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.

-Sarah Norris

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.
> Curtis
> -----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.

Sarah Norris 
Texas State Library and Archives Commission 
phone: (512) 463-5446 
fax: (512) 463-5430 
e-mail: [log in to unmask]