Dear Colleagues, Kodak Professional has a www site on
the Permanence and Handling of CDs
at http://www.kodak.com/global/en/professional/products/storage/pcd/techInfo/permanence.shtml
. This site includes information on More About Permanence
and Handling
Each of these above listed documents
has some very valuable information, which is revealing, and in some cases
brutally honest. Kodak does not forget to promote their own product, which is
to be expected. I wish to highlight some of the details
I have extracted for those whose interest in not to such a level so as to read
the 10 articles in total. A. CD Permanence in
Perspective States the following: 1. Ensuring the permanence of CD-ROMs and
writable CDs (and indeed of any modern information medium) is a combination of
manufacturers' and users' responsibilities. 2. This publication has surveyed the
currently available technology choices in CD media. It has explained the
special handling and storage requirements of CD discs which arise from their
physical nature as objects of plastic and metal. Beyond this, however,
permanence of the information stored on CDs depends as much or more on sound
archival practices than on the inherent long life of the discs. Users of CD
technology, whether in a personal or professional context, are the critical
links in assuring data survival in the long term. Master copies kept in
appropriate storage and handled only when necessary may last centuries. Copies
for use are likely to have considerably shorter lives. Therefore, a strategy of
master discs from which derivatives are made for use is fundamental to success. 3. Another
useful approach is to maintain data in more than one form or in more than one
type of storage media, perhaps in analog form, if possible. In addition, keeping
copies in different physical locations helps guarantee survival. 4. The
principal fact of life for all digital storage media is the rapid obsolescence
of hardware and software. Users of CD technology should be reassured by the
long physical life of CD discs, but they must not lose sight of the need to
maintain a viable path for migration of data to new hardware and software
platforms. Digital storage media impose a strict discipline that human-readable
records do not: their rapid evolution creates a continual progression of
technology that cannot safely be ignored for too long. However, armed with
knowledge of the physical nature of the media and of appropriate archival
approaches, users of CD technology can have the best of both worlds--the
functionality of digital media and a long life for stored data and pictures. B. So, How Long Can CDs Last? States
the following: 1. Leaving
aside scratches, fires, floods, and peanut butter sandwiches and concentrating
on the slow chemical changes that determine the inherent
life expectancy of a CD, extensive accelerated-aging tests suggest that Kodak
writable CD products, including Photo CD discs, will not reach a BLERmax of 50
for a period of around 200 years when kept in the dark at moderate storage
conditions. This long potential life expectancy is mainly a function of the
greater dark stability of the dye used in Kodak writable CD products.
Considering that BLERmax 50 is still not an unreadable level of error, Kodak
writable CDs have a very long life expectancy indeed. Similar research by the
3M Company shows that CD-ROM products made by them will not attain a block error
rate of 50 per second for more than 100 years in moderate storage conditions. 2. Accelerated aging is subject to
uncertainties, but it does rest on firm scientific footing. Behind the data is
the simple assumption that raising the
temperature causes the reactions of decay to happen faster--so fast, in fact, that
they occur within a few months, rather than decades. Readability, or When Is a CD's Life Really Over? States
the following: 1.
Still, nothing lasts forever, and it's
important to know what the inherent life expectancy of a CD disc actually is.
As mentioned, life expectancy at moderate storage conditions can be determined
by means of accelerated-aging tests, and most manufacturers provide such data
to consumers upon request. To understand the test results, one has to know what
was considered the "end of life" for test purposes and, of course,
what the predicted life expectancy turned out to be. End of life for a CD disc
seems a simple concept--when the disc can no longer be read, its useful life is
over. C. The Nature of CD-ROM Discs States
the following: 1. The
tendency of CD-ROM metallic layers to oxidize over time can be measured with
accelerated-aging tests. Today's CD-ROMs from quality manufacturers have a
predicted life of over one hundred years (in normal storage conditions) without
danger of data loss because of oxidation. 2. The
presence of obvious pinholes in the metallic layer is another undesirable
feature. 3. However,
physical damage from scratches or other kinds of mishandling is still a serious
threat. Proper handling, including appropriate storage containers for CDs, is a
vital contributor to CD longevity. D. The Nature of Writable CDs States
the following: 1. Both
the reading and writing are done from the bottom side of the disc, through the
full thickness of the polycarbonate substrate. This provides scratch resistance
as it does for the CD-ROM, but there is a catch. It takes a fair bit of energy for
the laser beam to make a "mark" on a writable CD. That's
why scratches and especially fingerprints or dirt are much more dangerous to a
writable CD BEFORE writing than afterward. The fingerprint, dirt,
or smudge can scatter the beam of the writing laser, perhaps weakening it to
the point where the mark it makes is too small or too light to be read. For
this reason, extra care must be taken in the handling of writable CDs before
the data is written. 2. Lightfastness
of Writable CDs Tests
show that the writable CDs of some manufacturers can be affected by casual
exposure to light (for example, by leaving a disc bottom-side up on a desk for
days). This can cause the dye to fade and the contrast between marks and lands
to disappear. The dye formulation in Kodak writable CD products, however,
provides much superior lightfastness, and the risk of accidental damage is much
lower. Nevertheless, as a general rule it is prudent to keep all writable CD
products in suitable enclosures (acrylic cases, caddies, or other opaque
enclosures) when not in use and never to leave them exposed to direct sunlight.
E. Do Not Fold, Spindle, or Mutilate: Safe Handling of CDs
States the following: 1. The worst
handling stresses for a CD are caused by severe flexing or application of a
sharp point to the top surface. These actions deform the substrate, wiping out
pits and causing areas of the disc to become unreadable. A sharp stylus--a
ballpoint pen, for example--can cause compression of the polycarbonate
substrate and the metallic reflecting layer in the area under the pen point.
This can happen to both CD-ROMs and writable CDs, but the danger is more acute
for writable CDs. The adhesion between the dye and the other layers in a
writable CD is weaker than the interlayer adhesion in a CD-ROM, where no dye is
needed. 2. Be
Careful Marking and Labeling CDs User-applied
labels of any kind may unbalance the CD and make it difficult for the player to
read. Also, labels may peel in humid conditions. Once a label is on the CD,
however, it is especially important not to try and
remove it. The act of peeling off a label creates a lever action that
concentrates stress in a small area. Such stress can cause delamination,
especially in a writable CD. If it is necessary to write on the top side of a
CD, a soft felt-tip marker is preferable to other writing instruments, but with
some solvent-based markers there may be a danger of the solvents migrating into
the protective lacquer. So, if a CD must be marked, it should be done in a way
that is approved by the disc manufacturer. F. Storage Conditions for CDs States
the following: 1. In
general, storage temperatures for CDs should not be any warmer than about 25°C
(77°F) when archival permanence is desired. Cooler temperatures (down to about
10°C or 50°F) will help ensure the longest possible life expectancy. Relative
humidity conditions should be in the range of 20% to 50% RH for best results. 2. Cycling
conditions, especially between extremes of temperature and humidity, can be
dangerous to CDs. Although moderate temperature and humidity changes ordinarily
do no harm, fast changes between very warm and wet conditions to cooler and
very dry conditions might produce warping and distortion. The recommended
maximum limit for temperature change is 15°C (27°F) per hour. For humidity, the
recommended maximum RH change per hour is 10%. In practice, such steep
gradients of temperature and RH are rare. The enclosures in which CDs are kept
provide the first line of defense against such cycling, damping the rate of
change in RH so that the CD inside its case or storage cabinet does not
"feel" fast changes occurring in the outside atmosphere. I trust that the information is if
interest and value. Regards, Laurie Mr Daniel
Lawrance [Laurie] Varendorff, ARMA Consultant/Trainer/Tutor/Presenter: Records and information Management Imaging and Micrographic Specialist with 25 years experience The Varendorff
Consultancy The Australian
Distributor for A&P International Scanner & Micrographic Resolution
Test Targets Visit our www site: http://www.pressenter.com/~apintl/ ABN: 77 836 801
165 27
Ronneby Rd, Lesmurdie, Western Australia 6076 - AUSTRALIA PHONE:
+61 8 9291 6925 - MOBILE: +61 0417 094 147 EMAIL: Laurie.Varendorff@Bigpond.com |