The Baumé is a hydrometric measurement of density developed by the French pharmacist Antoine Baumé in 1758 to measure the density of various liquids (originally brine); in this case we are using it for must or grape juice. The degree Baumé approximates to the potential alcohol resulting from the complete fermentation of the must.
The reason you have seen various pictures of us taking baumé readings is that this is what we use to determine when and how much brandy to add and also how we follow the progress of fermentation. The first reading we take is after the lagar has been trodden for the full 4 hours. A sample of must is taken from each of the 4 corners and mixed before taking the reading. This is what we call the initial baumé (i.e. the lagar has been trodden and mixed but is not yet fermenting), for example 13.5º, and it is from this value that our fortification calculations are based, telling us at what baumé to stop the fermentation, for example 8º, and how much brandy to add in order to achieve the final wine desired.
This initial value is therefore very important to measure correctly.
Once the fermentation has begun we will follow the progress of the fermentation by taking readings initially every 4 hours as the baumé gradually decreases from 13.5º getting ever closer to 8º. Once the baumé reaches 9º we begin taking readings hour by hour and even more often the closer it gets to 8º in order to add the brandy at exactly the correct moment.
Taking a baumé is no hassle at all; it’s just not that much fun when you have to do it at 3 or 4am in the morning!