fineness

Fineness is a measure of how fine or pure a substance is. In general, it describes the size distribution of particles or the proportion of a material that is the target substance relative to impurities. The meaning varies by industry, but it is typically expressed as a numerical value tied to either purity or particle size.

In precious metals, fineness expresses purity. It is commonly stated as parts per thousand or as karats. Fineness of 999 corresponds to 99.9% pure metal; 750 corresponds to 75% (18-karat). In jewelry, marks such as 999, 925 (sterling silver) indicate fineness. Fineness affects value, melting behavior, and corrosion resistance, and refining processes aim to increase fineness.

In cement and concrete, fineness commonly refers to particle size and specific surface area. Blaine fineness measures the surface area per unit mass, influencing hydration rate and strength. Fineness modulus, used for aggregates, summarizes the particle size distribution; higher numbers indicate coarser material and vice versa. Excessively fine cement increases water demand and shrinkage; excessively coarse aggregates reduce workability.

In powders and pharmaceuticals, fineness affects flow, compaction, and dissolution. It is assessed by sieve analysis for coarse powders or by laser diffraction and BET surface area for fine powders. Standards and descriptors vary by material and industry.

Because fineness is context-specific, the term should be defined alongside the method used to measure it.