All Glossary

LOI

Loss on Ignition is a number that appears on data sheets test results of ceramic materials and bodies. It refers to the amount of weight lost as it decomposes to release water vapor and various gases during firing.

Key phrases linking here: gas-generating, gassers, loi - Learn more

Details

LOI (Loss on Ignition) is the percentage of weight a ceramic material loses when heated to a specified temperature. It is an important property of ceramic raw materials, clay bodies and glaze recipes. The amount of weight lost can be surprising! Pure kaolinite, for example, loses almost 14% of its weight, calcium carbonate and dolomite more than 40%, barium carbonate about 22% and lithium carbonate almost 60% (these are theoretical values for complete decomposition to the corresponding oxides). Colemanite and Gerstley Borate also have high LOIs. Feldspars, by contrast, typically lose less than 1%. Commercial clays and other raw materials vary according to their mineralogy and impurities. Once a glaze has been fired and its gases expelled, the original LOI no longer applies to it.

These glazes have the same chemistry, but different recipes. The one on the left sources B₂O₃ from Gerstley Borate (which has an LOI). The one on the right from frits (which don't). The bubble clouding is from the decomposition behavior of GB.

Most of this weight loss occurs because of decomposition reactions that release volatiles, commonly water vapour, carbon dioxide and sulphur-containing gases. Some weight loss can also occur through the volatilization of glaze constituents at high temperatures. A material can even gain weight during firing if it takes up oxygen through oxidation. Thus, LOI is a net weight-change measurement, not a direct measurement of the quantity of gases generated. It does not identify which gases are released or the temperatures at which they escape.

A high LOI is not necessarily a problem. Some ceramic materials release most of their gases at relatively low temperatures, while the body is still porous and before the glaze begins melting. Others continue releasing gases at much higher temperatures, when the body is becoming dense or the glaze is beginning to seal the surface. This can cause bloating, bubbles, pinholes and blisters. The timing of gas release is often more important than the total amount. This explains why some bodies and glazes with relatively high LOIs fire without defects, while others having lower LOIs can be troublesome.

Technicians can determine the temperature ranges of weight loss using thermogravimetric analysis (TGA). This produces a weight-loss profile showing how a material's mass changes as temperature rises. A simpler approach is to fire weighed specimens to progressively higher temperatures, then cool and reweigh them to determine cumulative losses. Such tests can reveal the temperature ranges in which significant weight loss occurs. In periodic kilns, firing schedules can be adjusted to provide more time for gases to escape before vitrification or glaze melting becomes significant. Industrial fast-fire and tunnel kilns offer less flexibility, making the selection of raw materials, body formulation and firing profile especially important.

LOI can also be reduced by substituting materials. Some kaolin in a glaze recipe can be [calcined](https://digitalfire.com/glossary/calcination) to eliminate its chemically bound water. Wollastonite can supply CaO without the high CO₂ loss of calcium carbonate. Frits can supply boron and other oxides without the decomposition gases associated with some raw minerals. However, substitutions must account for the full chemistry and physical properties of the materials involved. Reducing LOI is not always necessary or even desirable, especially if it compromises plasticity, suspension properties, melting behaviour or other important characteristics.

For technicians calculating glaze chemistry, LOI has another important meaning. Think of it as the shells discarded from a bag of nuts. If we purchase 100 grams of a material having a 12.5% LOI, only 87.5 grams remain after firing. The other 12.5 grams are lost, usually as gases. To obtain 100 grams of fired residue from that material, we would need 100 / 0.875 = 114.3 grams of the raw powder. The LOI must therefore be accounted for when converting the raw recipe into its oxide formula.

Historically, Digitalfire Desktop Insight stored materials in its MDT (materials database) as oxide formulas. LOI was accommodated through the relationship between a material's formula weight and its fired oxide contribution. Insight-live.com stores material chemistries as percentage analyses, with LOI recorded alongside the oxides. This makes it easier to see how much of a raw material contributes to the fired chemistry and how much is expected to be lost. When the LOI of each ingredient is known, the expected LOI of a raw recipe can be calculated as a weighted average. Insight-live does this automatically when displaying recipe chemistry. For example, if a glaze contains 20% kaolin having a 12.5% LOI, that kaolin contributes 2.5 percentage points to the calculated LOI of the whole recipe. Other ingredients contribute according to their proportions and LOI values. Technicians can explore replacing high-LOI materials with alternatives having lower or zero LOI, while maintaining a similar calculated oxide chemistry. For example, the oxides Li₂O, BaO and B₂O₃ can potentially be sourced from suitable frits rather than high-LOI raw materials (where commercially available formulations permit it). Such changes can greatly reduce gas generation from the glaze itself.

However, calculated recipe LOI is only an estimate based on the material analyses entered into the database. Actual firing losses can differ.

Related Information

LOI horse race with a surprising winner and loser

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This chart compares the decompositional off-gassing (% weight Loss on Ignition) behavior of six glaze materials (there are many more). Each was heated through the range 500-1700F at 350F/hr. It is amazing how much weight some can lose on firing. For example, 100 grams of calcium carbonate generates 44 grams of CO2! Magnesium carbonate even more. But the main message of this chart is that some late gassers overlap early melters. That can be a problem (causing glaze bubbles, blisters, pinholes). The talc in this test is an example: It did not finish gassing until 1650F. Yet many fritted and Gerstley Borate glazes have already begun melting by then.

There are two takeaways here. The biggest weight loser is not the biggest troublemaker. What matters is how gas release overlaps glaze sealing. Calcium carbonate illustrates this; its high carbonate content can still produce a clear glaze (when gases can clear ahead of any melting). And the talc, although having a small LOI, is a potential troublemaker when the glaze melt cannot effectively clear the bubbles.

Same glaze chemistry, very different LOIs.

Look at the bubbles!

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These two GBMF test balls were melted on refractory tiles to compare their melting behaviour. Their calculated oxide chemistries are almost identical, but their recipes are very different. G2931F (left) contains 29% Ulexite, driving its calculated LOI to 14.3%. G2931K (right) sources its boron from frits, reducing the LOI to just 3.1%. The difference is striking! The high-LOI glaze is full of large bubbles that were unable to escape or clear before the melt stiffened during cooling. Although ulexite releases most of its chemically bound water before significant glaze melting, that early gas evolution can contribute to foaming and trapped bubbles as melting progresses. The fritted glaze is much clearer, although it still contains fine bubbles. These may be associated with the kaolin or other sources of entrained gas.

The GBMF test exaggerates gas-retention behaviour, which is precisely why it is useful for comparing recipes, but it does not necessarily predict the appearance of a thin glaze layer on ware.

Two glazes with the same chemistry but different melt flows

Hint: It is the LOI difference

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At low temperatures, potters want transparents with good clarity to support brilliant colors. Especially on terra cotta bodies. A melt flow test like this enables a comparison one cannot do on using glazed test tiles.

These are two low-temperature glazes being compared on a GLFL test: G2931F and G2931K. They have the same chemistry. But F gets its boron from Ulexite, a material with a high LOI (notice how the escaping and bubbling gases of its decomposition have disrupted the downward flow). The frit-sourced boron version on the right flows cleanly and contains almost no obvious bubbles.

Does this mean that the G2931F version is useless? Not exactly. LOI gases can often escape well from a thin glaze layer. I used the K version successfully on many pieces. In a thin layer, the ulexite seems to act as its own fining agent, like Gerstley Borate, and the bubbles do merge clear well enough.

Original glaze with Gerstley Borate

Improved version with frit

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These pieces are fired at the same temperature. The glaze on the left is a popular recipe found online, Worthington Clear (our code number G2931). The Gerstley Borate in it is "farting" as the glaze is melting (the calculated LOI of the glaze as a whole is 15%, mainly from that one material). Unless applied very thinly clouds of micro-bubbles appear and opacify it (this example is fired to cone 03). And strangely, it is crazing badly despite the low calculated thermal expansion. Using my account at insight-live.com I was able to source the B₂O₃ and MgO from a frit (actually two frits) to create the G2931K recipe. Although the thermal expansion calculates higher, it is strangely fitting better and is very glassy and clear (thick or thin).

Magnesium carbonate vs. oxide: One big difference

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How much light magnesium carbonate supplies the same MgO as 50 parts magnesium oxide? An Insight-live account enables calculating it. These two test recipes show how. Each contains the same 50 parts Ferro Frit 3134. In calculation mode, increase the magnesium carbonate in steps of 10 until the calculated oxide formulas match. But notice the recipe totals: 100 versus 170. Much of that extra weight escapes during firing as carbon dioxide and water vapor. The carbonate recipe has a calculated loss on ignition (LOI) of 40.2%, compared with zero for the oxide version.

However, the similar calculated chemistry does not guarantee similar firing behavior. At this exceptionally high proportion, light magnesium carbonate strongly promotes crawling: Its fine particles increase drying shrinkage, and its effect on the melt helps the glaze pull into islands. The high LOI adds a substantial gas burden. Glaze chemistry tells us what oxides materials supply, but their physical properties and decomposition behavior matter too.

Talc was making this glaze "puffy" - here is how I fixed it.

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The recipe on the right employs talc to source the MgO needed to create a silky matte texture. However, talc is a late gasser, potentially able to produce micro-bubbles in the glaze after it has begun melting. The reduction in the definition of contour edges on the tile in front and the reduction in melt fluidity tipped me off. Using my account at insight-live.com I did the calculations needed to source the MgO using dolomite instead, producing recipe G3933E. While dolomite has a far higher LOI than talc it starts releasing the gasses of its decomposition much earlier and finishes well before talc. The mug in the back confirmed my suspicions, firing with a much smoother surface (and with far better definition of the incising).

The LOI of Gerstley Borate is a big deal

Here is how much it can affect a glaze

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Fired at cone 6. The samples on the bottom tiles are from ten-gram balls that have melted down (in our GBMF test). 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 thus is more available for glass building.

What material makes the tiny bubbles? The big bubbles?

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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).

Bubble city with Seattle Midnight body:

With G3806C glaze at cone 01, 4 and 5

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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 melts 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.

A body containing manganese bubbles the glaze

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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).

High LOI materials can turn your glaze into Aero chocolate!

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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.

How much gas escapes firing from cone 03 & 04?

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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.

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

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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.

Imposing an LOI in INSIGHT

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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

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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.

Inbound Photo Links

Frits melting before carbon expulsion

Unoxidized carbon in unexpected glazes And melting earlier than expected

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