On 1/24/2016 2:05 PM, John Chester wrote:
> On 1/24/16 1:33 PM, John Haley wrote:
>> I have followed this long thread but frankly have found a lot of it
>> confusing. David, I have always assumed it is in fact the black
>> back-coating that is what has turned sticky and gums up the machine on
>> unbaked tapes. It wouldn't be the oxide layer coming off like that.
> In my experience, which is mostly with Ampex tape, both the oxide and
> the backcoating are sticky, and some of the oxide is coming off when an
> unbaked tape is played. The deposit that forms on heads and guides
> exposed to the oxide side of the tape is mostly oxide, although
> initially it seems to be somewhat darker in color than the oxide. I
> think some of the backcoating becomes embedded in the oxide.
> In the worst case, large chunks of oxide are so solidly attached to the
> backcoating that they are pulled off the base. (But I've never seen
> large chunks of backcoating pulled off the base.....)
> The deposit that forms on static surfaces exposed to the back of the
> tape is mostly backcoating.
John Chester's description accurately describes my experience. I think
where we went with this thread--at least where I went--was to better
understand how such different perspectives can be unified by better
understanding what actually happens.
(1) Both the mag coat and back coat are susceptible to hydrolysis.
(2) The back coat is more susceptible to hydrolysis and perhaps even
actively slurps up moisture from the air due to the large amount of
carbon black in the back coat.
(3) The back coat's higher moisture level directly in contact with the
mag coat accelerates and exacerbates the hydrolysis of the mag coat.
(4) Richardson's process of removing the back coat plus some added
processing to re-stabilize the mag coat appears to be working (although
I still hold to my contention that removing the back coat may be a risky
and time-consuming process, although Richardson holds to his contention
that baking is harmful, too).
(5) Baking causes the short, sticky chains to re-polymerize into longer
chains but not as long as the original chains.
(6--Richard's next conjecture) There is contention that multiple baking
cycles can degrade the sonic quality of a tape. What would be
interesting to note is the physical scale we are talking about with the
re-polymerization (as contrasted to wavelength) such that perhaps after
several cycles, the magnetic domains are moved enough (at random) to
start causing some gap-scatter-type losses (not dissimilar to azimuth
losses), although in this case it is the opposite: magnetic domain
scatter. I don't know if this is possible or if the measurement scale is
correct. This is really a question, because if it is possible, it would
neatly allow us to tie a bow around the theory of this process. We all
look for a unified theory of everything. This is on a scale and of such
a nature that we cannot easily observe the details.
As to John Chester's comment about large chunks of mag coat coming off,
it is from that experience that I take a very conservative approach and
bake anything that looks like it might need baking. I had large sections
of mag coat fly off in the last 1/8-inch of radius near the hub when a
client and I agreed the tape didn't look like it needed baking. It was a
rare recording of the late Stan Rogers live. The tape gods were with me
that day...all the flying off mag coat was Stan's guitar introduction
and we didn't miss a phrase of his singing. It was my worst experience
with this. I will NEVER again give in to the temptation of "lets try it
without baking...it doesn't look like it needs baking." Oh and it was on
a generic empty reel in a white box and the rest of the tapes in the
batch did not need baking.
Richard L. Hess email: [log in to unmask]
Aurora, Ontario, Canada 647 479 2800
Quality tape transfers -- even from hard-to-play tapes.