-live account) to substitute materials having lower or zero LOIs into glaze recipes without changing the chemistry (different materials source the same mix of oxides). Huge increases in glaze quality can be realized, for example, by sourcing oxides like Li2O, BaO, B2O3 from frits instead of the high-LOI raw materials that supply them.

If you have an analysis lacking an LOI figure, or suspect the accuracy of the analysis delivered by a lab, then you can weigh, fire, and weigh again to derive the actual LOI and compensate the analysis. Following is the mathematical method of applying a 5% measured LOI to an existing analysis which had no specified an LOI.

LOI-Adjusting an Analysis
100 - 95 = 5 / 100 = 0.95
--------------------------
K2O 7.3% x 0.95 = 6.9%
CaO 9.4% x 0.95 = 8.9%
MgO 1.0% x 0.95 = 1.0%
ZnO 1.0% x 0.95 = 1.0%
Al2O3 11.8% x 0.95 = 11.2%
SiO2 69.5% x 0.95 = 66.0%
LOI 5.0%
--------------------------
100.0% 100.0%


If the original already had specified a 2% LOI, for example, then the formula to calculate the multiplier would be:

98 - 95 = 3 / 100 = 0.97

Original glaze with Gerstley Borate vs. fixed version with frit

Original glaze with Gerstley Borate vs. fixed version with frit

These pieces are the same body and fired at the same temperature. The original glaze was found on the internet, and is popular. Materials within it are "farting" as the glaze is melting (they have a high LOI and the calculated LOI of the glaze as a whole is 15%). Unless it is applied very thinly this is the fired result (at cone 03): tons of micro-bubbles. And it is crazing. Using my account at insight-live.com I was able to source the B2O3 and MgO from a frit (actually two frits) and reduce the thermal expansion at the same time. As you can see, the product is a dazzling ultra clear (thick or thin) that fits perfect (it survives a 300F-to-ice-water shock test).

What material makes the tiny bubbles? The big bubbles?

What material makes the tiny bubbles? The big bubbles?

These are two 10 gram balls of Worthington Clear glaze fired at cone 03 on terra cotta tiles (55 Gerstley Borate, 30 kaolin, 20 silica). On the left it contains raw kaolin, on the right calcined kaolin. The clouds of finer bubbles (on the left) are gone from the glaze on the right. That means the kaolin is generating them and the Gerstley Borate the larger bubbles. These are a bane of the terra cotta process. One secret of getting more transparent glazes is to fire to temperature and soak only long enough to even out the temperature, then drop 100F and soak there (I hold it half an hour).

How much gas escapes firing from cone 03 & 04?

How much gas escapes firing from cone 03 & 04?

These were fired to cone 03 (upper) and 04 (lower). At cone 03 the loss in weight is 4.54%, at 04 it is 4.45%. That is 0.08% difference. If a mug weighs 250 grams, that is only 0.21 grams. Does not sound like much. But wait. Air weighs 0.001225g/cc. While this is not the exact weight of the gases escaping during firing it suggests that around 170cc of gases need to bubble up through the glaze if the piece was bisque fired at cone 04 and glaze fired to cone 03.

Examples of DFAC disk, SHAB bar, LOI bar for clay testing

Examples of DFAC disk, SHAB bar, LOI bar for clay testing

By preparing these three tests you can measure many properties of a clay body. These include drying shrinkage, fired shrinkage, porosity, drying performance, soluble salts content, water content and LOI.

Imposing an LOI in INSIGHT

Imposing an LOI in INSIGHT

Desktop Insight can calculate the LOI of a recipe based on the LOIs it knows of the individual materials in the recipe. But sometimes you need to impose an LOI to force a calculated analysis to match an actual measured LOI in the lab.

Desktop INSIGHT MDT dialog showing kaolin LOI

Desktop INSIGHT MDT dialog showing kaolin LOI

The LOI appears below the material name and alternative names (beside the weight). The formula that goes with that LOI is the bold numbers in the blanks beside the oxide names on the right.

Magnesium carbonate vs. oxide: One big difference

Magnesium carbonate vs. oxide: One big difference

Here is a screenshot of side-by-side recipes in my account at insight-live.com. It takes 120 mag carb to source the same amount of MgO as 50 mag ox. I just made the two recipes, went into calculation mode and kept bumping up the magcarb by 5 until the chemistry was the same. Note the LOI of the magcarb version is 40. This one would certainly crawl very badly.

LOI is not important? Think again!

LOI is not important? Think again!

This chart compares the gassing behavior of 6 materials (5 of which are very common in ceramic glazes) as they are fired from 500-1700F. It is a reminder that some late gassers overlap early melters. The LOI (loss on ignition) of these materials can affect your glazes (e.g. bubbles, blisters, pinholes, crawling). Notice that talc is not finished until after 1650F (many glazes have already begin melting by then).

Why is that transparent glaze firing cloudy? The balls test us.

Why is that transparent glaze firing cloudy? The balls test us.

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. Of course, the glaze must have a low enough surface tension to release the bubbles, that is a separate issue.

Melting range is mainly about boron content

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%).

Does this terra cotta clay have an LOI higher than kaolin? No.

Does this terra cotta clay have an LOI higher than kaolin? No.

These two samples demonstrate how different the LOI can be between different clays. The top one is mainly Redart (with a little bentonite and frit), it loses only 4% of its weight when fired to cone 02. The bottom one is New Zealand kaolin, it loses 14% when fired to the same temperature! The top one is vitrified, the bottom one will not vitrify for another 15 cones.

A body containing manganese bubbles the glaze

A body containing manganese bubbles the glaze

Laguna Barnard Slip substitute fired at cone 03 with a Ferro Frit 3195 clear glaze. The very high bubble content is likely because they are adding manganese dioxide to match the MnO in the chemistry of Barnard (it gases alot during firing).

More carbon needs to burn out than you might think!

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.

Orange-peel or pebbly glaze surface. Why?

Orange-peel or pebbly glaze surface. Why?

This is a cone 10 glossy glaze. It should be crystal clear and smooth. But it contains strontium carbonate, talc and calcium carbonate. They produce gases as they decompose, if that gas needs to come out at the wrong time it turns the glaze into a Swiss cheeze of micro bubbles. One solution is to use non-gassing sources of MgO, SrO and CaO. Or, better, do a study to isolate which of these three materials is the problem and it might be possible to adjust the firing to accommodate it. Or, an adjustment could be make to the chemistry of the glaze such that the melting happened later and more vigorously (rather than earlier and more slowly). The latter is actually the likely cause, this glaze contains a small amount of boron frit. Boron melts very early so the glaze is likely already fluid while gases that normally escape before other cone 10 glazes even get started melting are being trapped by this one.

G2931F Ulexite-based transparent bubbles, G2931K frit-based version does not

G2931F Ulexite-based transparent bubbles, G2931K frit-based version does not

I melted these two 9 gram balls on tiles to compare their melting (the chemistry of these is identical, the recipes are different). The Ulexite in the G2931F (left) drives the LOI to more than 14%. That means the while the ulexite is decomposing during melting it is creating gases that are creating bubbles in the glass. Notice the size of the F is greater (because it is full of bubbles). While this seems like a serious problem, in practice the F fires crystal-clear on most ware.

Glaze melt fluidity comparison between G2931F and fritted G2931K show the effect of LOI

Glaze melt fluidity comparison between G2931F and fritted G2931K show the effect of LOI

These two glazes have the same chemistry but different recipes. The F gets its boron from Ulexite, and Ulexite has a high LOI (it generates gases during firing, notice that these gases have affected the downward flow during melting). The frit-based version on the right flows cleanly and contains almost no bubbles. At high and medium temperatures potters seldom have bubble issues with glazes. This is not because they do not occur, it is because the appearance of typical glaze types are not affected by bubbles (and infact are often enhanced by them). But at low temperatures potters usually want to achieve good clarity in transparents and brilliance in a colors, so they find themselves in the same territory as the ceramic industry. An important way to do this is by using more frits (and the right firing schedules).

Insight-Live comparing a glossy and matte cone 6 base glaze recipe

Insight-Live comparing a glossy and matte cone 6 base glaze recipe

Insight-live is calculating the unity formula and mole% formula for the two glazes. Notice how different the formula and mole% are for each (the former compares relative numbers of molecules, the latter their weights). The predominant oxides are very different. The calculation is accurate because all materials in the recipe are linked (clickable to view to the right). Notice the Si:Al Ratio: The matte is much lower. Notice the calculated thermal expansion: The matte is much lower because of its high levels of MgO (low expansion) and low levels of KNaO (high expansion). Notice the LOI: The matte is much higher because it contains significant dolomite.

Decomposing manganese granular particles in a buff stoneware causing it to bloat

Decomposing manganese granular particles in a buff stoneware causing it to bloat

A cone 6 stoneware with 0.3% 60/80 mesh manganese granular (Plainsman M340). Fired from cone 4 (bottom) to cone 8 (top). It is normally stable to cone 8, with the manganese it begins to bloat at cone 7. The particles of manganese generate gases as they decompose and melt, these produce volumes and pressures sufficiently suddenly that closing channels within the maturing body are unable to vent them out.

Bubble city with Seattle Midnight body and G3806C glaze at cone 01, 4 and 5

Bubble city with Seattle Midnight body and G3806C glaze at cone 01, 4 and 5

This black body is made by Seattle Pottery Supply. Surfaces like this are obviously not functional, but for decorative ware? Yes! How does this happen? This body contains a material that is adding to its LOI (likely raw or burnt umber). Not just that, but the gases are being expelled at the wrong time. How is that? The glaze is fluid at cone 6 and begins melting way down around cone 04. It is melting long before the gases of decomposition from the body are finished being expelled. So they have to bubble up through the glaze, creating the effect you see here. This body is actually over-mature and brittle at cone 5, but at cone 01 its strength is fairly good.

Out Bound Links

  • (Articles) Organic Matter in Clays: Detailed Overview

    A detailed look at what materials contain organics, what its effects are in firing (e.g. black core), what to do to deal with the problem and how to measure the amount of organics in a clay material.

  • (Glossary) Unity Formula

    A "unity formula" is just a formula that has been retotaled so that the RO group of oxides totals one. This is also called a Seger formula and this standard provides one basis for comparing glazes. The three column format of expressing a formula was first used by Hermann Seger. The unity is normally...

  • (Glossary) Mole%

    Mole% is a way of expressing the oxide formula of a fired glaze or glass (technicians can extrapolate fired properties like melting temperature, thermal expansion, hardness, resistance to leaching, etc. by examining the chemistry of a glaze). Mole% is preferred over the Seger unity formula by many t...

  • (Glossary) Analysis

    Conceptually we consider fired glazes as being composed of 'oxides'. Materials supply those oxides to the melt. The ten major oxides likely make up 99% of all base glazes (and materials we use). The oxide formula of a glaze "explains" many details about the way the glaze fires. Thus we can view m...

  • (Hazards) Sulfur Dioxide Toxicity

    - UnDescribed

  • (Articles) Glaze Chemistry Basics - Formula, Analysis, Mole%, Unity, LOI

    Part of changing your viewpoint of glazes, from a collection of materials to a collection of oxides, is learning what a formula and analysis are, how conversion between the two is done and how unity and LOI impact this.

  • (Articles) How INSIGHT Deals With Unity, LOI and Formula Weight

    INSIGHT enables you to enter material analyses as recipes as a first step to inserting them into the materials database. Imposing an LOI and understanding how to set unity and its connection for formula weight are important concepts.

  • (Glossary) Colloid

    Colloidal particles are so small and light that they do not settle in water. Milk is colloidal. In true colloidal suspensions the movement of water molecules is enough to keep them from settling. Bentonite contains colloidal particles, but it also contains larger ones which also stay in suspension. ...

  • (Tests) LDW - LOI/Density/Water Content

In Bound Links

  • (Glossary) Carbon Burnout

    Ceramic bodies and glazes contain materials that release carbon as they decompose on heating. Clays, gums, plasticizers are examples. These can still be burning out at higher temperatures than most people realize (cone 04 or higher). That burning generates gases, if glazes are beginning to melt befo...

  • (Glossary) Formula Weight

    Quite simply, the weight of a formula. Typically, in glaze chemistry, when we refer to formula weight it is assumed we are talking about the weight of the fired formula of a glaze (without LOI and volatiles). However is is possible to also talk about the formula weight of a material (although materi...

  • (Project) Stains

    We make no attempt to classify or compile stains available here, there are too many. Individual stain manufacturers offer huge ranges of different colors and color systems (the same color can often be...

  • (Videos) Formulating Substitution Rules for Calcium Carbonate and Wollastonite

    How to use Digitalfire Insight software to determine how much wollastonite to substitute for whiting to maintain the same SiO2 in the glaze. Covers rationalization of the relative merits of materials,...

  • (Glossary) Glaze Bubbles

    As glazes melt, gases from decomposition of organics, carbonates, sulphates and hydrates are generated (if the body was glazed green, or unbisqued, many more of these gases will be present). If glazes are already melting while the gases are being generated, bubbles form and suspend in the glass melt...

  • (Glossary) Si:Al Ratio

    Conceptually we consider fired glazes as having a structure of oxides held together by molecular bonds. Ten major oxides likely make up 98% of all base glazes. Each oxide contributes specific characteristics to the glass and they interact in predictable ways. By rationalizing their absolute values a...

  • (Glossary) Blisters

    Glaze blisters are a surface defect in fired ceramic glazes. They have caused every potter and company grief at one time or another. The problem is often erratic in nature and counter intuitive measures may be needed to resolve it (we have a page in the trouble-shooting section dedicated to this def...

  • (Articles) Firing: What Happens to Ceramic Ware in a Firing Kiln

    Understanding more about changes are taking place in the ware at each stage of a firing and you can tune the curve and atmosphere to produce better ware

  • (Glossary) Oxidation

    A firing where the atmosphere inside the kiln has sufficient supplies of oxygen to react with the glaze and clay body surfaces (and thus produce the colors characteristic of this). Electric kilns are synonymous with oxidation firing. However these kilns lack the air flow of their gas counterparts. S...


By Tony Hansen




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