A certificate of analysis is only useful if it describes the batch in front of you. Most diamond powder COAs list the right numbers and leave out the two things that make those numbers comparable: how the sample was prepared, and whether the figures are batch data or a typical range. Here is what a COA should contain, and what its absence tells you.
What A COA Is, And What It Is Not
A COA is a batch record. It states what was measured on a specific lot, on a specific date, by a specific method. It is not a product brochure, and it is not a specification sheet.
The distinction matters because a specification describes what a grade should be, while a COA describes what this drum actually is. Suppliers who send a specification and call it a COA are not necessarily hiding anything, but they are not giving you a batch record either.
The Six Things That Must Be There
Batch number. Without it the document cannot be tied to the material. It should match the label on the drum.
Grade and size band. Including the base grade family and the nominal size, written the same way as on the order.
Particle size data. D10, D50 and D90 at minimum. A full distribution curve is better, because three points can be reproduced from a curve but a curve cannot be reconstructed from three points.
Measurement method. Laser diffraction or sieve, and for laser, the instrument type and the sample preparation.
Impurity data. Ash content and magnetic content, or an explicit statement that they were not measured.
Date and signature. When it was tested, and who released it.
The Two Things Usually Missing
Sample preparation is the first. For laser diffraction, the dispersion method changes the result more than most people expect, especially below 10 microns. A COA that says “D50: 3.2 microns” without saying whether that was after ultrasonication, and for how long, is a number you cannot compare with your own incoming test.
The second is whether the figures are batch data or typical values. Many COAs show a range copied from the grade specification, with the batch number filled in afterwards. The tell is a suspiciously round set of numbers, or a distribution that never varies across shipments.
Strength Data On Tool Grades
For grades that go into bonded tools, particle size alone does not predict performance. Single-particle compressive strength and toughness index are the numbers that separate a grade that survives a saw segment from one that does not.
Not every grade needs them, and a supplier who measures strength on polishing powder is wasting effort. But on a tool-grade grit, a COA without any mechanical data is incomplete, and the buyer has no way to compare one lot with the next.
Morphology Images
An image is not a measurement, but it answers a question numbers cannot: are these crystals angular and sharp, or rounded and frosted? A morphology image of the actual batch, not a reference sample, is the fastest way to see whether a shipment looks like the last one.
Where the surface is the specification, ask for it. Where the powder goes into a bond and never touches the workpiece directly, the size and strength data matter more.
How To Read One Quickly
Check the batch number against the drum label first. Then look for the method line. Then check whether the size figures come with a preparation description. Then look at whether there is any impurity data at all.
Four checks, under a minute. A COA that passes all four is a working document. One that fails any of them is a sales aid, and you should ask for the batch record behind it.
Next Step
Our own COAs name the method next to each figure and report the full distribution, not three points. Grades with batch records: HDY, HDG, HDH.
Testing detail is on quality assurance, and the document set that ships with an order is on certifications. For how the numbers are produced, see quality control and PSD.