The Dirty Truth About Industrial Grade Lye
Part 3
The Purity Problem:
What "Industrial Grade" Actually Means
Let's start with a chemistry reality check delivered with love and only mild alarm.
Food-grade sodium hydroxide (NaOH) is manufactured to meet stringent standards set by the FDA and the Food Chemicals Codex (FCC), with a minimum purity of 99% — and often as high as 99.5–99.9%. The key word isn't just the percentage — it's what the standards guarantee isn't in there. Food-grade certification means tight controls on heavy metals including lead, mercury, arsenic, cadmium, iron, nickel, and copper.
Industrial or technical-grade lye is a different beast. As one soap ingredient supplier puts it plainly: "Tech grade lye (drain cleaner) could range from anywhere between 90–97% purity." That means up to 10% of what's in your lye bucket could be something other than sodium hydroxide. And that "something other" is where things get uncomfortable.
The manufacturing process matters enormously. Sodium hydroxide is produced commercially via electrolysis of brine, and there are three main methods: the Mercury Cell Process, the Diaphragm Cell Process, and the Membrane Cell Process. The first two — which are still in use in many industrial facilities globally — raise specific concerns about trace contamination with mercury and asbestos. The Membrane Cell Process is the cleanest, but industrial-grade lye doesn't come with a label telling you which process was used.
On top of the manufacturing method, there's the cross-contamination problem. As one supplier explains it, a chemical plant makes many different chemicals using the same equipment. Just like how a chocolate bar might "contain traces of nuts," industrial lye may pick up traces of other heavy metals like lead from the production environment. Those impurities — iron, nickel, copper, lead, mercury, and chlorides — are documented in technical-grade NaOH specifications and are present at significantly higher parts-per-million than in food or pharmaceutical grades.
Essential Depot, one of the most trusted food-grade lye suppliers in the soap-making world, states it directly: "The grade of Sodium Hydroxide matters. In soap making as well as in food making. Drain cleaners are typically technical grade lye with greater parts per million of heavy metal impurities."