Thank you Richard Hess, you do rock! On Thu, Apr 9, 2015 at 5:03 PM, Jeff Willens <[log in to unmask]> wrote: > 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. >