Corey Bailey
Audio Engineering
USEFUL INFORMATION
BAKING ANALOG AND DIGITAL AUDIO TAPE (OR NOT)
Disclaimer:
Neither I or my company has profited in any way from the people or organizations
mentioned in this article. This would include the links at the end of this article.
The opinions expressed herein are mine alone and do not reflect upon the people
or organizations mentioned in this article.
Preface:
First of all, I want to be very clear: In this article, I am only talking about polyester
base audio tape. This would exclude all acetate base audio tape. Acetate base
audio tape will not benefit from the processes described in this article. In fact, you
can do harm or permanent damage to acetate base audio tape by heating it in any
manor.
A considerable amount has already been published on this subject and one can
find a wealth of information available on the internet simply by plugging the title of
this article into your favorite search engine. There are several research papers that
require membership in specific organizations in order to gain access. This,
obviously, can become quite expensive. However, there is a wealth of information
that is public information. There are links at the end of this article to some in-depth
reading on the subject for those who wish to delve into the science.
I would like to thank all of those who reviewed this document before it was posted.
Some background:
A surprising number of well respected people, whom have authored articles, are
making the assumption that audio tape can be successfully baked without any
consequences. I tend to disagree and will only bake audio tape (Analog or Digital)
as a last resort because changes in the oxide take place that are both measurable
and often permanent. To be fair, there are certain brands and types of analog audio
tape that will only respond to baking and there are a few that simply won’t respond
to any of the processes listed in this article. I will first test a polyester (PET) based
tape for playability. If I find any evidence of Sticky Shed Syndrome during the initial
testing phase, I will then decide (Based on years of experience) if lubrication or
baking is best. I have found that my method of lubrication, coupled with the type of
lubricant that I use, is about 90% effective. That said, there are those instances
where the only solution to making a tape suffering from “Sticky Shed Syndrome”
playable or ready to lubricate, is to bake it.
Sticky Shed Syndrome is actually the result of a chemical process known to
scientists as “Binder Hydrolysis”. The process of baking analog audio tape was first
proposed by AMPEX Corp. in the late 1980’s. The process was patented by
AMPEX (Patent #5,236,790).
The recording industry (Film, Records, etc.) was quick to adopt the process
described in the AMPEX patent because everyone involved had experienced the
problem of degrading audio tape. To my knowledge, AMPEX has never pursued
anyone for patent infringement.
Binder Hydrolysis:
About 80% of all polyester base analog tape manufactured from the 1970’s
through the turn of the 21
st
century will suffer from some form of binder hydrolysis.
The problem worsens as time marches on. The type and severity of binder
hydrolysis is as varied as the different types of tapes themselves. The most
common indication of the problem is an unusual amount of oxide buildup on all of
the stationary parts of a tape transport. Tapes suffering from binder hydrolysis will
often “squeal” while playing. Another indication is the tendency of the oxide layer to
stick to the stationary parts of the tape transport while the tape is at rest but
threaded up, ready to play. Some tape transports will be unable to go into play
mode if the stiction problem is bad enough. Severe cases will have the layers of
tape adhering to each other, causing the oxide from one layer to stick to the
backside of an adjacent layer.
Successive baking may have detrimental effects. Those of us who have baked a
tape more than once, and then transferred it, have heard a type of audible
degradation that we cannot adequately define. There has been very little research
on the downside of baking analog and digital audio tape. Most of the early
research on tape degradation was done on data tapes, mainly because that is
where the funding came from.
One result of the earliest research was to store the tape at very low temperatures
and low humidity. Apparently, Hydrolysis reactions (Sticky Shed Syndrome) can
actually be reversed, if the Relative Humidity is below 24 percent and the
temperature is below 65° Fahrenheit.
In 1995, at the fourth Joint technical Symposium (JTS), a research paper was
submitted by Kevin Bradley of the National Library of Australia. The research paper
was titled: “Anomalies in the Treatment of Hydrolyzed Tapes.” What caught my
attention about this paper was that the research was carried out on 1/4” analog
audio tapes using test equipment designed to analyze the same. What was
discovered is that when baking audio tape above 50°C (122° Fahrenheit), there
was a measurable increase in the dropout rate and a slight frequency loss (1/2dB
above 10kHz). The high frequency loss increased slightly as the temperature was
increased. Both trade-offs can be acceptable when baking is the only way to
recover the audio signal.
In 2015, Goran Finnberg of The Mastering Room AB, posted to the Association of
Recorded Sound Collections (ARSC) Listserv, that he had measured increases in
the AM (Amplitude Modulation) distortion starting with the first baking cycle and
increasing with each subsequent baking of the same tape. Goran, to my
knowledge, has yet to publish a peer reviewed paper on his findings so, for now,
his research will have to be considered anecdotal. However, his findings tend to
bear out what myself and many other audio engineers have experienced when
transferring an audio tape that has been baked.
The vast majority of procedures and processes involved with the recovery of the
information contained in degrading media is from the personal experiences of
those mired deep in the problem and is therefore anecdotal. Although a few of us
have pulled back the curtain to take a deeper look at the problem, very little
scientific research has been done as to the possible effects of those anecdotal
processes, either short or long term. Some of which have become industry
standard! While I’m sure that the engineers at AMPEX Corp. observed the
scientific process when doing their research on baking, there is no mention in the
patent of possible chemical alteration or side effects. In fact, mention is made in
the patent that the baking process may indeed stabilize the tape being baked.
A situation that has since proven to be just the opposite.
An alternative to baking is offered by Charles Richardson. He is perhaps the only
person to have conducted actual laboratory research on Sticky Shed since the
1980's. According to his journal papers, his work which was performed by
independent forensic chemists, discovered several previously unrealized factors,
including:
(1.) That the carbon black back coating is the primary cause of hydrolysis in
modern tapes, though oxide binders can also suffer hydrolysis due to poor
formulations.
(2.) How and why back-coating hydrolysis affects the oxide layers.
(3.) He defined the damaging chemical effects of exposure to elevated heat levels
based on well established knowledge of chemical behavior.
Mr. Richardson subsequently developed a no-heat process to remove the backing
material from both the back of the tape and the residue from the oxide as well.
I personally know of two people who have had their Sticky Shed tapes treated with
this process and the problem has apparently disappeared. One person had this
process done by Mr. Richardson more than five years ago and his tapes are still
stable. Charles Richardson claims that tapes he restored in 2006 not only remain
stable, but are sonically superior to any other treatment.
So, Mr. Richardson may indeed be onto something. Currently, he is not offering his
process of removing the back coating as a sale or service. However, Mr.
Richardson is busy getting the word out to various professional organizations so,
we’ll see. There is a link to his website at the end of this article.
That said, I have encountered Sticky Shed Syndrome (SSS) on non-backcoated
tapes as well, although the situation is rare as non-backcoated audio tape tends to
be vary stable. Some European manufactures also back-coated their audio tape
products but instead of using a carbon based coating, they used a plastic material
consisting of very fine plastic particles. I have encountered SSS on some of these
tapes as well. One set, in fact, couldn’t be saved no matter what we tried.
Fortunately, the owner had another source for the audio.
Assessing a polyester base reel of tape for Binder Hydrolysis:
The first thing I do with 7" reels (and smaller) is to insert a pencil into spindle hole
and un-spool several layers by hand. This will tell you if the layers have the
tendency to stick together. If you are dealing with a 10-1/2" (NAB Hub) reel, you
will, most likely, have to mount the reel on the tape deck. Hopefully, the reel brakes
on the tape deck will be disengaged, allowing for easy un-spooling of the tape. I
have a couple of TEAC (Brand) NAB hub adapters (TEAC Part # TZ-612) that have
a metal flange at the base which allows them to mount to an NAB hub off the tape
deck. Then, I can use a pencil threaded through the spindle hole to un-spool a few
layers of tape the same as smaller reels.
Carefully un-spool several layers by hand, and observe how the tape comes off the
reel. The problem, if it exists, may not show itself on the first few wraps. The tape
should un-spool effortlessly without the layers wanting to stick together. If you are
concerned about the tape being on the floor, use a clean empty bucket or
wastebasket to catch the tape as you un-spool it. If you do have to mount the reel
on your tape deck, you can use the other reel to take up the slack. When I check
audio tape reels on a tape deck, I will thread the tape directly from one reel to the
other, avoiding the rest of the transport entirely and use both hands to rotate the
reels which gives a better feel for the unspooling process.
Once you have convinced yourself that layer-to-layer adhesion is not a problem for
the first few wraps, backwind the tape onto the reel, mount the reel on your tape
deck, and attach about 4 feet of leader to the head end of the tape. The type of
leader (Paper or Plastic) is a matter of individual preference however, non acidic
paper leader is the only one considered to be archival quality. Leader has several
benefits: It allows for thread-up and run-up to speed on a transport before the
beginning of the tape to be transferred passes across the play head. Consumer
decks tend to have shorter tape paths and some were quite good at recording
modulation to the very end of the tape. Leader also keeps the end of the tape
protected at the outer edge of the reel and protects the tape from the unevenness
of slotted hubs at the center of the reel. Eventually, leader will be applied to both
ends of the tape. Having leader at both ends of the tape allows for the transfer of
the entire tape, end-to-end.
Know that sometimes layer-to-layer adhesion won’t rear it’s ugly head until you are
at the last few wraps near the end of the supply reel. This problem most often
occurs on reels with smaller center hubs (5” Reels, 7” Reels, etc.) So, this is why
you need to pay attention to how the tape comes off of the supply reel until the
very end.
If, you suspect that layer-to-layer adhesion is present, STOP and consult a
professional. Solving the layer-to-layer adhesion can be tricky and remediation is
best done by someone who knows how to deal with the problem. Often, baking
(done properly) is the answer. If any of the oxide has come off and stuck to the
back of the adjacent layer, it's “game over” for that section of audio tape.
Once leader is attached to the outer edge of the tape, with the tape threaded up,
play into the tape for about 30 seconds. Stop the tape deck and disable the
transport (As though you are going to edit the tape). Pull the tape away from the
transport and observe all of the stationary parts of the transport (heads, guides,
idler arms, etc.). There should be NO evidence of oxide build-up on any of the
stationary components. Then, with the tape deck is in STOP mode and the tape
threaded up, rotate the reels back and forth by hand. There should be no
resistance except for the reel brakes. No tendency for the tape to stick to any of
the stationary parts of the tape path. Pull the tape away from the transport and
clean everything. Use the purest alcohol you can find. Medical grade alcohol is
best, if you can get it. Isopropyl alcohol can be found that is 99% pure (or greater).
Rubbing alcohol that you get at the drug store usually contains a certain
percentage of water and any water is generally not good for the tape path. I use
denatured alcohol. Inspect the Q-Tips or cloth that you used for any oxide residue.
Presuming that you have a very clean tape path, play the tape for about 30
seconds again. While the tape plays, observe how it comes off the supply reel.
There should be no tendency for the tape to stick to an adjacent layer. Listen to the
tape while it is playing past the heads (put your ear up close to the head stack).
There should be no squealing or any suspicious sounds of any kind. Stop after 30
seconds or so, pull the tape away from the transport and inspect all of the
stationary surfaces (heads, guides, idler arms, etc.) for any sign of oxide deposit.
Clean the entire tape path again and yes, inspect the Q-Tips or cloth that you used
for any oxide. With the tape threaded up, ready to play, rotate the reels back and
forth by hand in between each “play test” to check for any signs of stiction. By now,
you have played about a minute or more of the tape and should not have observed
anything out of the ordinary. It is at this point that I will back-wind* the tape to the
beginning and lubricate the entire length of the tape. When I have finished with the
lubricating process, I will attach a leader to the end of the tape and I am ready to
begin the transfer after a rigorous cleaning and inspection of the entire tape path. I
will often transfer side B first to avoid the unnecessary rewinding of the tape even
though, on my particular tape deck, the tape only comes in contact with ball
bearing surfaces for any operation other than playing the tape.
The tape deck that I use for this process (An Otari MTR-15) has been modified so
that the tape itself only comes in contact with rotating bearing surfaces in any
mode other than play. Even the tape lifters have been sleeved so that the sleeves
act as rotating bearings. Note that the picture below is without lifter sleeves. When
the lifter sleeves are installed, I will only use the Fast-Forward/Rewind functions to
position the tape prior to transfer. Fast-Forward and Rewind functions are then
accomplished using "shuttle" mode with the tape threaded around a reversing idler,
away from any stationary parts of the transport. Some tape decks, such as the
Studer 80 series, are designed with only rotating bearing surfaces that come in
contact with the tape for all operating modes.
There are some things that you can do that will help things:
1) Cover the Erase and Record heads on your tape deck with Teflon tape (Fig. 1).
This is relatively easy and there is a link to Teflon tape at the end of this article.
ONLY use wooden or plastic tools for attaching and removing the tape. Metal tools
run the risk of damaging the heads.
Fig. 1
Showing Teflon tape covering the
Erase and Record heads.
Some Teflon taping tips:
I use ½” tape and trim the width to fit the head stack if necessary. It’s OK to attach
the ends of the tape to the outside of the head shield (Notice the Record Head in
the picture). This actually makes the tape easier to remove for shielded heads.
Make sure that the tape adheres flat to the area of the head where the audio tape
will come in contact. You should have no wrinkles and squeegee out any air
bubbles. I use my little finger for this process. If the tape application is done
correctly, there will be about a 10% reduction in scrape flutter. An added bonus!
2) You can sleeve the lifters so that the sleeves act as bearings. (In the picture
above, the lifter sleeves have been removed) Making lifter sleeves is not so easy
to do, usually requires the services of a machine shop, and has operational
consequences, so you may want to skip this procedure. If you do add sleeves to
the lifters, know that full speed rewind or fast-forwarding is OUT. With sleeves over
the lifters, only slow movement of the tape with the lifters engaged is
recommended. Mainly use the Fast-Forward and Rewind buttons for positioning
the tape prior to playback. Otherwise, use alternate modes for shuttling the tape.
Know that every time the tape lifters are engaged, you are scraping the
valuable oxide with a stationary object. If the Fast-Forward or Rewind
functions are used, you are scraping the oxide at very high speeds!
3) An alternative to having sleeves manufactured is to wrap the lifters with
the same Teflon tape that you would use to cover the unused heads. This
operation is a bit tedious (a PITA, actually). The results are much better than
bare lifters. That said, I still would advise against using the deck for normal
fast-forward or rewind operations. However, if you use those operations only
for positioning before listening or transfer, the Teflon taped lifters will help
considerably.
And now, for the baking process:
So, consider the baking of analog and digital audio tape as a last resort because
the end result may be somewhat compromised. If you are at all unsure about what
to do, consult an experienced professional.
If you do have to resort to baking, the advice from the experts (myself included) is
to properly bake a tape and then transfer it to the digital domain as soon as
possible. Mainly because the stabilization effect offered by baking is only
temporary, lasting a week or two at best.
AMPEX Corp. recommended baking at a temperature of 54°C. (129-130°F.)
I prefer to use a lower temperature of 48°C. (118°F.). Anywhere between 43.5°C
and 49°C (115° F. to 120° F.) should be fine. The lower temperature usually
requires a longer baking time so I will usually let 4 or 5 reels of 1/4” tape go 24
hours before checking them. For fewer reels, I will check the tape after about 12
hours. For a single reel, 8 hours is the check point. I use a dehydrator and a digital
thermometer that will reach the tape itself for checking actual temperature.
National Public Radio (NPR) has over 155,000 analog tapes and a special
laboratory oven, where they bake 1/4” reel-to-reel tapes at a temperature of
approximately 51°C or 125°F for about 8 hours.
The industry standard for the feature film companies is to bake their 1/4” reels at
between 120-130°F. for 8 to 12 hours. (Longer when necessary) For wider tapes,
the baking time is simply adjusted to allow for the width and number of tapes being
baked.
While I have personally witnessed the use of temperatures higher than
54°C. or 130°F., I certainly don’t recommend it. At higher temperatures, you are
running the risk of permanent damage. Think: “Lower and slower.”
The cooling process.
Once your tapes have completed the baking process, they have to be cooled to
room temperature. Plan ahead for the removal and acclimation process. Use
cotton gardening gloves (at least) for handling the hot reels. Have a safe space
ready to receive those hot reels that is in or the same temperature and humidity as
your transfer facility. Since I use a dehydrator for baking and generally acclimate
the tapes overnight, I will often transport the dehydrator into the studio and let the
tapes cool and acclimate inside the dehydrator.
Room Temperature may be an ambiguous reference but the bottom line is that the
tapes, once baked, need to be at the same temperature and humidity as your
transfer facility, whatever that environment is. The time it takes to acclimate to your
work environment will vary. Some facilities are climate controlled, some are not.
For those that are not, seasonal variations will play a large role. One factor is the
amount of tapes that have been baked. I will only process the amount of tapes that
I can comfortably transfer in one day. My studio is in Southern California (The arid
Southwest). Acclimation time for the amount of tapes that I can transfer in one day
is usually 3 to 4 hours. However, I will usually plan things so that the tapes have
overnight to acclimate to the studio room.
Many companies with large holdings will “Bake first, ask questions later.” Mainly
because it’s simply the most cost effective way to migrate their large inventories of
analog product to the digital domain. That said, these same companies employ
knowledgeable people who know how to properly assess the product. Like NPR,
these companies have extensive transfer facilities and still are playing “catch up.”
Adding to the problem of degrading analog tape is that digital audio tape is
showing signs of SSS. This is something that those of us working in the trenches
never expected because the manufacturing process for analog audio and digital
audio tape is completely different.
The digital audio formats that have exhibited problems (so far) are: DA88, DAT and
reel-to-reel digital. DA88’s and DAT’s are particular problems because both are
cassette based and, being rotary head formats, can ruin your expensive playback
deck while you figure out the problem.
When preparing to transfer any cassette based tape, fast-forward and rewind the
tape first. Not only does fast-forwarding and rewinding exercise a tape that has
been stored for a long time, but by learning just how a tape in good shape sounds
during this process, you will perhaps save an expensive repair bill.
There are some video tape formats that are degrading as well. Those being: Hi-8,
Digital 8, 3/4” analog and some 1/2” professional digital formats.
In general, if you are to consider baking digital audio or video tape, use my
recommended temperatures for analog audio tape as a starting point (48°C.,
118°F.) or slightly lower temperatures and longer baking times if you want to be
extra safe. The reason for the extra caution is because both the base film and the
oxide layer are much thinner than analog tape. Adding to the problem is that many
of the digital audio (and video) formats are in cassettes so the results are really
variable. I have noted about a 50% success rate when baking cassette based
formats. Reel-to-reel digital is much better.
So, in the end, there is no good news here. As I point out at the home page of this
website: “Not only is magnetic media degrading, but the equipment and expertise
needed to retrieve those aging recordings is becoming increasingly more difficult to
find.”
Some Links:
Charles Richardson’s website:
http://www.rezerex.com/
From Josephson Engineering (Microphones):
http://www.josephson.com/bake_tape.html
The Library of Congress:
http://www.loc.gov/preservation/scientists/projects/sticky_shed.html
Magnetic Media Preservation, Library of Congress:
Here a list of likely suspects for SSS. The website is provided by Richard Hess and
the entire site is very informative:
http://richardhess.com/notes/formats/magnetic-media/magnetic-tapes/analog-
audio/degrading-tapes/
Storing and handling of magnetic media CLIR:
https://www.clir.org/pubs/reports/pub54/5premature_degrade/
Teflon tape:
https://www.mcmaster.com/#tape-(made-with-teflon-ptfe)/=1bcy6ti
Return to TOP of page
© Corey Bailey Audio Engineering