Standards, extraction force & taint control
What can actually be measured on a piece of bark.
A straight cork is a cylinder of bark, typically 44 mm long and 24 mm in diameter, driven into a bottle and held by compression. Because no two pieces are identical, the industry built a standards family around it: ISO 9727 for physical tests, ISO 16420 for still-wine requirements, ISO 17727 for sampling and ISO 20752 for releasable TCA. A cork order naming none of these is not really specified.
The dimension standard is separate from the spec
ISO 9727-1 defines how diameter, length and ovalization are measured; ISO 16420 sets the mechanical and physical requirements for still-wine stoppers. One gives the method, the other the limits, and a purchase order citing only a size in millimeters has specified neither of them.
Extraction force is a protocol, not a feel
ISO 9727-5 conditions the corks for 24 hours, drives them into filled bottles, lays those bottles horizontal for eight days and then measures the maximum force needed to pull each one. That eight-day wait is the point: a cork that pulls easily on day one can grip hard once it has relaxed into the neck.
Humidity is a controlled property
ISO 9727-3 covers moisture content, which sounds academic until a batch arrives dry and brittle or damp and prone to mold. Cork moves with the atmosphere it was stored in, so the figure belongs in the incoming check rather than being inferred from how the corks look on arrival.
TCA is measured in nanograms per liter
ISO 20752 macerates stoppers in a 12% hydroalcoholic simulant for 24 hours and quantifies releasable 2,4,6-trichloroanisole by SPME with GC-MS. The numbers matter: sensory detection sits around 1–2 ng/L and consumer rejection near 3 ng/L, while below roughly 0.5 ng/L is generally imperceptible.
Oxygen ingress is slow and variable
A natural cork admits on the order of one to two milligrams of oxygen per year, which is what allows slow bottle development. The spread between individual corks is the trade-off, and it is why producers who need a predictable figure look at technical or synthetic closures instead.
Sampling is how QC stays affordable
ISO 17727 sets a sampling plan for cork-stopper quality control, so a buyer inspects a defined subset rather than every stopper. Agreeing that plan up front turns a subjective argument about a handful of bad corks into a documented accept-or-reject decision.
