How to Perform Loss on Ignition (LOI) Tests on Quartz?

Loss on Ignition (LOI) is one of the most fundamental quality control parameters in quartz and silica powder production. It measures the total mass loss when a sample is heated to a high temperature — typically 950–1000°C — and reflects the combined content of adsorbed moisture, crystal water, organic matter, carbonates, sulfides, and other volatile constituents. For high-purity quartz used in electronics, semiconductors, and advanced ceramics, LOI is a critical purity indicator: values above specification signal contamination that can compromise downstream performance.

At JACAN, we process high-purity quartz and silica powder to ppm-level impurity standards, and LOI testing is an integral part of our incoming material inspection and finished product release. Below is a complete, step-by-step guide to performing accurate and reproducible LOI tests on quartz.

1. What LOI Measures in Quartz

When quartz is heated to 950–1000°C, several processes contribute to mass loss:

• Adsorbed moisture (H₂O⁻): Surface water and pore water, typically removed below 110°C.
• Crystalline water / hydroxyl groups (H₂O⁺): Water bound in mineral structures (e.g., clay minerals, hydrous silica phases), released between 400–800°C.
• Organic matter and carbon: Combustion of carbonaceous impurities, typically 300–600°C.
• Carbonate decomposition: Calcite (CaCO₃), dolomite, or other carbonate minerals release CO₂ above ~600°C.
• Sulfide oxidation: Pyrite and other sulfides oxidize, releasing SO₂.
• Other volatile constituents: Halides, boron compounds, etc.

Note that LOI is a net value: some reactions cause mass gain (e.g., oxidation of Fe²⁺ to Fe³⁺), so the measured LOI is the algebraic sum of all mass changes. For high-purity quartz, this is rarely significant, but for impure feedstocks it should be considered.

2. Applicable Standards

Several standards govern LOI testing for quartz and silica:

For general quartz quality control, 1000°C for 30–60 minutes to constant mass (per IS 1917-1) is the most widely accepted procedure.

3. Equipment and Materials

Required Instruments

• Muffle furnace: Capable of reaching 1050°C with temperature stability of ±25°C. The furnace should have good temperature uniformity across the working zone.
• Analytical balance: Readability 0.1 mg (0.0001 g) or better, calibrated and verified with reference weights.
• Drying oven: Capable of maintaining 105 ± 5°C.
• Desiccator: With fresh desiccant (silica gel with indicator, anhydrous magnesium perchlorate, or molecular sieves). The desiccator must be airtight.
• Crucibles: Platinum crucibles are preferred for high-purity work (inert, non-contaminating). Porcelain or high-alumina crucibles are acceptable for routine testing but may contribute trace contamination. Crucibles should be 20–50 mL capacity with lids.
• Crucible tongs: Tip-coated (platinum or nickel-chromium) to avoid damaging crucibles.
• Mortar and pestle or laboratory mill: For sample preparation (agate or alumina to avoid contamination).
• Sieve: 150 μm (100 mesh) or 75 μm (200 mesh) for sample size reduction.

Safety Equipment

• Heat-resistant gloves
• Face shield for furnace operation
• Lab coat and safety glasses

4. Sample Preparation

Proper sample preparation is essential for reproducible results.

4.1 Representative Sampling

• Take a representative sample from the bulk lot using standard quartering or riffle-splitting techniques.
• The laboratory sample should be at least 100–200 g to ensure representativeness.

4.2 Size Reduction

• Crush and grind the sample to pass a 150 μm (100 mesh) sieve. For ultra-fine or high-purity quartz, 75 μm (200 mesh) is preferred.
• Use an agate mortar, alumina mill, or tungsten carbide mill to avoid iron contamination. Do not use steel equipment for high-purity quartz.
• Mix the ground sample thoroughly to ensure homogeneity.

4.3 Pre-Drying

• Dry the prepared sample at 105 ± 5°C for a minimum of 2 hours (or overnight) to remove adsorbed moisture.
• Transfer the dried sample to a desiccator and cool to room temperature before weighing.
• Note: If the LOI result needs to include adsorbed moisture (some standards report “as-received” LOI), skip this pre-drying step. Most industrial quartz specifications report LOI on a dry basis.

5. Step-by-Step Test Procedure

Step 1: Prepare and Pre-Ignite Crucibles

1. Clean crucibles thoroughly and mark them with a unique identification number (use a ceramic marker or engrave on the base).
2. Place empty crucibles (with lids ajar) in the muffle furnace.
3. Ignite at 1000°C for 30 minutes to remove any residual contaminants and stabilize the crucible mass.
4. Remove from the furnace, place in a desiccator, and cool to room temperature (typically 30–45 minutes).
5. Weigh each crucible to the nearest 0.1 mg. Record as W₁ (mass of empty crucible).
6. Repeat the ignition–cooling–weighing cycle until constant mass is achieved (consecutive weighings differ by ≤ 0.5 mg).

Step 2: Weigh the Sample

1. Transfer approximately 1.0000 g (IS 1917-1 method) or 10 g (SAMSA method) of the dried, cooled sample into the pre-ignited crucible.
   • For high-purity quartz with very low LOI, a larger sample mass (5–10 g) improves measurement precision.
   • For impure quartz with expected LOI > 2%, 1 g is sufficient.
2. Weigh the crucible + sample to the nearest 0.1 mg. Record as W₂.
3. Calculate the dry sample mass: m_sample = W₂ − W₁.

Step 3: Ignition

1. Place the crucible (lid ajar to allow gas escape) in the muffle furnace at room temperature or below 200°C.
2. Ramp the temperature slowly — approximately 5°C per minute — to 1000°C. Rapid heating can cause spattering (sample ejection) from volatile release, especially in samples with high carbonate or moisture content.
3. Once at 1000°C, maintain the temperature for 30 minutes (IS 1917-1) or 60–90 minutes (SN/T 0483, SAMSA).
4. After the hold period, turn off the furnace. Open the furnace door slightly (1–2 cm) and allow the crucibles to cool inside the furnace to below 200°C (approximately 30–60 minutes). This prevents thermal shock and reduces moisture uptake during transfer.

Step 4: Cooling and Weighing

1. Transfer the crucibles to a desiccator using crucible tongs.
2. Allow to cool to room temperature (30–45 minutes). Do not cool for excessively long periods, as the ignited sample may absorb atmospheric moisture.
3. Weigh each crucible + ignited sample to the nearest 0.1 mg. Record as W₃.

Step 5: Constant Mass Check

1. Return the crucibles to the furnace and ignite at 1000°C for an additional 30 minutes.
2. Cool and weigh as above.
3. Repeat until consecutive weighings differ by ≤ 0.5 mg (or ≤ 0.1% of sample mass, whichever is larger).
4. Record the final constant mass as W₃(final).

6. Calculation

The loss on ignition is calculated as follows:

LOI (%) = [(W₂ − W₃) / (W₂ − W₁)] × 100

Where:

• W₁ = mass of empty, pre-ignited crucible (g)
• W₂ = mass of crucible + dried sample before ignition (g)
• W₃ = mass of crucible + sample after ignition to constant mass (g)

Example Calculation

• W₁ (empty crucible) = 25.3421 g
• W₂ (crucible + sample) = 26.3456 g
• W₃ (crucible + ignited sample) = 26.3389 g
• Sample mass = 26.3456 − 25.3421 = 1.0035 g
• Mass loss = 26.3456 − 26.3389 = 0.0067 g
• LOI = (0.0067 / 1.0035) × 100 = 0.67%

7. Typical LOI Values for Quartz

A sudden increase in LOI from a supplier’s typical baseline often indicates a change in ore source, inadequate washing, or contamination — making LOI an excellent incoming inspection parameter.

8. Advanced: Stepwise LOI (Thermogravimetric Approach)

For troubleshooting or detailed characterization, a stepwise LOI procedure can identify the source of mass loss:

This is performed by weighing the sample after each temperature hold, then calculating the mass loss between steps. A thermogravimetric analyzer (TGA) automates this process with continuous mass recording.

9. Common Errors and How to Avoid Them

Error 1: Sample Spattering

Cause: Rapid heating, especially for samples with high moisture or carbonate content.
Solution: Ramp at ≤ 5°C/min. For high-carbonate samples, pre-heat at 500°C for 30 minutes before ramping to 1000°C. Use crucible lids with a small gap.

Error 2: Moisture Reabsorption During Cooling

Cause: Cooling in open air or using an exhausted desiccant.
Solution: Always cool in a desiccator with fresh desiccant. Weigh within 1 hour of removal from the desiccator. Use crucible lids during cooling.

Error 3: Incomplete Ignition

Cause: Insufficient time, sample too coarse, or furnace temperature below setpoint.
Solution: Always verify constant mass. Grind samples to ≤ 150 μm. Calibrate furnace temperature periodically with a thermocouple.

Error 4: Crucible Contamination

Cause: Using porcelain crucibles that react with the sample, or reusing crucibles without proper cleaning.
Solution: Use platinum crucibles for high-purity work. Pre-ignite all crucibles before use. Clean crucibles with acid (HCl or aqua regia for platinum) between samples if cross-contamination is suspected.

Error 5: Furnace Temperature Non-Uniformity

Cause: Hot spots or cold zones in the muffle furnace.
Solution: Place all crucibles in the central zone. Avoid overcrowding. Calibrate furnace temperature at multiple locations annually.

Error 6: Balance Drift

Cause: Uncalibrated balance, static electricity, or air currents.
Solution: Calibrate the balance daily with reference weights. Use a draft shield. Ground the balance and handle samples with tweezers to avoid static.

Error 7: Oxidation Gain Masking Loss

Cause: Samples containing Fe²⁺, sulfides, or other oxidizable species gain mass during ignition, partially offsetting true LOI.
Solution: For such samples, report “apparent LOI” and note the potential for oxidation effects. TGA with gas analysis can distinguish true volatilization from oxidation.

10. Quality Control and Verification

To ensure reliable LOI results:

• Run duplicates: Analyze every sample in duplicate. Duplicate results should agree within ± 0.05% absolute for LOI < 1%, or ± 5% relative for LOI > 1%.
• Use reference materials: Analyze a certified reference material (CRM) with known LOI alongside samples. Suitable CRMs include quartz sand standards from NIST, BCR, or JCM.
• Blank correction: Run empty crucible blanks through the full procedure to account for any crucible mass change or balance drift.
• Record all conditions: Document furnace temperature, hold time, sample mass, crucible type, and ambient humidity for every test batch.
• Maintain equipment: Calibrate the balance monthly, verify furnace temperature quarterly with a certified thermocouple, and replace desiccant when the indicator changes color.

11. The JACAN Quality Control Perspective

At JACAN, LOI testing is not merely a compliance checkbox — it is a frontline quality control tool that protects the integrity of our high-purity quartz and silica powder processing:

• Incoming inspection: Every batch of raw quartz is tested for LOI before acceptance. Elevated LOI flags clay, carbonate, or organic contamination that would compromise downstream purification.
• Process monitoring: LOI is measured after each processing stage — crushing, magnetic separation, acid leaching, and grinding — to verify that purification steps are performing as designed.
• Finished product release: Final LOI must meet the specification for the target grade (electronic-grade, fused quartz, industrial filler) before the product is released.
• Traceability: Every LOI result is linked to a specific batch, processing line, and operator, enabling full traceability and rapid root-cause analysis if deviations occur.

For producers seeking to improve their quartz quality, establishing a rigorous LOI testing protocol is one of the simplest and most cost-effective steps — it requires only basic laboratory equipment yet provides immediate insight into material purity and process performance.

Performing an accurate LOI test on quartz requires attention to detail at every stage: representative sampling, proper size reduction, pre-drying, controlled ignition at 950–1000°C to constant mass, moisture-free cooling, and precise weighing. The standard procedure — ignite 1–10 g of ≤ 150 μm sample at 1000°C for 30–90 minutes to constant mass, cool in a desiccator, and calculate mass loss percentage — is straightforward but unforgiving of shortcuts.

The most common failure modes are spattering from rapid heating, moisture reabsorption during cooling, incomplete ignition from insufficient time or coarse particle size, and crucible contamination. With proper technique, duplicate agreement of ± 0.05% absolute is readily achievable — sufficient to distinguish between high-purity quartz (LOI < 0.1%) and standard industrial grades (LOI 0.1–0.5%).