At a given moment, the number of read/write errors increased very fast. Careful examination showed us that a "chewing gum"-like film was deposited on the read/write heads in several units. Trend analysis showed that the problems correlated to units that processed tapes coming from a certain user. These tapes contained measurements made on board of one of the Royal Navy vessels. Certainty about the problem was obtained when the magnetic layer of one of the tapes came loosely from the tape; one could look through the tape. Cleaning of the magnetic tapes - which is running the tape over a saphire and scrape dirt from it - did not resolve the problem.
After clearing a unit very carefully, experiments with unused magnetic tapes that had been on board of the Navy vessel, made it clear : These tapes caused the read/write errors. Other tapes from the same series, that had not been on board did not show the sticky gum problems.
The tapes of those series magenetic tapes had a smooth silicon finishing layer.
As the summer had been very hot, it was feasible that the tape cartridges had
been left in the full sunshine or in the trunk of a car causing exposures to
high temperatures. This might have caused desintegration of the silicon finishing
layer. The manufacturer did not react to questions... and was subsequently removed
from the list of suppliers to the Laboratory.
Using a couple of magnetic tapes which were placed for a short period in full sunshine and a control set of tapes from the same batch of tapes which was kept cool in the central computer room. A short period later, we could proof that the sunshine heating had caused the problems (a real "TNO research project"!).
In the mean time, the "chewing gum"-plague had spread as a sticky layer problem across the total magnetic tape set. Especially affected were the set of backup tapes, which could result in serious problems when a recovery was needed if a system calamity would occur.
It was decided to copy about one hundred of tapes to a new set of tapes from another manufacturer to have a "clean" copy. The set of dirty tapes was processed only on one (slow) tape unit in order to keep the other units clean. This unit was cleared a couple of times a day. At the same time, the backup tapes were replaced.
After these rigourous measures, a couple months later we could see from the trend analysis that we overcame the "sticky" problem. After half a year, the special measurements could be lifted.