Purging is the step in solvent-based extraction where leftover solvent is removed from the finished concentrate. After butane, propane, ethanol or another solvent has dissolved the cannabinoids and terpenes, most of it is recovered, but some remains mixed in the extract. Purging removes as much of that as possible.
Extractors usually purge using gentle heat and a vacuum oven. The vacuum lowers the boiling point of the solvent so it evaporates at a temperature that does less damage to terpenes. Purge time and temperature also affect texture. A still, undisturbed purge tends to make shatter, while stirring or whipping can produce badder or sugar.
A well-purged concentrate should contain only trace amounts of residual solvent, well within regulatory limits. That is exactly what the residual solvents section of a COA measures, often in parts per million (ppm).
You may hear people judge a purge by color, bubbles or how a concentrate looks. Those are not reliable signs; only lab testing can tell you how much solvent is left.
Solventless products like rosin and hash do not need purging because no solvent is used. If you want to know more, ask to see the product's COA.