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.
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.
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 |
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 |
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.
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.
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.
