The liqueur de tirage is a mix of wine, sugar, and yeasts, which is added to still wine to induce the second in-bottle fermentation and obtain sparkling champagne.
In French, the definition of tirage is drawing off, and tirage in champagne means drawing off the blended wines into bottles.
Once the bottles are filled with the blended wine obtained during the assemblage, the winemaker adds the liqueur de tirage to kick-start the second fermentation.
This champagne liquor is made with a base of still champagne wines that also includes natural and/or mineral adjuvants (tannins, bentonite, alginates) that make the sediments of fermentation heavier and easier to remove later.
The bottles are hermetically sealed with a bidule, a hollow plastic cylinder that will later hold the dead yeasts and sediments for their removal before commercialization, held in place by a metal crown cap–the same used for beer bottles.
Some houses use corks instead, like it was done in the past, in general for their higher-end champagnes.
The bottles are then transferred to the cellar and stacked horizontally sur lattes (on slats), row upon row where they will undergo the second fermentation that will last several weeks, even several months.