|Monthly Tech-Tip |
Loss on Ignition is a number that appears on the data sheets of ceramic materials. It refers to the amount of weight the material loses as it decomposes to release water vapor and various gases during firing.
Simplistically, LOI is the percentage of weight a ceramic material loses on firing. Thus, glaze materials have an LOI, but the fired glaze does not. 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 sulphur). Many raw stoneware, earthenware and fireclays can have high sulphur 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%+ 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 and 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 difference). Insight-live.com now stores material chemistries 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 automatically 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
This chart compares the decompositional gassing behavior of six materials as they are heated through the range 500-1700F. These materials are common in ceramic glazes, it is amazing that some can lose 40%, or even 50%, of their weight on firing. For example, 100 grams of calcium carbonate will generate 45 grams of CO2! This chart is a reminder that some late gassers overlap early melters. That is a problem. The LOI (% weight loss) of these materials can affect your glazes (causing bubbles, blisters, pinholes, crawling). Notice talc: It is not finished gassing until 1650F, yet many glazes have already begun melting by then (especially fritted ones). Even Gerstley Borate, a raw material, is beginning to melt while talc is barely finished gassing. And, there are lots of others that also create gases as they decompose during glaze melting (e.g. clays, carbonates, dioxides).
Fired at cone 6. The samples on the bottom tiles are from ten-gram balls that have melted down. These glazes have the same chemistry, but the one of the left sources its B2O3 from Gerstley Borate (which has a high LOI). The one on the right gets it from a frit. Because the fritted version has less gases of decomposition to expel, the glass is much smoother. Curiously, the fritted version is flowing less and the red color has been lost. Why? This could be because the Al2O3, which stabilizes glazes against excessive fluidity, is being dissolved into the melt better and more available for glass building.
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).
These are two 10 gram GBMF test 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).
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.
G1916Q and J low fire ultra-clear glazes (contain Ferro Frit 3195, 3110 and clay) fired across the range of 1650 to 2000F (these were 10 gram GBMF test 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.
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).
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.
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%).
These two samples demonstrate how different the LOI can be between different clay minerals. 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.
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.
The smooth surface of this blistering glaze has been ground off to reveal how serious the bubble problem really is. If the body or glaze itself is generating gases of decomposition at the wrong time, and the glaze has too little melt fluidity to pass the bubbles, this can happen. Opacifiers (especially zircon) and stains can reduce melt fluidity dramatically. Even glossy smooth glazes can have bubbles lurking just below the surface, wear and tear on a piece can open them up, creating micro holes that roughen the surface.
I melted these two 9 gram GBMF test 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.
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.
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.
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.
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.
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 IWCT test).
Both pieces are the same clay, same glaze. The one on the left went to cone 4. Notice how full of holes and bubbles the glaze is. The one on the right went to cone 6 using the C6DHSC firing schedule. It is perfectly smooth and glassy.
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.
|Oxides||LOI - Loss on Ignition|
Blistering is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production. And be difficult to solve.
Ceramic materials, especially clays, often contain carbon and organic compounds. When they are fired in a kiln, these must burn out, often producing complications.
The ratios of individual or group oxide molecule numbers are indicators of things like fired gloss, durability, melting temperature, balance, tendency to craze, etc.
In ceramics, this term is most often used to refer to kilns firing with an atmosphere having available oxygen to react with glaze and body surfaces during firing
This term refers to critical thinking ability that potters and technicians can develop to recognize recipes having obvious issues and merit, simply by seeing the materials and percentages.
In ceramics some clays of are of such exceedingly small particle sizes that they can stay in suspension in water indefinitely. But unlike common colloids, clays have a secret weapon.
The chemistry of ceramic glazes are normally expressed as formulas. A unity formula has been retotaled to make the numbers of flux oxides total one.
Mole% is a way of expressing the oxide formula of a fired glaze or glass. It is a preferred over the formula by many technicians who use glaze or glass chemistry.
In ceramics, raw material chemistry is expressed a chemical analyses. This is in contrast to fired glaze chemistries which are expressed as oxide formulas.
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them.
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
How desktop INSIGHT Deals With Unity, LOI and Formula Weight
INSIGHT enables you to enter material analyses as recipes. This is a first step to inserting them into the materials database. Imposing an LOI and understanding how to set unity, and its connection with formula weight are important concepts.
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.
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.
Sulfur Dioxide Toxicity
This gas can be produced when clays are fired in a kiln. What can you do?
Creating Rules for Calcium Carbonate - Wollastonite Substitution
How to use Digitalfire Insight software to determine how much wollastonite to add and silica to remove to substitute for calcium carbonate in the glaze. Create substitution rules.