1. What “LOI” really means

Loss-on-Ignition is the percentage of mass a sample loses when it is heated to a specified high temperature. That loss represents everything in the powder that will volatilise or burn off:

• physically adsorbed water
• crystal (structural) water
• CO₂ from carbonates
• organic matter, oils, binders, or processing aids
• sulfur or other low-boiling compounds

Because the furnace drives off only volatile species, the result is a quick fingerprint of purity and thermal stability.

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2. Standard test outline

Step	Typical conditions	What happens
a. Weigh dry sample (m₀)	Sample dried at 105 °C, cooled in desiccator	Removes free moisture so the furnace step reflects volatile other than surface water
b. Ignite (m₁)	950 °C ± 25 °C for 2 h in a muffle furnace (ASTM E1755 / ISO 787-5)	Carbonates decompose, organics burn, lattice water leaves
c. Cool & re-weigh	Desiccator, room temp	Prevents the hot, anhydrous residue re-absorbing moisture
d. Calculate	LOI % = [(m₀ − m₁) / m₀] × 100	The single number reported on your COA

Some industries use a lower ignition temperature (e.g., 550 °C for clays to keep carbonates intact) – always state the temperature beside the LOI value.

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3. Typical LOI benchmarks for fillers

Material	Expected LOI (%)	Main contributors
High-purity talc	4 – 7	Structural OH in talc layers (Mg₃Si₄O₁₀(OH)₂)
Dolomitic or calcite-rich talc	15 – 30	CO₂ from Ca/Mg carbonates + OH
Ground calcium carbonate (GCC)	43 – 45	CO₂ only (CaCO₃ → CaO + CO₂)
Calcined kaolin	< 1	Already de-hydroxylated in the kiln
Natural kaolin	12 – 15	Lattice water given off above 600 °C
Barite (BaSO₄)	< 0.5	Virtually inert; high LOI flags contamination

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4. Why LOI matters in real applications

Concern	Effect of high LOI
Plastic compounding & extrusion	Volatiles turn into bubbles → voids, poor mechanical strength, die build-up
Paints & coatings	Higher film porosity as CO₂ or water escapes during baking
Ceramics & sintering	Gas release causes blisters; fixed carbon lowers oxidation potential
Pricing & freight	You pay freight on water/CO₂ you can’t sell; lower LOI = more usable solids per tonne
Purity claims	Elevated LOI can reveal undeclared carbonate dilution or residual process oil

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5. Interpreting a talc LOI specifically

• < 8 % → Cosmetic / polypropylene grades – essentially pure magnesite talc
• 8 – 15 % → General filler grades – slight carbonate or serpentine content, good for paints & putties
• 15 % → Ceramic / agricultural talc – dolomitic or mixed ore, colour and thermal release less critical

If the LOI is unexpectedly high, cross-check the CaO + MgO + CO₂ line in the chemistry table; a jump there usually confirms carbonate dilution.

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6. Best-practice reporting format

Loss-on-Ignition (950 °C, 2 h, ISO 787-5): 6.1 % ± 0.3 %

State temperature, duration, and standard method every time—LOI cannot be compared unless the conditions are identical.

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7. Key take-aways

• LOI is a one-number snapshot of everything volatile in your powder.
• Run the test in two stages (105 °C pre-dry, then 950 °C ignition) to isolate true volatiles.
• For talc, LOI below 8 % signals high-purity cosmetic/plastic grade; much higher means carbonates or organics.
• Elevated LOI affects processing (bubbles, blisters) and economics (paying freight on gas).
• Always quote the temperature/time along with the percentage so your customers can make an apples-to-apples comparison.

With that understanding, you can glance at the LOI line on any Certificate of Analysis and instantly know if the filler matches the performance window your product needs.