Tom and Forrest -
Good thoughts about shrink wrapping.
I have another: If an object is shrink-wrapped and sits for awhile before
being frozen, larvae of various critters I mentioned in an earlier e-mail can
easily chew their way out of the wrapping. If there is space between the
object and the bag, larvae are not inclined to chew their way out as easily.
Tom Parker
-----Original Message-----
From: tom.str...@pch.gc.ca
To: pestlist@museumpests.net
Sent: Wed, Jun 17, 2009 11:09 am
Subject: Re: [pestlist] Wrapping infested objects for freezing -- why you don't
need to shrink wrap
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ear Forrest,
Shrink wrapping or otherwise decreasing the empty volume of the bagged
aterial before low temperature treatment for disinfestation is a frequent
uestion, but it is generally unnecessary for the following reasons:
1) Causes unintended distortion of the object. Mitigating this problem with
upport frames adds empty space (defeats shrink wrapping) and takes time
nd resources better spent on the next few objects. You do not want to flex
bjects when they are cold, to prevent damage to any embrittled polymers.
ormally such polymers will go through low temperature treatment without
roblems.
2) The film is put in intimate contact with the object - and condensation
f it occurs will be c
lose to the object. Condensation occurs on the
oldest surface first - and freely suspended film meets that criteria as
he object supplies heat as the package cools so any small amount of
ondensation is away from the object surface (warm) and on the film surface
cold). The amount of moisture condensing from room temp air at 50%RH is
ather small and generally managed by the (cool) film surface, or
dsorption with the organic portion of the object.
It is instructive to know the real numbers involved. The exchange we are
alking about is 8.6 grams water per cubic meter at 20C, 50%RH, going down
o 0.5 grams per cubic meter at -20C, 50%RH. 20 drops of water is roughly a
ram (ml, cc) which translates to less than 20 drops of water in 100 litres
f air (approx. the same in quarts) or one drop per 5 litres of air. This
s my definition of 'rather small'. Compare this to common moisture
ontents of 8% of the objects mass, which will likely measure in
ilograms/pounds. The total amount of water being condensed is trivial
ecause it is spread over the bag surface, let alone being absorbed by the
bject.
On rewarming, the coldest spot over time will be the object. This sounds
erilous, but it is not. The bulk of the air is demanding more moisture at
his point as it warms, as the heat is taken on slowly by the object
ompared to the free suspended part of the film which changes temperature
uickly so any moisture deposited on the surface of the free film will
atisfy the headspace air quickly and
mitigate demand on the thinner fast
esponding portions of the object. This is why I recommend a single sheet
f tissue, or fabric as buffer wrapping if you suspect the surface is very
ensitive to moisture. It will intercept the problem and respond quickly.
lso the object will warm on its outside surface, and organics self
nsulate, cutting off the problem of the large thermal gradient and the
riving force behind the condensation risk.
3) More time to do than it is generally worth. Better to spend the time
rotecting delicate objects headed for low temp treatment from handling
amage by boxing etc.
A great test to show yourself the real scope of the problem is to make
worst case scenarios'. Fill an empty polythene bag with room air (don't
low into it to make it) and then put it in the freezer. Look for the
ondensation. Play with adding organic matter. By 10 kg (5 pounds) per
ubic meter of air volume (cubic yard) you will have reestablished
ignificant control of the cavity moisture. That is still a pretty spacious
ag given the common density of wood, paper etc.
Sincerely,
om Strang
Canadian Conservation Institute | Institut canadien de conservation
enior Conservation Scientist | Scientifique principal en conservation
nstitut Canadien de Conservation | Canadian Conservation Institute
ttawa, Ontario, Canada, K1A 0M5
tom.str...@pch.gc.ca
éléphone | Telephone 613-998-3721 (extension 239)
élécopieur | Facsimile 613-998-4721
éléimprimeur (sans frais) 1-888-997-3123 | Teletypewriter (toll-free)
-888-997-3123=0
D
"Forrest St.
Aubin"
<forr...@saintaub To
inbce.com> pestlist@museumpests.net
Sent by: cc
pestlist-ow...@mu
seumpests.net Subject
Re: [pestlist] Wrapping infested
objects for freezing
06/12/2009 03:09 =2
0
PM
Please respond to
pestl...@museumpe
sts.net
orry, Tom. What I had in mind was one of the vacuum cleaner-operated
torage bags, with which you could control the amount of evacuation and
hrinkage.
Forrest E. St. Aubin, BCE
hair, ESA-ACE Oversight Committee
iaison, ESA/NPMA
2835 Pembroke Circle
eawood, Kansas 66209
hone: 913.345.2114
ax: 913.345.8008
-mail: forr...@saintaubinbce.com
ebsite: www.saintaubinbce.com
"Innovation distinguishes between a leader and a follower".
Steve Jobs, Apple Co-founder
----Original Message-----
rom: bugma...@aol.com
ate: 06/12/2009 01:46 PM
o: pestlist@museumpests.net
ubject: Re: [pestlist] Wrapping infested objects for freezing
Forrest -
True shrink-wrapping requires a heat gun; not a good idea with artifacts.
ven if no heat gun were used, such a tight-fitting wrap may damage an
rtifact.
Tom Parker
-----Original Message-----
rom: Forrest St. Aubin <forr...@saintaubinbce.com>
o: pestlist@museumpests.net
ent: Fri, 12 Jun 2009 1:55 pm
ubject: Re: [pestlist] Wrapping infested objects for freezing
Tom, Would there be any pluses or minuses to shrinkwrapping prior to
reezing?
Forrest E. St. Aubin, BCE
hair, ESA-ACE Oversight Committee
iaison, ESA/NPMA
2835 Pembroke Circle
eawood, Kansas 66209
hone: 913.345.2114
ax: 913.345.8008
-mail: forr...@saintaubinbce.com
ebsite: www.saintaubinbce.com
"Innovation distinguishes between a leader and a follower".
Steve Jobs, Apple Co-founder
----Original Message-----
rom: bugma...@aol.com
ate: 06/11/2009 04:27 PM
o: pestlist@museumpests.net
ubject: Re: [pestlist] Wrapping infested objects for freezing
Randi -
The key to successful freezing is to take the object from room temperature
nd crash the temp. You are attempting to reach 32 F in four hours. That
eats any insect or egg to its self-defenses. If you're not sure you've
een20successful, then remove the item after 72 hours, let it warm to room
emperature for a day or so, and then repeat the process.
Tom Parker
----Original Message-----
rom: randi_sm...@fws.gov
o: pestlist@museumpests.net
ent: Thu, 11 Jun 2009 10:13 am
ubject: Re: [pestlist] Wrapping infested objects for freezing
Hi!
Something I have heard that I've not seen mentioned.
We bag items to be frozen in plastic bags- usually they are paper based in
ecord size boxes. For routine, precautionary freezing before going to
torage we do not bag until just before freezing, as I have heard that
nsects can react to the change in atmosphere from bagging and prepare
hemselves to withstand freezing more easily. It is like they make
ntifreeze in response. We bag and load in less than an hour, not bagging
oo much at one time.
I don't remember where I learned this. Has anybody else heard this?
Randi Sue Smith
urator
C Booth Historic National Fish Hatchery and Archives
S Fish and Wildlife Service
23 Hatchery Circle
pearfish, SD 57783
05-642-7730 ext. 215
ax 605-642-2336
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