Fineness testing of chemical products is a core quality-control measurement that determines the particle size and degree of dispersion of pigments, coatings, inks, dyestuffs, fillers, and specialty powders. This article explains the internationally recognized standards, sampling requirements, test items, and laboratory methods used for fineness evaluation. It is a practical reference for manufacturers, formulators, and buyers who need reliable fineness data for product release and supplier qualification.
Technical Overview
Fineness is a fundamental physical characteristic of chemical products. In liquid systems such as paints, varnishes, and printing inks, fineness—commonly called "fineness of grind"—describes the size of the largest pigment agglomerates remaining after the dispersion and milling stages. In powders such as pigments, extenders, and powder coatings, fineness is expressed as sieve residue or as a particle size distribution characterized by percentile values such as D10, D50, and D90.
Fineness directly influences end-use performance. In coatings, inadequate fineness reduces gloss, hiding power, color strength, and film smoothness, and can cause visible seediness, roughness, or nozzle clogging during spray application. In printing inks, poor fineness causes dot loss, plate wear, and inconsistent color reproduction. In powders, fineness affects flowability, packing density, reactivity, and sedimentation stability. Because of this, fineness is specified in most product data sheets and purchase specifications, and it is routinely verified in factories and in accredited third-party laboratories.
The principal test methods are internationally standardized. Grindometer (Hegman-type gauge) testing is covered by ISO 1524, ASTM D1210, ASTM D1316, GB/T 6753.1, and the JIS K 5600 series. Sieve residue and coarse-particle determinations are covered by ISO 787-7, ISO 787-18, ASTM D185, ISO 3310-1, and ASTM E11. Laser diffraction particle size analysis is governed by ISO 13320, GB/T 19077, and ASTM B822. Results may be reported in micrometers, Hegman units, NPIRI units, mils, mass percent residue on a specified sieve, or percentile diameters, depending on the method and the product specification.
Testing Samples
Liquid Coatings and Varnishes
Architectural coatings, industrial coatings, automotive OEM and refinish paints, wood coatings, and marine and protective coatings are the most common samples. Typically, 100 mL to 500 mL of a representative, well-mixed sample is submitted. Samples must be protected from skin formation, solvent loss, pigment settling, and temperature extremes before testing.
Printing Inks and Pigment Dispersions
Offset, gravure, flexographic, and screen-printing inks, as well as pigment concentrates and tinting pastes, are tested for fineness of grind. Because inks are usually specified at very low fineness values, gauges with shallow ranges such as 0–25 µm are often required. Sample amounts of 20 g to 100 g are generally sufficient.
Pigments, Dyestuffs, and Extenders
Organic and inorganic pigments, titanium dioxide, iron oxides, carbon black, calcium carbonate, talc, kaolin, and barium sulfate are evaluated for sieve residue and particle size distribution. Powder samples of 50 g to 200 g, taken from multiple locations of the batch and reduced by quartering, ensure representativeness.
Powder Coatings and Polymer Powders
Thermosetting powder coatings based on epoxy, polyester, hybrid, and polyurethane chemistries, as well as polymer powders for rotational molding and masterbatch carriers, are tested for particle size distribution. A sample of 100 g to 500 g, obtained with a sampling thief or from the classification stage, is typical.
Specialty and Fine Chemical Powders
Ceramic raw materials, battery electrode materials, catalysts, abrasive grains, and agrochemical powders such as wettable powders are also subject to fineness and particle size control, since their performance depends strongly on particle size distribution and the absence of oversize agglomerates.
Testing Items
Depending on the physical form of the product and the governing specification, the following items are typically determined and reported:
- Fineness of grind, expressed in micrometers, mils, Hegman units (H), or NPIRI units.
- Sieve residue, expressed as mass percent retained on a specified sieve, commonly 45 µm (325 mesh), 38 µm (400 mesh), or 25 µm (500 mesh).
- Particle size distribution parameters: D10, D50, D90, D99, and span, on a volume basis.
- Maximum particle size and oversize (coarse particle) content for critical applications.
- Count and size of discrete oversize particles, gels, or seed particles visible on the gauge channel.
- Dispersibility-related evaluation, when required, using the dispersion and assessment procedures of the ISO 8780 and ISO 8781 series.
Testing Methods
Grindometer Method (Fineness of Grind)
The grindometer is a hardened steel block containing one or two channels precision-ground with a slope from zero to a known depth, commonly 0–25 µm, 0–50 µm, or 0–100 µm, with an engraved scale. The sample is conditioned at 23 ± 2 °C in accordance with ISO 3270, placed at the deep end of the channel, and drawn toward the shallow end with the scraper held perpendicular to the block at a steady speed of a few seconds per stroke. The channel is then viewed at an oblique angle against a light source, and the reading is taken at the scale position where a significant number of particles first appear on the surface of the film. The median of at least three readings is reported.
Acceptance criteria, apparatus calibration, and gauge selection requirements are defined in ISO 1524, ASTM D1210, ASTM D1316, and GB/T 6753.1. A gauge should be selected so that the expected reading falls within the reliable upper portion of its range. On the Hegman scale, 0 H corresponds to 100 µm and 8 H corresponds to 0 µm, so each Hegman unit equals 12.5 µm; on the NPIRI scale used with a 0–100 µm gauge, each unit equals 10 µm.
Sieve Analysis and Sieve Residue
For wet sieving, a weighed test portion is washed with water (or a suitable aqueous or organic liquid with dispersant when required) through a calibrated test sieve conforming to ISO 3310-1 or ASTM E11. The retained residue is dried to constant mass and reported as a mass percentage of the original test portion. The manual water method is described in ISO 787-7 and the mechanical flushing procedure in ISO 787-18, while ASTM D185 covers coarse particles in pigments, pigment pastes, and paints. For free-flowing powders, dry sieving with a mechanical shaker, lid, and receiver is widely used for routine control, although wet methods are preferred for powders that tend to agglomerate electrostatically.
Laser Diffraction Particle Size Analysis
Laser diffraction, standardized in ISO 13320, GB/T 19077, and ASTM B822, measures the angular distribution of scattered light and converts it into a volume-equivalent particle size distribution using Mie or Fraunhofer theory. Measurements may be performed with a wet dispersion module, using water or an organic medium together with ultrasonic energy and a surfactant to break weak agglomerates, or with a dry air-jet dispersion module for robust powders. The method requires correct refractive index and absorption inputs, verification with certified reference materials, and control of dispersion conditions. The reported D10, D50, and D90 values should satisfy the repeatability limits of ISO 13320, typically within a few percent for the median.
Microscopy and Image Analysis
Optical microscopy and scanning electron microscopy are used to confirm the presence of oversize particles, gels, or agglomerates, to assess particle shape, and to validate anomalies found by other methods. Static and dynamic image analysis, covered by the ISO 13322 series, provides number-based particle size and shape information and is particularly useful for needle-shaped or flake-shaped pigments.
Supplementary Techniques
Where specifications require additional information, gravitational sedimentation (ISO 13317), the electrical sensing zone method, dynamic light scattering for submicron dispersions, and air permeability (Blaine) fineness measurement for sub-sieve powders may be applied. These techniques complement grindometer and laser diffraction data and help explain discrepancies between laboratories.
Applications
Fineness testing supports quality assurance across the entire chemical product value chain. Typical applications include:
- Milling endpoint control during the dispersion of paints, inks, and pigment concentrates, allowing production batches to be released from the mill as soon as the target fineness is reached.
- Incoming inspection of pigments, dyestuffs, and mineral fillers against supplier specifications for sieve residue and particle size distribution.
- Finished product release testing against product data sheets for architectural, industrial, automotive, wood, and protective coatings.
- Particle size control of powder coatings, where D50 and oversize content determine fluidization behavior, transfer efficiency, and final film appearance.
- Process control for ceramics, battery electrode materials, and abrasive products, where D50 consistency governs pressing, sintering, coating, and cutting performance.
- Agrochemical formulation development, where fineness of wettable powders and suspension concentrates determines suspensibility and biological efficacy.
- Failure investigation of film defects such as seediness, roughness, loss of gloss, poor color development, and spray nozzle blockage.
- Independent verification by accredited third-party laboratories for buyer–supplier dispute resolution, export documentation, and supplier qualification programs.
FAQ
What is the difference between fineness of grind and particle size distribution?
Fineness of grind is a single-point measure that identifies the size of the largest remaining agglomerates in a dispersed liquid system, obtained with a grindometer. Particle size distribution, usually measured by laser diffraction, characterizes the full range of particle sizes in a powder or suspension and reports percentile values such as D10, D50, and D90. The two parameters are complementary: fineness of grind is fast and ideal for in-process control, while particle size distribution gives a complete quantitative description.
How are Hegman units converted to micrometers?
On the standard 0–100 µm gauge, the Hegman scale runs from 0 H at 100 µm to 8 H at 0 µm, so each Hegman unit corresponds to 12.5 µm. For example, a reading of 6 H is equivalent to 25 µm, and 4 H is equivalent to 50 µm. The NPIRI scale on the same gauge runs from 0 to 10, with each unit equal to 10 µm.
Which standard should be used for paints and inks?
For fineness of grind, ISO 1524 and its national adoptions such as GB/T 6753.1 and the JIS K 5600 series are the primary references, while ASTM D1210 and ASTM D1316 are the established North American equivalents. For coarse particles and sieve residue of pigments and paints, ISO 787-7, ISO 787-18, and ASTM D185 apply. The choice should follow the specification agreed between the manufacturer and the customer.
How much sample is required and how should it be taken?
A few grams are sufficient for grindometer testing, 20 g to 100 g for sieve residue, and 100 g to 500 g for laser diffraction of powders. The critical requirement is representativeness: liquid samples must be homogenized before subsampling, and powders should be sampled from multiple points of the batch and reduced by quartering or a sample splitter to avoid segregation errors.
Why do different laboratories sometimes report different fineness values?
Differences usually arise from inadequate homogenization or dispersion before measurement, gauge or sieve calibration differences, worn scraper edges, operator interpretation of the particle appearance point, and the use of different test standards. Conditioning the sample at 23 ± 2 °C, using calibrated apparatus, following the specified scraping speed and viewing geometry, and reporting the median of repeated readings all improve interlaboratory agreement.
How often should fineness be tested in production?
Best practice is to test fineness at incoming raw material inspection, at regular intervals during milling until the dispersion endpoint is confirmed, and again on the finished product before release. Critical applications such as automotive topcoats and gravure inks typically require tighter fineness limits and more frequent verification than general industrial coatings.