Coating ignition residue testing, also known as ash content determination, quantifies the inorganic residue remaining after a paint or coating is carbonized and ignited under controlled laboratory conditions. As a core indicator of pigment and extender loading, this test is widely used for quality control, formulation verification, incoming inspection, and failure analysis of liquid coatings, powder coatings, and cured coating films, and is routinely performed by accredited third-party laboratories worldwide.
Technical Overview
Ignition residue, commonly referred to as ash content, is the mass fraction of non-combustible inorganic matter that remains after a weighed test portion of a coating is dried, carbonized, and ignited at a specified temperature until a constant mass is reached. During the test, organic components such as binders, resins, solvents, coalescing agents, organic additives, and organic pigments decompose and burn off as volatile combustion products, while inorganic pigments (for example titanium dioxide and iron oxides), mineral extenders (calcium carbonate, talc, kaolin, barium sulfate), and inorganic additives remain as the residual ash.
Ignition residue should not be confused with non-volatile matter content, which is determined by drying at a moderate oven temperature in accordance with ISO 3251 or ASTM D2369 and retains both binder and pigment. Ignition residue instead approximates the inorganic fraction of the coating, and under the ashing-method approach described in the ISO 14680-3 methodology and adopted in China as GB/T 1747.2, it serves as a practical estimate of pigment and extender content.
It should be noted that thermal decomposition can alter the residue mass during ignition. Carbonates may decompose into oxides with loss of carbon dioxide, hydrated minerals release bound water, and certain compounds such as zinc salts or halide-containing additives may partially volatilize at elevated temperatures. For this reason, test standards specify the ignition temperature, heating rate, and endpoint criteria, and results are interpreted as the residue under the stated conditions rather than as an absolute pigment content.
Testing Samples
Liquid Coatings and Varnishes
Solvent-borne, water-borne, and emulsion-based architectural and industrial coatings, including primers, topcoats, anti-corrosion coatings, and clear varnishes, can be tested directly after homogenization. A representative sample of typically 100 to 250 mL in the original sealed container is recommended.
Powder Coatings
Thermosetting epoxy, polyester, polyurethane, and hybrid powder coatings are suitable for ignition residue testing. Samples should be dry, sealed against moisture pickup, and thoroughly blended before weighing.
Cured Coating Films and Coated Products
Fully cured films removed from coated panels, electrophoretic coating films, coil coating layers, and conformal coatings on electronic assemblies can be tested after being cut or ground into small pieces to ensure uniform charring and complete combustion.
Coating Raw Materials
Resins, polymer dispersions, pigment pastes, extenders, and functional additives may also be submitted for ash determination as part of incoming material inspection or supplier qualification.
Sampling should be carried out in accordance with ISO 15528 or GB/T 3186, and sample examination and preparation should follow ISO 1513 to ensure the test portion is representative of the entire batch.
Testing Items
- Ignition residue (ash content) at a specified ignition temperature, typically 700 to 800 °C
- Pigment and extender content by the ashing method in accordance with the GB/T 1747.2 approach
- Sulfated ash, where the specification requires treatment with sulfuric acid before ignition
- Loss on ignition for cured films and inorganic-rich samples
- Duplicate determinations with repeatability assessment and, where required, reproducibility checks
- Optional follow-on elemental analysis of the residue by X-ray fluorescence spectroscopy, ICP-OES, or AAS for restricted-substance screening or filler identification
Testing Methods
Reference Standards
- ISO 14680-3, determination of pigment content by the ashing method, which forms the technical basis for GB/T 1747.2
- GB/T 1747.2-2008, paints and varnishes, determination of pigment content, ashing method
- ISO 3451 series, determination of ash, general procedures frequently adapted for polymeric binders and coating materials
- ASTM D2584 and ASTM D5630, ignition loss and ash content of cured resin and polymeric materials
- ISO 3251 and ASTM D2369, non-volatile matter content, as complementary determinations
- ISO 15528, GB/T 3186, and ISO 1513 for sampling and sample preparation
Apparatus
- Muffle furnace capable of maintaining the specified temperature, typically up to 900 °C, with control of approximately ±25 °C
- Analytical balance with a readability of 0.1 mg
- Porcelain, quartz, or platinum crucibles with lids
- Desiccator with effective desiccant for cooling crucibles without moisture pickup
- Ventilated drying oven at 105 ± 2 °C for pre-drying test portions
- Hot plate or gas burner for gentle charring prior to furnace ignition
Test Procedure
- Homogenize the sample thoroughly and, if necessary, strain or condition it according to ISO 1513.
- Pre-ignite the empty crucible at the test temperature, cool it in the desiccator, and weigh it to obtain the crucible mass.
- Weigh a suitable test portion, typically 1 to 5 g depending on the expected ash level, into the crucible and record the combined mass.
- Pre-dry the test portion at 105 ± 2 °C to remove solvents and water, which prevents spattering and uncontrolled flaming during charring.
- Char the sample gently on a hot plate or burner until smoking ceases, taking care to avoid mechanical loss of material.
- Transfer the crucible to the muffle furnace and ignite at the specified temperature, commonly 700 to 800 °C, holding for 30 to 60 minutes.
- Cool the crucible in the desiccator to ambient temperature and weigh it, then repeat the ignition, cooling, and weighing cycles until constant mass is achieved, typically when consecutive mass differences do not exceed 0.5 mg.
- Perform the determination in duplicate and report the mean result together with the ignition temperature and drying conditions.
Calculation and Reporting
The ignition residue is calculated as the mass of residue divided by the mass of the original test portion, expressed as a percentage. In formula form, the ash content equals the mass of crucible plus residue minus the mass of the empty crucible, divided by the mass of crucible plus sample minus the mass of the empty crucible, multiplied by 100. The test report should include the sample identification, ignition temperature, drying and charring conditions, individual and mean results, and any observations such as residue color, sintering, or incomplete burn-off.
Interferences and Quality Control
- Spattering or foaming during charring can eject material and bias results low, which is why controlled pre-drying and gentle heating are essential.
- Incomplete combustion leaves black carbon specks in the residue and requires additional ignition cycles until the residue is uniform in color.
- Decomposition of carbonates and hydrated extenders, and volatilization of zinc, antimony, or halide compounds, can shift the residue away from the true inorganic pigment mass, so the selected temperature must match the specification.
- Hygroscopic residues absorb moisture rapidly; crucibles should be weighed promptly after cooling in the desiccator.
- Crucible material should be chemically compatible with the ash to avoid fusion or corrosion of the crucible and contamination of the residue.
- Reagent and apparatus blanks, calibrated balances, and verified furnace temperature profiles support traceable, reproducible results.
Applications
- Batch-to-batch quality control of pigment and extender loading in architectural, industrial, and protective coatings
- Verification of formulation consistency when comparing different production batches, production sites, or raw material suppliers
- Incoming inspection of resins, pigment dispersions, extenders, and functional additives
- Estimation of pigment content in accordance with the GB/T 1747.2 ashing approach for product conformity assessment
- Failure analysis, where the isolated ash is further characterized by X-ray fluorescence, ICP-OES, or scanning electron microscopy to identify fillers, contaminants, or unexpected inorganic species
- Research and development support for high-solids, low-VOC, and cost-optimized filler formulations
- Regulatory screening workflows, in which the combustion residue serves as a concentrated inorganic fraction for subsequent elemental analysis
FAQ
What is coating ignition residue?
It is the percentage by mass of non-combustible inorganic material remaining after a dried coating sample is charred and ignited to constant mass at a controlled high temperature. It primarily reflects the inorganic pigment, extender, and inorganic additive content of the coating.
How is ignition residue different from non-volatile content?
Non-volatile content is determined by drying at roughly 105 to 125 °C per ISO 3251 or ASTM D2369 and includes both binder solids and pigments. Ignition residue goes much further, burning off all organic matter at several hundred degrees Celsius so that only the inorganic ash remains.
Which ignition temperature is used?
Most specifications call for 700 to 800 °C, although lower temperatures in the range of 450 to 550 °C may be selected when the formulation contains temperature-sensitive pigments or extenders prone to decomposition or volatilization. The chosen temperature must always be reported with the result.
Does the ash equal the pigment content of the coating?
Not exactly. The ash closely approximates pigment and extender content, but thermal decomposition of carbonates and hydrates, as well as volatilization of certain metal compounds, can cause the residue mass to differ from the original inorganic loading. Standards therefore define the result as residue under specified ignition conditions.
How much sample is required and how long does the test take?
A practical submission of 100 to 250 mL or 100 to 250 g of homogeneous material is usually sufficient, including allowance for duplicate determinations. Testing is typically completed within three to seven working days at accredited third-party laboratories, depending on drying requirements and the number of ignition cycles needed to reach constant mass.
Can the residue be analyzed further?
Yes. The isolated ash is an ideal concentrated inorganic fraction for subsequent elemental analysis by X-ray fluorescence, ICP-OES, or AAS, supporting filler identification, heavy-metal screening, and failure investigations.
Which standards govern this test?
The principal references are the ashing method basis of ISO 14680-3 as implemented by GB/T 1747.2, supplemented by the general ash procedures of the ISO 3451 series, ASTM D2584 and ASTM D5630 for cured resin materials, and the sampling standards ISO 15528, GB/T 3186, and ISO 1513.
