Richard, 

You rock!

I'm not sure that either Partial Erasure or Magnetostriction are involved
here simply because this issue ran across more than one reel coming from
different batches, different studios and different years, yet all were
Ampex, mid-1970s. But who knows, since all those tapes may have been pulled
for the same type of project years before. 

Really the most likely causes I'd think would be:

(4) loss of mag coat due to normal unbaked sticky shed deposits on heads and
guides

(5) increased spacing loss due to warping or embossing of the tape or other
mechanical/chemical defects

And possibly this, or some variation thereof:

(7) Loss of magnetic information where the magnetic particles stop being
permanent magnets. 

Has this ever been investigated as a side effect of too much cumulative bake
time in an oven over a tape's lifespan? 

I had a 2" multi that I had to bake in order to transfer to make a safety
copy. No matter how long I put it in the oven (hours, days, etc.), it still
would not play without shedding. I was finally able to get exactly one pass,
and that was it. There was just no way this tape was going to withstand
another baking regimen. 

Jeff




On Wed, 8 Apr 2015 08:58:36 -0400, Richard L. Hess
<[log in to unmask]> wrote:

>Jeff,
>
>That is scary.
>
>There are two magnetic-related failure modes I can think of that MIGHT
>have contributed. I'm mentioning them to see if you think either (or
>both) might be at play.
>
>(1) Partial erasure: this is the more common and better understood
>failure mode. It involves relatively low-level magnetic fields. These
>are also applied for "skimming" to reduce print through and can be
>unintentionally applied by magnetized heads and guides, though uncommon,
>can happen.
>
>(2) Magnetostriction: This is less-well understood and the only case I'm
>aware of that it was consistently applicable was on cassettes and since
>this is arguably a wavelength dependent failure mode, related to sharp
>bends of short-wavelength recordings, is probably not applicable.
>
>Then, as you (or someone) mentioned, there are the physical damage modes:
>
>(3) loss of mag coat due to ripping out by an unbaked playback attempt
>
>(4) loss of mag coat due to normal unbaked sticky shed deposits on heads
>and guides
>
>(5) increased spacing loss due to warping or embossing of the tape or
>other mechanical/chemical defects
>
>(6) poor azimuth performance due to country lane-ing usually resulting
>from poor slitting but can be warped in that direction as well due to
>poor winding and storage.
>
>(7) Loss of magnetic information where the magnetic particles stop being
>permanent magnets. This is called the "Curie temperature" or "Curie
>point" (Tc) and is usually not considered a factor. For two common iron
>oxide formulations, it is:
>Iron(III) oxide (Fe2O3) 	948 K  675 °C  1,247 °F
>Iron(II,III) oxide (FeOFe2O3) 	858 K  585 °C  1,085 °F
>
>BUT, Chromium dioxide's Tc is lower:
>CrO2 				386 K  113 °C    235 °F
>While that is above the recommended baking points (by more than a factor
>of 2), it is not out of the question that these temperatures could have
>been reached without destruction of the tape base film. The glass
>transition temperature (Tg) of PET is about 67-81 C and that of the mag
>coat is lower, sometimes below room temperature. The Tg is the point
>where the polymer becomes rubbery. The melting point of PET is at least
>250 °C and the boiling/destruction point is at least 350 °C. While I
>quickly grabbed these PET numbers from Wikipedia, I recall that they are
>in keeping with what I've seen from other sources.
>
>Anyway, I'd appreciate your thoughts on how these magnetic records were
>damaged (or rendered not playable) at that point in time.
>
>Cheers,
>
>Richard
>
>
>On 2015-04-08 1:06 AM, Jeff Willens wrote:
>> Worn out tapes are absolutely in the vaults right now, and have been for
>> some time. I've come to find out (and perhaps others can correct me), that
>> there is a finite number of hours a polyester-backed tape may be baked
>> cumulatively before it stops working on the binder and/or it stops giving
>> the fidelity it used to offer.
>>
>> As for tapes in record company vaults, having worked at more than one major
>> label, I can say that you just don't know who's been on a tape before you
>> get hold of it. Some engineer may have tried to play a shedding Ampex 456
>> tape for a transfer without baking it, and lost a ton of oxide in the
>> process years before you. If so, there went your high end, and all your
detail.
>>
>> I had to do tape transfers of a very prominent 70s R&B band (who will remain
>> nameless) for an outside mastering engineer doing a CD compilation. They HAD
>> to be from the "original masters". All the masters were on Ampex 456 and 406
>> 1/4". All had been baked and used scores of times over the years. I checked
>> and baked the tapes for our usual amount of time, set up my machines, did
>> the azimuth, etc. All was set to go. I put up the tapes. And they sounded
>> like crap. Every last one of them. I double checked my entire set up. I
>> checked the tape path. I had other engineers checking everything. Nothing
>> made sense. All the tapes sounded like I was playing them from the outside
>> -- muffled, muddy, and smeared. Still, I transferred them flat and sent them
>> on.
>>
>> 2 days later, an extremely irate mastering engineer called me up and
>> demanded to know why I transferred the tapes inside out. When I explained
>> what happened, he couldn't believe the tapes could just "wear out" like
>> that. But that is what happened. They were utterly shot. No amount of
>> baking, EQ, azimuth tweaking, or sonic wizardry at the time was going to
>> bring these things back from the dead. The ME used other sources.
>>
>> So it's not necessarily "shelf life" that contributes to a reel of tape's
>> demise, but the use and handling it may get over the years that does the
>> ultimate damage. And that damage can be irreparable.
>>
>>
>>
>>
>>
>> On Tue, 7 Apr 2015 19:06:13 -0400, Robin Hendrickson
>> <[log in to unmask]> wrote:
>>
>>> I had a conversation once with someone who worked in the
>>> Capitol/EMI/Universal system, and this person told me that the "the
>>> old tapes are wearing out" and that all tape has a maximum shelf life
>>> of 30 – 40 years.
>>>
>>> That sounded like BS to me; I know there are older tapes out there
>>> that still work fine. (No guarantees, of course.)
>>>
>>> This Tom Petty story made me wonder whether there is some fallacious
>>> conventional wisdom out there that would lead one to rush to a verdict
>>> of "Yep, no way these old tapes are gonna work, we have to use
>>> something else."
>>>
>>> The mind reels. Excuse the pun, this is a serious matter indeed.
>>>
>>> Robin
>>>
>>>
>>>
>>> On Tue, Apr 7, 2015 at 5:26 PM, Richard L. Hess
>>> <[log in to unmask]> wrote:
>>>> On 2015-04-07 3:18 PM, Eric Jacobs wrote:
>>>>>
>>>>> 2-inch tapes with sticky shed may not respond as well to baking as
>>>>> 1/4-inch.
>>>>>
>>>> That is true in the sense of in the same or similar time frame.
>>>>
>>>> I don't think there are precise formulae for predicting the time either to
>>>> achieve thermal equilibrium or to achieve moisture equilibrium in a tape
>>>> pack. Vos (1994) inspired me to develop a rule of thumb that moisture
>>>> equilibrium appears to take 1500 times as long as thermal equilibrium in a
>>>> one-inch tape, based on my extrapolations from his curves.
>>>>
>>>> We have long suspected that the width of the tape was a large modifier of
>>>> this ratio. I based my estaimate on Vos's graphs which seemed to indicate
>>>> that a 1-inch tape pack, might achieve thermal equilibrium might in 100-200
>>>> minutes while it might take 100-200 DAYS to achieve moisture equilibrium. I
>>>> felt that a factor of 1440 implied far too much precision in the
>>>> calculation, so I rounded it to 1500.
>>>>
>>>> Further pointing to this is what Stuart Rohre has reported on the Ampex
>>>> mailing list and elsewhere. He has been responsible for retrieving the most
>>>> information possible from some 1-inch instrumentation tapes which are
>>>> 15-inch diameter tape packs on glass precision Corning reels with no
>>>> windows. The windowless reels further slow moisture diffusion. He had
>>>> originally said they were baking for several days and could get through
>>>> about half the tape and then had to rebake, but they also had to run the
>>>> tape through their Bow tape cleaners. Partially at my suggestion and
>>>> partially on his own initiative, Stuart found that if he baked the
tapes for
>>>> 30 days, they would play through without the need for any tape cleaning or
>>>> re-baking and he was getting very clean signals off the tapes at that
point.
>>>>
>>>> So, are the two-inch tapes not responding to baking or simply in need of
>>>> more of it?
>>>>
>>>> One 7-inch reel of 1/4-inch tape that had been exposed to high humidity
>>>> cycles overnight had a very easy-to-remove mag coat when first inspected.
>>>> When it was stored in my air-conditioned home (minus the economizer cycle
>>>> bringing in Los Angele's famed "Marine Layer" of "night and morning low
>>>> clouds") for 3-4 months, the same test that initially showed mag coat
>>>> removal could not be duplicated and the tape binder seemed very secure at
>>>> that point.
>>>>
>>>> The "more baking" concept pertains to tapes like Ampex 456, 406, and 407 as
>>>> well as the instrumentation tapes made by Ampex at about the same time. It
>>>> may also apply to Scotch 226 and 227 and possibly Scotch 250. It probably
>>>> does not apply to Agfa tapes which have some of their own nastiness.
>>>>
>>>> This web page attempts to categorize tapes by degradation modality, and
>>>> degradation modalities are currently described more by what can ameliorate
>>>> their effect than by the actual chemical/mechanical failure modes. My
>>>> decade-long goal of a "pool-test kit" for tape degradation measurement is
>>>> farther in the distance than it was when I started the quest.
>>>>
>>>>
>>
http://richardhess.com/notes/formats/magnetic-media/magnetic-tapes/analog-audio/degrading-tapes/
>>>>
>>>>
>>>> Cheers,
>>>>
>>>> Richard
>>>> --
>>>> Richard L. Hess                   email: [log in to unmask]
>>>> Aurora, Ontario, Canada                             647 479 2800
>>>> http://www.richardhess.com/tape/contact.htm
>>>> Quality tape transfers -- even from hard-to-play tapes.