More about the NAHARI ink problem - IMPORTANT
POST UPDATED WITH THE LETTER BELOW
Any sofer using NAHARI ink purchased in the past 6 months or so should stop using it immediately.
It seems that there are three problems, which I outline below:
1. The recent batches of ink are hard to write with and the ink can spread and expand after a letter is written (not sure how long after this happens) making the letter thick and ugly and sometime can loose its shape.
2. The ink bleeds to the back and is visible on the back of the parchment.This becomes is visible only after a few weeks. I have already seen it on items sent to me from sofrim in Israel (see picture).
This is a problem for mechikos, because places where the ink was rubbed out, the erased ink will reappear a few weeks later under (in brown or grey) under the new writing. So if you make a mechikah, even if you erased it well, the original writing will come back lightly and it cannot be erased.
3. If parshiyos and mezuzos or written yeriyos are placed on top of one another (or parshiyos rolled together), an imprint of the ksav underneath will appear on the back of the klaf that it touches, almost like a stencil copy, although lighter. (See above picture as well, its hard to see but that is the light brown letters on the back.) I do not think that this is a halachic problem but it looks terrible especially on a sefer Torah, and cannot be erased. It can be chalked up and covered but that's not a great solution.
Therefore, the best solution for the moment is to stop using NAHARI ink until they come out with a new batch that does not have this problem.
The Nahari Ink Company issued a statement that they did nothing differently with this batch, and are blaming it on the klaf, saying this only is happening of late only on certain klaf. From my understanding, it may be a mixture of both, because other ink brands do not have this problem, on any klaf, as far as we know.
UPDATED: Yesterday a sofer lost four yeriyos of work for his sefer torah, in which the letters swelled up and became really distorted in some places.
If anyone has any updates, please post in the comments.