Addressing the aging issue that Pam has observed, several factors could be at work. First is the nature of the plastic. Generally, polyethylene is what is meant by poly bags. However, many other types of poly-plastic are possible. There are other types of plastic used for bags as well, and the options are ever-expanding as manufacturer try to find the strongest yet thinnest material for trash bags. For artifact storage, stay with polyethylene. A second factor is that not all polyethylene is the same. The conversion process from ethylene (single molecule) to polyethylene (chain of molecules) generally requires a polymerizing agent. I don't know exactly what is used and in what quantities by different manufacturers, but all or most of the polymerizing agent is used up in the process. Ideally, you want bags where all of it is used up and none remains. How you determine this is unclear. Hopefully, you can call the manufacturers of products you are considering and ask. You will probably need to speak to a technical specialist. The other manufacturing concern is whether the bags are made of virgin material or recycled material. My understanding is that recycled is less stable, probably due to chemicals used to break down the used polyethylene. I would avoid recycled polyethylene.
How do you determine that you have a "good" polyethylene? I'm not aware of a study, and if there was one, you can be almost certain that manufacturers have changed their processes or formulations since then. The best route is to ask the manufacturers of products that you are considering. I would assume (but don't know for sure) that "food-grade" would imply a higher level of purity and stability, so that designation may help. You do not want any treatments like anti-static. Once you find a specific type of bag that looks good, I suggest that you buy a reasonable quantity to hold you for a number of years. You can't assume that they are the same product in the future, even if the name is exactly the same, and you will have to re-investigate when you buy more.
Finally, polyethylene is very susceptible to UV degradation, as are most organic materials. It will yellow and turn brittle from UV exposure. Usually this is due to daylight, but fluorescent lights also can have this effect, as can any other light with high UV content. Polyethylene in daylight (even indoors through windows) can become brittle and crumble in less than a year, so be sure to keep your bags out of strong light.
Marc
American Conservation Consortium, Ltd.
4 Rockville Road
Broad Brook, CT 06016
www.conservator.com
860-386-6058
*Collections Preservation Consultation
*Conservation Assessments & Surveys
*Low-Tech Environmental Control
*Moisture Management Solutions
*Collections in Historic Structures
*Collections Care Grant Preparation
*Conservation Treatment of:
Furniture
Painted Wood
Horse-Drawn Vehicles
Architectural Interiors
Marc A. Williams, President
MS in Art Conservation, Winterthur Museum Program
Former Chief Wooden Object Conservator, Smithsonian Institution
Fellow, American Institute for Conservation (AIC)
----- Original Message -----
From: Pamela Silvestri
To: [log in to unmask]
Sent: Monday, August 13, 2007 11:35 PM
Subject: Re: [MUSEUM-L] artifact bag thickness
Kelli,
I agree with the info in the other posts but would like to add that, I have now seen these bags (poly) -regardless of thickness and proper storage, after a decade+ and even lesser periods of time...appear to 'break down'. They harden and seem to become fragile.
There are many factors, other than thickness alone which may determine how permeable they are, such as how they are sealed and resealed (firmly/airtight..if there's any moisture in the bags to begin with) and if they are stored tightly with other bagged objects and in a container which allows for airflow, how often they are handled, etc.
That being said...one of the other benefits of clear bags of course, is that you can view the contents without opening the bag and removing them. However, with the exception of lithics (generally), I have always lightly wrapped objects in acid free, ph neutral and/or cotton batting (depending on the composition of the object) as a buffer to further protect/preserve/stabilize the bagged objects.
I have re-bagged objects using newer bags in as few as 3-4 years..but as I recall they had been stored in a too cold (even freezing) environment.
Pam
In a message dated 8/13/2007 4:39:11 P.M. Eastern Daylight Time, [log in to unmask] writes:
For my second question of the day, when bagging artifacts (of any kind) for
storage, is there a museum standard for the thickness of the bag? 2mm, 3mm,
4mm, etc.? Thanks again for any responses.
Kelli
*********************************
Kelli Bacon
Nebraska State Historical Society
1500 R St.
PO Box 82554
Lincoln NE 68501-2554
402-471-4766
[log in to unmask]
www.nebraskahistory.org
------------------------------------------------------------------------------
Get a sneak peek of the all-new AOL.com.
========================================================= Important Subscriber Information:
The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/ . You may obtain detailed information about the listserv commands by sending a one line e-mail message to [log in to unmask] . The body of the message should read "help" (without the quotes).
If you decide to leave Museum-L, please send a one line e-mail message to [log in to unmask] . The body of the message should read "Signoff Museum-L" (without the quotes).
=========================================================
Important Subscriber Information:
The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/ . You may obtain detailed information about the listserv commands by sending a one line e-mail message to [log in to unmask] . The body of the message should read "help" (without the quotes).
If you decide to leave Museum-L, please send a one line e-mail message to [log in to unmask] . The body of the message should read "Signoff Museum-L" (without the quotes).
|
