> Ok, I gotta ask. How does NIST source a large supply of mud that's stable in composition over a long period of time, both in terms of supply chain and storage

> Ok, I gotta ask. How does NIST source a large supply of mud that's stable in composition over a long period of time, both in terms of supply chain and storage? Technically, they don't. Obviously, they have some storage (this is one of those things you don't want to use just-in-time manufacturing). Instead, NIST (and other analogous standards organisations) have batches of materials and really, really expensive machines (that are hard to mass-produce) that can measure out the precise quantities of whatever thing they need to measure backed by processes that intrinsically is just precise (and accurate, for pedants), for example, electron microscopes or atomic force microscopes, which is then used by others (like in this case for measuring mud) to machines that are easy to mass-produce but can measure badly if not checked regularly. For some things though (like drug testing in urine), they instead have a pure substance laced with precise amounts of substances you've expect to find, but that's relatively easier than most SRMs where you're measuring things you've already have (whether it's mud, crude oil, whale blubber or peanut butter).