Yellowing of archival plastic bags and tags
September 27, 2013 at 3:56 pm #4924
I was wondering if anyone has noticed that archival quality polyethylene plastic bags and acid-free tags will sometimes yellow after a few years when in contact with certain archaeological materials.¬† I have noticed it the most with organics, but today, I have noticed it with chert debitage.¬† It does not have a smell and¬† the artifacts don’t seem “bothered” but it makes seeing through the supposedly clear plastic bags a pain.¬† I ask the people who worked with me to always double bag organics and materials that will continue to deteriorate like prehistoric sherds, metals especially iron based ones so there is no contact with the acid free tag.¬† But what’s most troubling is that the archival quality 4 mil¬† and 2 mil polyethylene bags are doing this.¬† The collection I am working with today has only been stored for about 10 years.
Please feel free to post to other lists if you think that might help get an answer.
MarybethSeptember 30, 2013 at 12:47 pm #4928
I’m so glad you brought this up. We have seen this phenomenon as well. I’ve noticed it particularly with lithics (chert and volcanic) as well as charcoal.¬†Ours were all¬†archival quality 4mil and 2mil; I’m¬†wondering if the bags aren’t really archival? Perhaps the method of manufacture incorporates some sort of non-archival layer between the layers of plastic that reacts with salts or acidic sediments?
LauraSeptember 30, 2013 at 1:23 pm #4933
I have no idea what is going on.¬† I sent an email to the Smithsonian; maybe they can help, unless of course the govt gets shut down tonight.September 30, 2013 at 1:45 pm #4935
Jane E KlingerParticipant
We also have seen this phenomenon. The first time was with bags that came from Europe with a variety of materials housed in them. But, and this is worrisome, we have also discarded old bags that were never used because they had yellowed considerably. The bags were stored in powder coated steel drawers for a few years. Due to needing to replace the drawers they were then placed in commercial plastic drawer units for temporary housing. To add to the confusion as to why, not all the bags stored this way yellowed.September 30, 2013 at 1:59 pm #4940
I feel your pain — I can’t explain it because there seem to be so many contributing factors!October 1, 2013 at 7:14 pm #4953
I had this happen with a group of mylar enclosures that were packaged in plastic bags that were bad. The plastic bags turned yellow and in turn caused the mylar to discolor around the edges. I complained to the manufacturer and got replacements.October 2, 2013 at 9:29 am #4957
It sounds to me like your problem was one of impure chemical composition in the polyethylene.¬† Have you ever noticed the reaction in objects?
MarybethOctober 2, 2013 at 11:05 am #4959
As a ¬†video game museum, plastics are (sadly) at the heart of our collection. The discoloration of console cases, especially Nintendos, is a common problem. ¬†The best article I have read about plastic discoloration is this: ¬†http://www.vintagecomputing.com/index.php/archives/189/what-happened-to-my-snes-case-discoloration-in-classic-machines¬†. Benj Edwards (who is on our advisory board) has done an excellent job of researching the issue and presented it in a very accessible way for non-chemists.
Plastics are sourced from all over the world, more and more from China and other developing countries, where controls are not strict, so it’s not surprising that these bags are often not what they are promised to be. ¬†I think it’s essential that you contact your supplier, if possible, and alert them of the problem. ¬†The response would be instructive: ¬†while replacing defective merchandise is all very nice, are they able to track the lot that you bought and alert all the customers who received bags from that lot that there is a problem and offer replacement? ¬†That would be the difference between customer service and service to the museum community. We all have thigs that go into storage and are not seen again for years. ¬†If these bags are degrading, the nature of plastics outgassing is certainly degrading the artifacts around them.
Sorry for writing a book, but I spend way too much time thinking about plastic! Oh, and do’t read the “what can I do about it” part of the article above – it will give you nightmares.October 2, 2013 at 11:16 am #4960
Great article.¬† I also have a book about caring for plastics that is very useful.¬† However, this thread was started not because of the inherent chemistry of plastics, but what might be causing a reaction between the plastic and objects in our collections.¬† In our case I really think its a matter of residual chemicals on the artifacts that are reacting to the potentially impure plastic.¬† I think most conservators would rather us not scrub the materials to a shine, but there may have to be some way of preventing this discoloration.
Love to hear from more people and their experiences.¬† It seems like this is a common problem and yet, no one has answers.October 3, 2013 at 2:35 pm #4979
I have been in touch with conservators¬† at the University of Texas at Austin and the University of Arizona. Nancy Odegaard, Conservator, Arizona State Museum suggested that what is happening is that the plasticizer in the polyethylene bag is degrading probably in the presence of acid in the artifacts. ¬† So, the question is, do we just replace bags at certain intervals, do we test the soil from which the artifacts came to provide a warning about which materials may degrade faster, or some combination of this and other methods.¬† I am mostly concerned about organic materials that we package in plastic or tin foil (carbon samples).¬† If we package them in acid/lignen free buffered paper are we added something to the organics that will throw off future analyses?¬† Should we go to Nalgene vials or bottles and what is that cost?¬† I am reluctant to go to glass vials and bottles because these are archaeological collections and archaeologists, myself included are not always gentle when moving boxes or bags of artifacts.
Love to hear from you all out in cyberspace.
October 4, 2013 at 1:26 am #4984
Marybeth – I think that the storage bags were probably not Polyethylene or were very bad PE. However, I did use a few of the mylar enclosures that had yellowed on the edges and they did not seem to affect their contents even after ten or so years. I just check to see how things were doing. ;^)
SusanOctober 4, 2013 at 8:57 am #4985
I agree with you that the potential for the bags to be bad is very high.¬† However, the problem I am concerned with is a “contact” yellowing.¬† Therefore I find it hard to ignore that it might be something contained on the artifact or in the organic matrix that is initiating the yellowing.¬† I am going to buy glycerin and put it on pH strips in multiple types of artifact bags (that is organics and inorganics) and see what happens.
Also, I think that we all need to monitor our collections more closely, like we have the time, yes, I know. . .I will also be initiating a system of testing soil from the sites to see if we can predict which collections may yellow the bags over time.
MarybethOctober 6, 2013 at 12:49 pm #5005
Of course, you are correct – materials in an artifact can cause even “archival” materials to deteriorate. It is a problem I see often in small museums – it is hard to think that archival enclosures can deteriorate especially if you paid a premium to get and use the best materials. Everything ages even if it is an an archival enclosure or if it is an archival enclosure. We have to keep an eye on the aging of the enclosures as well as the artifacts. Certainly, archaeological materials can be very complicated composites – especially once you factor in the matrix and soil. In that case, it might be good to think of having several types of materials in enclosures – say a tissue wrapping before placing the sample into a bag. This last, is just a thought.
SusanOctober 7, 2013 at 6:30 am #5017
Would using Tyvek bags help? ¬†Just curious – we use them because they protect things and they breathe, which poly won’t do.
JudithOctober 7, 2013 at 11:51 am #5018
Judith: Poly does breathe albeit slower than the Tyvek and allows slow migration of temperature and humidity changes.¬† I don’t know if I would want to trade the polyethylene for Tyvek
Susan: considered the tissue paper idea, but the problem, is money and labor which you know all of us don’t have.¬† We are thinking about a “basic” wash for the stone.¬† With these materials you just don’t know what someone might want to do with them in 50 years and if the notes get lost of what we did to them — well, I think you see where this is going.
All: So no easy solutions, but a fact of the business we are in — I think monitoring and changing the polyethylene bags is the best we can do.
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