This is not what we would expect. Leaving the lid on the French Press should keep the water hot longer, which would increase the extraction. We recommend that you leave the French Press lid off at least until the crust has broken. This will maximize extraction.
This effect was already observed in previous experiments to determine the best time for breaking the crust during cupping. To ensure that cupping did not depend on the crust insulating its surface, we covered the cupping cups with polystyrene covers. We found that the extraction rate was lower the longer the polystyrene lid was left on.
The longer the cupping bowl is covered, the less extraction occurs. See the original experiment for more information.
Matt initially thought that condensation from the lids might have been dripping onto the crust, partially breaking it. However, after publishing these two experiments, two Barista Hustle readers almost simultaneously, suggested a different explanation. They suggested that allowing the surface cool and leaving the lid off was increasing extraction and helping convection currents form.
The French Press’s top portion of the liquid will cool down if you don’t close the lid. This creates a temperature gradient with hotter water at its bottom and cooler water at its top. Hot water is denser than cold water, so it rises. The cooler water at its top sinks to the bottom. Convection is a result of this circular current in liquid.
We set up a simple experiment to see if convection is causing this odd effect or if drops of condensation are interfering with crust. We brewed coffee using cupping bowls. Some cups were covered with concave dishes. The lids could be filled with hot water to keep it warm or with ice to keep it chilled.
To test the theory, Matt has setup an interesting experiment which proved that the reason for the increased extraction with a non-covered beaker was indeed convection. Read the the experiment here.