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Dear Forrest, Shrink wrapping or otherwise decreasing the empty volume of the bagged material before low temperature treatment for disinfestation is a frequent question, but it is generally unnecessary for the following reasons: 1) Causes unintended distortion of the object. Mitigating this problem with support frames adds empty space (defeats shrink wrapping) and takes time and resources better spent on the next few objects. You do not want to flex objects when they are cold, to prevent damage to any embrittled polymers. Normally such polymers will go through low temperature treatment without problems. 2) The film is put in intimate contact with the object - and condensation if it occurs will be close to the object. Condensation occurs on the coldest surface first - and freely suspended film meets that criteria as the object supplies heat as the package cools so any small amount of condensation 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 rather small and generally managed by the (cool) film surface, or adsorption with the organic portion of the object. It is instructive to know the real numbers involved. The exchange we are talking about is 8.6 grams water per cubic meter at 20C, 50%RH, going down to 0.5 grams per cubic meter at -20C, 50%RH. 20 drops of water is roughly a gram (ml, cc) which translates to less than 20 drops of water in 100 litres of air (approx. the same in quarts) or one drop per 5 litres of air. This is my definition of 'rather small'. Compare this to common moisture contents of 8% of the objects mass, which will likely measure in kilograms/pounds. The total amount of water being condensed is trivial because it is spread over the bag surface, let alone being absorbed by the object. On rewarming, the coldest spot over time will be the object. This sounds perilous, but it is not. The bulk of the air is demanding more moisture at this point as it warms, as the heat is taken on slowly by the object compared to the free suspended part of the film which changes temperature quickly so any moisture deposited on the surface of the free film will satisfy the headspace air quickly and mitigate demand on the thinner fast responding portions of the object. This is why I recommend a single sheet of tissue, or fabric as buffer wrapping if you suspect the surface is very sensitive to moisture. It will intercept the problem and respond quickly. Also the object will warm on its outside surface, and organics self insulate, cutting off the problem of the large thermal gradient and the driving force behind the condensation risk. 3) More time to do than it is generally worth. Better to spend the time protecting delicate objects headed for low temp treatment from handling damage 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 blow into it to make it) and then put it in the freezer. Look for the condensation. Play with adding organic matter. By 10 kg (5 pounds) per cubic meter of air volume (cubic yard) you will have reestablished significant control of the cavity moisture. That is still a pretty spacious bag given the common density of wood, paper etc. Sincerely, Tom Strang Canadian Conservation Institute | Institut canadien de conservation Senior Conservation Scientist | Scientifique principal en conservation Institut Canadien de Conservation | Canadian Conservation Institute Ottawa, Ontario, Canada, K1A 0M5 [email protected] Téléphone | Telephone 613-998-3721 (extension 239) Télécopieur | Facsimile 613-998-4721 Téléimprimeur (sans frais) 1-888-997-3123 | Teletypewriter (toll-free) 1-888-997-3123 "Forrest St. Aubin" <forr...@saintaub To inbce.com> [email protected] Sent by: cc pestlist-ow...@mu seumpests.net Subject Re: [pestlist] Wrapping infested objects for freezing 06/12/2009 03:09 PM Please respond to pestl...@museumpe sts.net Sorry, Tom. What I had in mind was one of the vacuum cleaner-operated storage bags, with which you could control the amount of evacuation and shrinkage. Forrest E. St. Aubin, BCE Chair, ESA-ACE Oversight Committee Liaison, ESA/NPMA 12835 Pembroke Circle Leawood, Kansas 66209 Phone: 913.345.2114 Fax: 913.345.8008 E-mail: [email protected] Website: www.saintaubinbce.com "Innovation distinguishes between a leader and a follower". Steve Jobs, Apple Co-founder -----Original Message----- From: [email protected] Date: 06/12/2009 01:46 PM To: [email protected] Subject: Re: [pestlist] Wrapping infested objects for freezing Forrest - True shrink-wrapping requires a heat gun; not a good idea with artifacts. Even if no heat gun were used, such a tight-fitting wrap may damage an artifact. Tom Parker -----Original Message----- From: Forrest St. Aubin <[email protected]> To: [email protected] Sent: Fri, 12 Jun 2009 1:55 pm Subject: Re: [pestlist] Wrapping infested objects for freezing Tom, Would there be any pluses or minuses to shrinkwrapping prior to freezing? Forrest E. St. Aubin, BCE Chair, ESA-ACE Oversight Committee Liaison, ESA/NPMA 12835 Pembroke Circle Leawood, Kansas 66209 Phone: 913.345.2114 Fax: 913.345.8008 E-mail: [email protected] Website: www.saintaubinbce.com "Innovation distinguishes between a leader and a follower". Steve Jobs, Apple Co-founder -----Original Message----- From: [email protected] Date: 06/11/2009 04:27 PM To: [email protected] Subject: Re: [pestlist] Wrapping infested objects for freezing Randi - The key to successful freezing is to take the object from room temperature and crash the temp. You are attempting to reach 32 F in four hours. That beats any insect or egg to its self-defenses. If you're not sure you've been successful, then remove the item after 72 hours, let it warm to room temperature for a day or so, and then repeat the process. Tom Parker -----Original Message----- From: [email protected] To: [email protected] Sent: Thu, 11 Jun 2009 10:13 am Subject: 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 record size boxes. For routine, precautionary freezing before going to storage we do not bag until just before freezing, as I have heard that insects can react to the change in atmosphere from bagging and prepare themselves to withstand freezing more easily. It is like they make antifreeze in response. We bag and load in less than an hour, not bagging too much at one time. I don't remember where I learned this. Has anybody else heard this? Randi Sue Smith Curator DC Booth Historic National Fish Hatchery and Archives US Fish and Wildlife Service 423 Hatchery Circle Spearfish, SD 57783 605-642-7730 ext. 215 fax 605-642-2336 ------------------------------------------------------------- To send an email to the list, send your msg to [email protected] To unsubscribe from this list send an email to [email protected] and in the body put: "unsubscribe pestlist" Any problems email [email protected]
