Batch distillation is largely synonymous with pot stills, and represent the simplest and oldest form of distillation. Given how old this technology is, there is huge variation found in pot still designs and this is definitely the case when it comes to Rum. The Caribbean is home to a vast amount of unique stills, sometimes built hundreds of years ago and still operational today.
Pot stills work like large kettles, and are usually made from copper because of its high thermal conductivity, malleability and purifying effect on a distilled liquid. Sometimes pot stills are heated with direct fire, but more commonly with steam as it provides a more evenly dispersed heat source.
Distillation is typically done in 2 batches, where the first distillation yields a spirit of 25-30% abv. This is called low wines and must be distilled a second time to produce a liquid that can be aged or brought to market. The hearts cut of the second distillation comes off the still at 65-75% abv, while the heads and tails will often be added to the next batch of low wines to be distilled, in order to maximise the extraction of both ethanol and congeners.
In the rum-world, pot stills rarely operate in isolation, and often have a pair of “retorts” connected between the pot and the condenser. A retort is basically an additional pot that conducts further distillation, thus increasing the strength of the liquid and removing the need to do a second batch run.
A huge number of variables in this process will affect the final character of the liquid, including but not limited to; the heat source, the design of the still, and the width of the hearts cut.