Foreign Material

Foreign material is not a fixed physical category the way "weed seeds" or "stones" might sound like on their own. It is defined by what a specific grading standard says remains in a sample after a specific, prescribed procedure — typically cleaning by defined equipment to remove the readily separable fraction (dockage), followed by inspection of what is left. Because the prescribed equipment, the cleaning intensity, and the categories counted all differ between standards and between commodities within a standard, the resulting percentage is only meaningful in reference to the exact standard it was generated under. Reading a foreign material figure from one country’s wheat standard as equivalent to another country’s figure, or to a different commodity’s standard, is a common and misleading error.

Dockage

Material readily removable from the lot by the prescribed cleaning equipment specified in the applicable standard. Assessed and reported separately, then deducted from the lot weight before grading proceeds.

Foreign material

Material that remains mixed with the grain after the dockage-removal step — not readily separated by the prescribed cleaning — and that is itself assessed as a grade factor.

The practical effect is that foreign material and dockage describe two different stages of the same cleaning process, not two competing measurements of the same thing. A lot can carry substantial dockage that is efficiently removed and deducted, while still carrying a foreign material figure that reflects only what the prescribed equipment could not separate out. In United States corn standards specifically, foreign material is not reported alone; it is assessed together with broken corn kernels as a single combined factor, because the two are difficult to separate cleanly during inspection.

Because foreign material is reported as a proportion of total mass, it treats a tablespoon of sand and a single fragment of glass as comparable only insofar as they weigh the same. That arithmetic is appropriate for a commercial grading factor, where cleanliness and processing burden are the concern, but it is the wrong lens entirely for hazard. Some contaminants that fall under the foreign material umbrella — ergot sclerotia, toxic weed seeds such as Datura, glass, metal, and stones — are dangerous at levels far below whatever threshold moves a mass percentage. A lot could pass comfortably on an aggregate foreign material figure while still containing an ergot sclerotium or a shard of glass that a mass-based figure would never flag on its own.

Foreign material is typically assessed by sieve analysis, which physically separates material by size and density, combined with visual and manual inspection of the retained fractions to categorize what has been separated out. The result an entire lot is assigned, however, depends heavily on how representatively the tested sample was drawn from the bulk in the first place: probe sampling determines what portion of the lot the laboratory actually sees, and sampling error in that step commonly exceeds the error of the sieving or inspection method itself. A rigorous sieve analysis performed on an unrepresentative sample still produces a misleading foreign material figure for the lot as a whole.