I'll dip my toe in with an observation. Most of my experience has been 
with 3/4" u-matic tape from the early 80's.

I have always been amazed (jealous) when I read results here about 
recovering audio after 8 hr or 1 day of baking.

When I encountered U-matic tapes going (gone) bad I started out trying 
to bake a day or two but quickly came to the conclusion that I was 
wasting my time re-assembling the tape housing and testing in a video 
machine. As opposed to audio tape machines, testing video and 
re-cleaning the machines is more laborious (my guess).

Now when I confirm a bad tape I stop the play back immediately. Shells 
are open-up  and then I feed de-humidified air into the back of an oven 
with a blower set to 135 F for a min of one week at which point I may 
test a tape  or two and if it fails, the another full week.

The longest I baked material was 4 weeks at which point the heads were 
still clogging up after 20 min, so I cleaned and continued and stitched 
the video together afterwards.
I would typically bake 12 u-matics at a time, preferably in the fall or 
winter.

Observations-
1) do not worry about ruining high frequency content on audio tape. 
Video tape is ALL high frequency (Mhz) and the baking allows the signals 
to be recovered .
2) I suspect the width of the tape is a big factor as well as humidity. 
If temp alone mattered then baking a 1/4 in tape or a 3/4 in tape would 
reach thee same temp within an hour easily).
3) I suspect it takes exponentially longer for the dry air to have an 
affect wider tapes.

Guess -
1) I suspect a gadect which continually wound a tape back and forth 
between two reels slowly in an oven would speed up the process for video 
tapes.

Off to try and give away a working Webbcor 210-1c in the GTA (free)


On 9/1/2017 2:02 PM, Richard L. Hess wrote:
> Lou,
>
> What Peter wrote is key to understanding sticky-shed. My chemist 
> mentor, Ric Bradshaw, formerly chief chemist at IBM tape division, 
> suggests that the original matrix is never recreated by baking so the 
> repolymerized longer chains are not as long as the original chains, 
> but no longer short enough to be sticky.
>
> A few years ago, I visited the Library of Congress and happened to 
> mention that I was baking tapes for two days (1/4 and 1/2 inch, mostly).
>
> The people I met with wanted to know more. They bake for about eight 
> hours and have good results. I rarely achieve my goals with that short 
> of a baking cycle.
>
> LoC confirmed they were transferring their "captive" tapes which had 
> spent some time in their controlled storage.
>
> I receive tapes from "the wild" and often they have been miserably 
> stored. There is a lot of humidity in many parts of North America.
>
> Stuart Rohre bakes tapes 30 days now. these are 15-inch windowless 
> glass reels of 1-inch instrumentation tape which had been sent to sea 
> in a buoy to record a string of hydrophones. The machines were exposed 
> to the sea atmosphere just prior to launch. He can make a full pass on 
> one of these reels after the 30-day baking. So, to me, that seems to 
> indicate that those of us who bake two days still have some "headroom" 
> left before things get really bad.
>
> I rarely go back and re-transfer tapes that I've baked, and if I do, 
> it's within a few weeks, usually (to see if I can improve a specific 
> aspect of a transfer).
>
> If I have NR encoded tapes, I save both a raw and decoded version so I 
> don't have to go back to try and correct NR anomalies. In fact, I 
> provide both to the client. I started doing that on a 16T 1/2-inch 
> tape that had some tracks recorded with NR and some not and no 
> documentation as to which was which, and it was "new" music, so I just 
> sent both sets to the composer and said (nicely), "you figure it out." 
> He was ecstatic as the original machine when they did the analog mix 
> would not allow them track-selective NR switching.
>
> Cheers,
>
> Richard
>
>
>
>
> On 2017-09-01 1:40 PM, Lou Judson wrote:
>> This is quite a wordy explanation, and I do not doubt or argue with 
>> it. My only interest is in recovering tapes people bring me to 
>> transfer, and so far have had excellent success, baking 8 - 10 hours, 
>> cooling the some amount of time, and playing them then. I have not 
>> examined them later, the storage comment was third-hand anecdotal, 
>> not personal or scientific.
>>
>> But - can you please specify what you mean by “Short-term baking?” 
>> and is long-term better? How short, how long and what temperatures? 
>> Also, what do you recommend for long term storage after the treatment?
>>
>> Thanks. Peter.
>> <L>
>> Lou Judson
>> Intuitive Audio
>> 415-883-2689
>>
>> On Sep 1, 2017, at 10:27 AM, lists <[log in to unmask]> wrote:
>>
>>> Pardon for the very late posting (have been massively busy with 
>>> disaster
>>> recovery projects) but the posting concerning "sticky shed" truly 
>>> needs some
>>> clarification as the way it is stated is very misleading.
>>>
>>> Sticky shed is caused by binder hydrolysis.  This is indeed a 
>>> function of
>>> chemistry- but the chemistry involved is the interaction of water 
>>> (often
>>> absorbed from humid air) with the long-chain polymers in the tape 
>>> resulting
>>> in the polymers breaking down into short-chain, low-molecular-weight
>>> oligomers.
>>>
>>> While this is a chemical reaction, the reaction is very dependent on 
>>> the
>>> moisture content of the air in the environment in which the tapes are
>>> stored.  The assertion that " they return to the sticky state 
>>> eventually,
>>> even in perfect storage" is not correct.  Should this happen, they 
>>> have not
>>> been placed in "perfect storage".  It has, in fact, been proven that
>>> "sticky" tapes (without baking) will become less sticky if stored in 
>>> stable,
>>> low-RH environments.  If you store polyester-base tapes in an 
>>> environment of
>>> approximately 68 degrees and an RH of 20% or less, within a year, 
>>> most (some
>>> take longer) sticky tapes are no longer sticky and further testing 
>>> 2, 3 and
>>> 5 years down the road, show that the tapes continue not to exhibit 
>>> "sticky
>>> shed".  As such, the "perfect storage" referred to in the earlier 
>>> post is
>>> not actually "perfect" storage for polyester-base magnetic tapes.
>>>
>>> Another issue could be the method used for "baking" and how soon the 
>>> tapes
>>> are tested after they have been returned to storage.  Many people 
>>> perform
>>> short-term baking.  The issue with this is how hydrolysis affects 
>>> the tape.
>>> When hydrolysis occurs, polymers in the tape matrix as well as 
>>> polymers on
>>> the tape surface are effected.  The oligomer residue created inside 
>>> the tape
>>> matrix may partially migrate to the tape surface.  Heating the tape 
>>> during
>>> short-term baking primarily causes some of the oligomer residue to be
>>> re-absorbed into the tape matrix leaving less on the surface and 
>>> making the
>>> tape, temporarily, playable.  It has little effect on the oligomers 
>>> other
>>> than their absorption into the tape and away from the surface.  As 
>>> soon as
>>> the tape begins to cool, these oligomers (slowly) start to migrate 
>>> back to
>>> the surface again.  This is one of the reasons that individuals who do
>>> short-term baking state that you must play back the tape as soon after
>>> baking as possible.  More sustained treatment (whether by "baking" or
>>> exposure to very low RH environments or a vacuum) actually forces
>>> cross-linking of the oligomer residue back into polymers.  As such, 
>>> there is
>>> no great abundance of oligomer residue to migrate back to the 
>>> surface and
>>> the tapes (so long as they are not exposed to elevated humidity) remain
>>> playable for an extended time.
>>>
>>> This is likely more information about the subject than most people 
>>> really
>>> want to know but to state that "sticky shed" is not a function of 
>>> "storage"
>>> is extremely misleading.
>>>
>>> Just as background, I was one of the primary authors of the National 
>>> and
>>> International Standards about magnetic tape storage and magnetic tape
>>> handling  published by ANSI, AES and the ISO.
>>>
>>>
>>> Peter Brothers
>>> SPECS BROS., LLC
>>> 973-777-5055
>>> [log in to unmask]
>>> Audio and video restoration and re-mastering since 1983
>>>
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: Association for Recorded Sound Discussion List
>>> [mailto:[log in to unmask]] On Behalf Of Lou Judson
>>> Sent: Thursday, June 29, 2017 11:44 AM
>>> To: [log in to unmask]
>>> Subject: Re: [ARSCLIST] Cassette repair question
>>>
>>> I believe it has been more or less proven that sticky shed is a 
>>> function of
>>> chemistry, not storage, as even after baking they return to the 
>>> sticky state
>>> eventually, even in perfect storage.
>>>
>>> <L>
>>> Lou Judson
>>> Intuitive Audio
>>> 415-883-2689
>>>
>>> On Jun 29, 2017, at 5:11 AM, Steve Greene <[log in to unmask]> 
>>> wrote:
>>>
>>>> But bear in mind, the loss of the felt suggests that the tape probably
>>>> wasn't stored in the best conditions, be concerned about sticky-shed,
>>>> or other binder problems in your future.
>>>
>>

-- 
John Gledhill
BIT WORKS Inc.
905 881 2733
[log in to unmask]
www.bitworks.org