Having published a sticky shed paper with review from Richard Hess and Ric Bradshaw - and many thanks to them both - I may be able to summarize an understanding of baking that contrasts with the Bertram / Cuddihy view. Ric could certainly do this more thoroughly, but I will make an effort, as it seems pertinent to the discussion.
Bertram / Cuddihy would say that baking works because increased temperature and reduced relative humidity reverse the hydrolysis reaction by driving off water. This actually re-makes the binder that holds media in a matrix, at least for a short time.
Bhushan / Bradshaw would likely counter that this is not the case. Instead, two distinct processes occur during baking. Increased temperatures soften the binder and allow greater particle mobility within the coating slurry. Meanwhile, reduced relative humidity drives off water and opens up hydrogen bonding sites between the binder molecules and the media particles. Though the degraded binder is NOT re-made, the slurry does become more cohesive, at least for a short time.
One critical difference between these two models is the roles played by temperature and RH. In the Bertram / Cuddihy model, temperature and RH both achieve the same goal: driving off water. In the Bhushan / Bradshaw model, temperature and RH achieve different goals: temperature softens binder while RH drives off water. Both tools are required.
My paper (ARSC Journal 41 No.2, Fall 2010) followed the Bertram / Cuddihy model and attempted to achieve baking-like results with only one of two seemingly redundant tools, RH. I did not achieve predictable results, at least in the time span of the test. There are several possible readings as to why this happened: one is a support of the Bhushan / Bradshaw model.
Date: Mon, 1 Apr 2013 17:25:32 -0400
From: "Richard L. Hess" <[log in to unmask]>
Subject: Re: Sticky SHRED
My theory is there is more degradation product which needs to be
"evaporated" out or otherwise processed. I don't think anyone has
definitively described the process to a good deal of satisfaction. Ric
Bradshaw made it plain to me that he doesn't buy the reversible nature
of the hydrolysis reaction as described by Bertram and Cuddihy. He
indicated that it just doesn't happen that way in a filled matrix--the
chain ends can't find their way back through the other material and join
up. But we know baking works.
20 °C and 33 % won't fully stop degradation. I do not know what the LoC
vaults for tape are kept at, but 20 °C is not that cold. 33 % is pretty
dry. But your tapes are better off than most.
I don't do enough rebakes to know. I have followed Tom's advice for a
long time. Get a good transfer and be done with it. Ric Bradshaw was
unequivocal about this as well.
The best reason to keep the original tapes is that we have not recovered
what Jamie Howarth calls "Mechanical Metadata" from them yet. He's made
strides towards recovering the mechanical metadata even without
processing it. There are certainly materials that are worthy of this
approach. I don't know if there is a business case for this for me or
other independent restorers.
On 2013-04-01 5:00 PM, Peoples, Curtis wrote:
> We have stored our tapes for ten years in an environment averaging 68 degrees F and a RH of about 33%. It used to take 4-6 hours to bake a tape. We are now at 8 hours. I am interested to know why these tapes are now taking longer.
> -----Original Message-----
> From: Association for Recorded Sound Discussion List [mailto:[log in to unmask]] On Behalf Of Tom Fine
> Sent: Monday, April 01, 2013 3:42 PM
> To: [log in to unmask]
> Subject: Re: [ARSCLIST] Sticky SHRED
> Any theories as to why baking times are increasing?
> Also, are you finding you need the longer baking times for re-bakes as well as first-time bakes?
> -- Tom Fine
Richard L. Hess email: [log in to unmask]
Aurora, Ontario, Canada 647 479 2800
Quality tape transfers -- even from hard-to-play tapes.
Texas State Library and Archives Commission
phone: (512) 463-5446
fax: (512) 463-5430
e-mail: [log in to unmask]