Question about DECtape formulation

Gary Oliver go at
Mon Jan 24 11:56:09 CST 2022

On 1/24/22 7:30 AM, Paul Koning via cctalk wrote:
>> On Jan 23, 2022, at 8:40 PM, Noel Chiappa via cctalk<cctalk at>  wrote:
>>> From: Gary Oliver
>>> I've always thought the physical tape wound on a DECtape spool was a
>>> fairly conventional 'sandwich' of mylar/oxide/mylar ...
>>> Was there some kind of 'lubricating' coat on the data side? It makes
>>> sense, but none of my DEC documents or Googling has any mention of
>>> lubrication ...
>>> If someone has some detail information on the tape construction, I'd am
>>> curious to see it.
>> Dunno if you know of this:
>> but it doesn't mention any lubrication, just a "Protective Overlay" layer,
>> over the "Coating" (which I assume is the oxide). I'm a bit surprised that
>> "some of the data side of the tape came off on the wipe", though, unless the
>> "various concentrations of isopropanol/water" dissolved the Protective
>> Overlay.
>> 	Noel
> Depending on how much the tape has been used, it's possible that the top layer has worn through.  I have known that to happen on highly used tapes, though not many got enough to wear that far.
> It also might be oxide tranferred from one tape to the head to other tapes, if the heads weren't cleaned enough.
> 	paul
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.


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