Having established that 20 v.% is an optimal amount of alcohol when ethanol is used, see Figure , with a molar Base/Au ratio around 4, and that temperature is not a convenient knob to achieve size control, we used these conditions to further optimize the synthesis at RT. Over several repeats in different laboratories and countries, it became clear that the synthesis performed in different environments tends to lead to slightly different results. Certainly, the purity of the chemicals can play a role, but for a same set of chemicals and same stock solutions, it became also clear that relatively higher ʎspr values were measured for syntheses performed on cloudy days, compared to relatively lower ʎspr values measured for syntheses performed on sunny days, see section S8. To the best of our knowledge this sensitivity has not been reported or commented on for glycerol-mediated syntheses.