Where did erasing take place in the Mishkan?

Erasing was one of the 39 melachos used in order to construct the Mishkan. The uniqueness of this melacha is that, together with a selected few other melachot, it is a destructive melacha in that it is not productive on its own. Although erasing is destructive, in the Mishkan it was utilized for constructive purposes: Chazal tell us that the beams of the Mishkan were numbered and if a wrong number was written on a beam, it would be erased in order to write the correct number. This is constructive erasing. Nowadays as well, in order for erasing to be a melacha d’oraisso it must be constructive.

How many letters must one erase in order to be liable to bring a korban?

The Melacha of writing – 'kotev'  in the Mishkan was implemented in order to write on the beams. Consecutive numbers were written on the beams in order to remember the correct place for each beam. Hence the minimum requirement is the writing of two digits. [1] Since the melacha of erasing is the diametrical opposite of writing, it too consists of the erasing of two digits in order to replace the two errant digits. [2]

Does the melacha only take place when two digits (or two letters) are erased?

The Bi’ur Halacha [3] says that the object of the melacha is to enable the writing of two letters and not the erasure per se of two letters. Consequently, erasing an ink blot, a smudge or basically anything large enough that occupies the space of two letters is an issur d’oraisso.

The Bach [4] holds that if an ink blot or wax falls onto letters obscuring them from sight and one erases the ink blot thus exposing the letters, he would be chayav on account of the melacha of writing. [5] It is likewise ossur to erase dirt pasted over even one letter or digit.

It seems that this was a practical problem. A scribe would sometimes write a sefer Torah by the light of a candle and occasionally wax would drip onto the sefer Torah. During the reading of the portion on Shabbos they would discover that some letters are covered in wax and are obscured from sight. Since it is a problem to “read” the Torah by heart they were obliged to remove the wax. The Bach holds, as mentioned, that it is a severe issur to do so. The Mishna Berura [6] rules that in such an event they must “read” those words by heart and not take out another sefer Torah.

Could this scenario be applicable in the home as well?

Yes, very easily. The Bi’ur Halacha [7] writes that if two pages are stuck to each other where letters are located, either with wax, dirt or glue, then separating the pages would be a problem of erasing. As many of us have experienced, the pages of a bentcher stick together at the letters with dried food, wine or such. Separating them generates this problem.

If two letters in a sefer Torah are touching, may one erase the point where they touch?

Two letters touching in a sefer Torah, tefillin or mezuzos render it possul and the letters must be corrected. [8] The halachos in such an event are many and the manner of correction depends on where the letters are touching. In certain cases it suffices to erase the point of contact.

In such a case, even though one is not erasing two letters or even an ink blot the size of two letters, nevertheless it is equivalent to the melacha of writing because by erasing the point of contact one is correcting the sefer Torah.