The Rama (459) records the Halacha not to bake Massa in a room with a window exposed to the sun. This would potentially cause extra heat to the dough and expedite leavening. This is more problematic on a cloudy day, as mentioned in Masechet Taanit, because the rays of the sun are dispersed and will enter, even if there is no direct sunlight. Therefore, any window in the Massa bakery should be covered.
It is highly advisable that in between each batch, all workers wash their hands to prevent residual dough from sticking to their palms and fingernails. That dough would become Hametz after 18 minutes and would render the subsequent rounds of dough unfit.
It is important that they dry their hands well to prevent tap water, which is not "Mayim Shelanu"-water that rested overnight-from being incorporated into the dough. There are opinions that if dough was kneaded with such water, the Massot are unfit.
All workers should remove their watches, to prevent dough from becoming stuck in the band. They should also refrain from using cell phones during the baking process. Besides for interrupting their concentration, residual dough may stick to the devices. This is also the reason the Poskim recommend that workers roll up their sleeves.
These Halachot illustrate how much caution should be exercised when baking the Massa. There is only a hairline difference between Hametz and Massa. This principle is illustrated by the Hebrew spelling of the words "Massa" and "Hametz." They both share the letters "Mem" and "Sadi," whereas Massa contains a "Heh" and Hametz contains a "Het." The form of the Heh and the "Het" are almost identical except for the small fracture in the leg of the "Het."
This is why Hacham Ben Sion once said that the most dangerous food to eat on Pesah is Massa. It is the most likely to be Hametz! Therefore, he personally limited the amount of Massa he ate on Pesah to the mandatory requirement.
SUMMARY
1. Massa should be baked in a room without windows.
2. All workers engaged in baking must wash and dry their hands in between each batch.
3. All workers must roll up their sleeves, remove their watches and refrain from using cellphones during the baking process.
