Back in 1971 or so, we had a PDP-5 with DECTapes, and bought 3/4" analog recorder tapes surplus to make up new tapes.? It was much thinner than DECTape, and we had print-through issues of tapes that were stored for a few weeks. That seemed to clear up if the tapes were then used again.? I guess the permeability was not as high as real DECTapes. Jon

On 1/24/22 7:30 AM, Paul Koning via cctalk wrote: I also have tried the alcohol wiping on a piece from a 'fresh' tape (never used - still with tape seal) and the results were the same. The procedure was to slightly dampen a piece of lint-less paper towel (e.g. kimwipe) with the alcohol solution and wipe with minimal force over a small portion of the 'data' side of the tape.? (I used 25%, 50% and full strength alcohol and all were identical except for time required to remove the layer.)? Removing the dark brown/grey 'coating', allows the brown oxide to show. After this step, the top and bottom of the tape looked identical (a medium color iron oxide.)? This 'exposed' oxide did not seem to be any easier to scrap off using a sharp exacto knife (no oxide residue when scraping, but clearly left scrape marks on the tape) suggesting there is still a robust protecting layer left on the tape. Also, after removal of the layer with alcohol, there was no apparent thickness change.? I haven't used a micrometer yet to verify, but the 'fingernail' test doesn't indicate any thickness layer boundary between the 'cleaned' and 'uncleaned'? portions. Paul - thanks for the bitsavers reference.? I thought I had copied all of the DEC documents relating to DECtape, but clearly missed that one. -- -Gary

It sounds more like some sort of sprayed-on coating of some sort of abrasion-resistent material. It might well be porous, which would be a possible explanation for the oxide coming off when rubbed with solvent. Is it possible we're looking at something called "dirt"? C

On 1/24/22 11:09 AM, Chris Zach via cctalk wrote: Not likely, as it's on all of my? tapes, including the four 'unopened' (well, there opened now...) tapes I had in my stash. Brand spanking 'new' in blue plastic boxes, still with seal on tape.? I even had an unused 'scotch' brand that had the seal on the tape, though? that one came in a paper box, so conceivably it could have been 'contaminated'.? But it still looks the same. Also, in reference to another comment: the 'oxide' isn't coming off exactly.? I did have one tape with some really ragged ends on it, and on that one, the oxide did flake a bit.? But that tape had other damage on the leader end.? For the rest of them (tried so far) all I can say is the oxide appeared 'exposed' but on several different tapes I tried 'scraping' this exposed layer with an exacto knife and all that happened is some scratches appeared on the 'overcoat' layer.? No oxide residue on the knife. Sorry if it appears I implied the oxide was degrading.? May on one or two, but this is not a feature of the other 100-ish tapes. I'm guessing it *is* some kind of spray-on 'extra' lubricating coating, so I'm just trying to figure out what it is.? Obviously it's extremely soluble in iso, but that's the 'strongest' thing I've tried.? As I think I mentioned, even water will degrade this layer with some rubbing, but alcohol takes it off quickly.? 'Dry polishing' the tape doesn't seem to degrade the coating, though I do seem some 'smudges' left on the kimwipe. This issue isn't standing in my way of completing the project - I was just checking the 'collective conscience' here if any knew some unpublished secrets or where they were published if not secret. Thanks for all the replies. -Gary

I thought Black Watch has coating on the back, as others mentioned. DECtape is not black but brown, so graphite doesn't seem to be involved. paul

So, can we assume that the words about a "tape sandwich" refer to a mylar base, oxide coating, and a lubricant/protective coating? That is not an oxide coating sandwiched between to layers of mylar. There was such a tape construction, used on the Datamatic 1000. (ca. 1955) --Chuck