ouillage in French, the operation of refilling any sort of wooden container to replace wine lost through evaporation. The container should be kept full or nearly full lest the ubiquitous acetobacter use oxygen from the head space to start the process of transforming wine into vinegar. The well-run winery will have a strict regime of topping up all wooden containers on a regular basis.
A barrel closed with a sound, new, inert plastic bung loses liquid by diffusion through the barrel’s staves. The head space created by this liquid loss is filled with water and alcohol vapours, together with traces of carbon dioxide. Since no oxygen can enter the head space, there is no danger of acetification and topping up is not necessary.