There is an time-honored custom among sofrim to immerse in a mikveh before writing STAM. This immersion is not a requirement, but it is considered standard when purchasing high-quality STAM.
There are several different variations of this custom mentioned in the sources. The earliest sources in the Rishonim only state that G‑d’s name should be written in a state of purity. A sofer could fulfill this requirement by immersing in a mikveh only when he is impure,1 assuming his planned writing for that day contains G‑d’s name. Another option – relevant only if the sofer is writing a Sefer Torah, megila, or navi2 – is to write an entire yeri’ah leaving blank spaces where the Shemos should be. When the yeri’ah is completed, he can immerse one time in the mikveh and write in all of the Shemos at once.3
There are later sources that recommend that all STAM be written in a state of purity (not just the names of G‑d), and some even advocate daily immersion in a mikveh regardless of the sofer’s status.4
There is no authoritative source behind the common belief that the sofer must immerse in the mikveh every time he writes G‑d’s name.5 Even if there were such a source, it is impractical to the point of being unrealistic.
To put it in perspective: A mezuza is written on twenty-two lines and has ten Shemos. Five of the Shemos are in the first ten words of the mezuza. If a sofer were to immerse in a mikveh before writing each and every Shem, he would have to write the first two words, immerse in the mikveh, write a Shem, immerse, write a Shem, immerse, write a Shem, write three more words, immerse, write a Shem, immerse, write a Shem… And that’s just in the first two lines of the mezuza! In a Sefer Torah it can be just as complicated, as there are columns in a standard Sefer Torah where G‑d’s name appears twenty times.
Although there may very well be sofrim who immerse in the mikveh every single time they write G‑d’s name, their behavior is not based on any of the classic halachic or kabalistic texts.
update: In light of the public health crisis caused by the covid-19 pandemic, some poskim have recommended that sofrim forgo immersion in the mikveh, and instead rely on a lower level of purification by pouring 9 kav of water (approximately 12 liters) over their bodies. A sofer who follows this recommendation is required to inform the buyer that he is doing so.
-= 8 =-
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Tevilas Ezra. ↩
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This option is not relevant for tefilin or mezuzos, which must be written kisidran. ↩
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Leaving out all the Shemos in a Sefer Torah and only filling them in after completing the entire sefer may be problematic, because the contrast between the older and newer ink might give the sefer the appearance of being menumar. ↩
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This custom is most common among Chasidim. ↩
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The origin of this misconception is probably the book All for the Boss (Shain, 1984), which erroneously ascribed this practice to the famous sofer Rav Nesanel Tefilinski. Although Rav Nesanel was scrupulous to only write G‑d’s name in a state of purity, Mrs. Shain apparently misunderstood the exact details of his behavior. ↩