Email digest for the Global Conservation Forum (ConsDistList) egroup. ------------------------------------------------------------------------------------------------------------
1. RE: Yellow-Green Staining on Polyethylene Bags 2. RE: Gilder's bole 3. RE: Yellow-Green Staining on Polyethylene Bags ------------------------------------------------------------------------------------------------------------ 1.From: Luisa Casella Posted: Saturday July 18, 2026 7:51 AM Subject: RE: Yellow-Green Staining on Polyethylene Bags Message: Thank you for this very informative exchange. I have often recommended food grade polyethylene bags to small collections and stewards with limited resources as a safe, durable, and low cost means of protecting materials from water damage. I was unaware of the oxidative degradation described, having assumed that polyethylene was comparable in long term stability to polyester. Given that the small institutions and individuals I advise are unlikely to replace these bags on a ten year cycle, should I reconsider this recommendation? Or does the risk lie primarily in the loss of the bag's protective function as a water barrier rather than in any harm to the enclosed objects, in which case periodic inspection and replacement of bags when yellowed might suffice?I would be grateful for any thoughts. ------------------------------------------- Original Message: Sent: 7/17/2026 12:46:00 PM From: Marybeth Tomka Subject: RE: Yellow-Green Staining on Polyethylene Bags Yvonne Shashoua Thank you so much for your reply. I did a double take when I saw your email. I have had your book on my shelf for over 10 years since I started seeing this plastic issue. I had also reached out to the Canadian Conservation Institute and Dr. Mark Kearney replied with the same advice and explanation as you did. He also provided these links to a few studies that are great illustrates of the experiments I wanted to do: (A preliminary investigation into the influence of archaeological material on the yellowing of polyethylene storage bags: AICCM Bulletin: Vol 42 , No 2 - Get Access <https://www.tandfonline.com/doi/full/10.1080/10344233.2022.2058245>; Safe Storage? An Assessment of Polyethylene for the Storage of Heritage Objects: Studies in Conservation: Vol 68 , No 6 - Get Access <https://www.tandfonline.com/doi/full/10.1080/00393630.2022.2092702?src=recsys>; Antioxidant-induced yellowing of textiles - ScienceDirect <https://www.sciencedirect.com/science/article/abs/pii/0141391095001565> I hope the readers of this list have the resources to access the presses because the PDFS are pricing if you do not. I do have one question about the 2 Australian studies if you are aware of them or read them. They both mention PE but do not specify if it is of high density polyethylene (HDPE; Number 2) or low density polyethylene (LDPE Number 4). I don't think it would make much of a difference except that the low density should degrade faster, correct? In a nutshell, what this means for collections managers, is that we must take the assessment of our collections much more seriously in archaeology and dedicate more funds toward the repackaging of our collections on ten-year cycles. Everything that the managers of archaeological budgets do not want or can do. Again, thank you for answering my question, your wonderful resource book, and any additional thoughts you may share. Marybeth ------------------------------ Marybeth Tomka Owner-Operator Converse, Texas United States ------------------------------ ------------------------------------------- Original Message: Sent: 07-17-2026 05:25 From: Yvonne Shashoua Subject: Yellow-Green Staining on Polyethylene Bags Yellowing of polyethylene bags, such as Ziplock or freezer bags is most frequently due to degradation by oxidation. The reaction between oxygen in air and polyethylene results in the formation of carbonyl chemical groups that result in darkening and yellowing. New polyethylene bags contain UV absorbers, like sun cream, and antioxidants that protect the plastic from light and air. When these are exhausted, yellowing and weakening are the result. No plastic lasts forever and, in my experience, yellowing is common in polyethylene bags after 10 years. Because oxidation cannot be reversed, replacing these bags is the solution. You mention plasticizer as a possible cause of yellowing. Polyethylene does not require plasticizer to be flexible, it is an inbuilt property, whereas PVC does and it too yellows with time. Yvonne Shashoua Research Professor, National Museum of Denmark 2.From: Sonia Santos Gómez Posted: Saturday July 18, 2026 4:59 PM Subject: RE: Gilder's bole Message: El bol de la casa Lefranc también funciona muy bien ------------------------------ Sonia Santos Gómez Professor, Paintings Conservation Universidad Compultense de Madrid, Conservación del Patrimonio Cultural Madrid Spain ------------------------------ ------------------------------------------- Original Message: Sent: 07-16-2026 07:07 From: Caroline Oliver Subject: Gilder's bole Hi everyone, My jars are getting empty and I've just been searching the usual suppliers to buy some bole clay for gilding. It's getting harder to find stock available, especially since Selhamin has been discontinued, and the colour range is ever shrinking. I'm sure there are studios around the country (I'm UK based) that have bole they no longer use. How do people feel about re-homing/selling unused supplies? Just a thought - it doesn't go off and there are gilders who would make good use of it. Caroline Caroline Oliver [email protected] <[email protected]>07739 069165 www.coconservation.co.uk <http://www.coconservation.co.uk> 3.From: Yvonne Shashoua Posted: Saturday July 18, 2026 4:59 PM Subject: RE: Yellow-Green Staining on Polyethylene Bags Message: Marybeth-thank you so much for your kind words about my book. I plan to update it within the next 2 years and your encouragement has given me a boost. Oxidation is the major cause of degradation of both high density and low density polyethylenes. Most freezer, ziplock and food bags are low density PE. Luisa-whereas polyester, also widely known as polyethylene terephthalate or PET (tradenames include Mylar and Melinex), is considered the gold standard in stability for general use plastics, polyethylene is at the other end of the scale. Its linear chemical structure compared to PET's network structure (washing line versus fishing net) makes it vulnerable to oxidation because a single bond breaking reduces its resistance and potential tensile strength. The presence of metals, particularly copper, nickel and iron accelerate oxidation. If only mild discolouration is present, replacement can wait, but breaks, holes and tears will follow. If it is possible to replace PE with PET bags, that would be a longer-term solution. Yvonne Shashoua, Research Professor National Museum of Denmark ------------------------------------------- Original Message: Sent: 7/18/2026 7:13:00 AM From: Luisa Casella Subject: RE: Yellow-Green Staining on Polyethylene Bags Thank you for this very informative exchange. I have often recommended food grade polyethylene bags to small collections and stewards with limited resources as a safe, durable, and low cost means of protecting materials from water damage. I was unaware of the oxidative degradation described, having assumed that polyethylene was comparable in long term stability to polyester. Given that the small institutions and individuals I advise are unlikely to replace these bags on a ten year cycle, should I reconsider this recommendation? Or does the risk lie primarily in the loss of the bag's protective function as a water barrier rather than in any harm to the enclosed objects, in which case periodic inspection and replacement of bags when yellowed might suffice?I would be grateful for any thoughts. ------------------------------------------- Original Message: Sent: 7/17/2026 12:46:00 PM From: Marybeth Tomka Subject: RE: Yellow-Green Staining on Polyethylene Bags Yvonne Shashoua Thank you so much for your reply. I did a double take when I saw your email. I have had your book on my shelf for over 10 years since I started seeing this plastic issue. I had also reached out to the Canadian Conservation Institute and Dr. Mark Kearney replied with the same advice and explanation as you did. He also provided these links to a few studies that are great illustrates of the experiments I wanted to do: (A preliminary investigation into the influence of archaeological material on the yellowing of polyethylene storage bags: AICCM Bulletin: Vol 42 , No 2 - Get Access <https://www.tandfonline.com/doi/full/10.1080/10344233.2022.2058245>; Safe Storage? An Assessment of Polyethylene for the Storage of Heritage Objects: Studies in Conservation: Vol 68 , No 6 - Get Access <https://www.tandfonline.com/doi/full/10.1080/00393630.2022.2092702?src=recsys>; Antioxidant-induced yellowing of textiles - ScienceDirect <https://www.sciencedirect.com/science/article/abs/pii/0141391095001565> I hope the readers of this list have the resources to access the presses because the PDFS are pricing if you do not. I do have one question about the 2 Australian studies if you are aware of them or read them. They both mention PE but do not specify if it is of high density polyethylene (HDPE; Number 2) or low density polyethylene (LDPE Number 4). I don't think it would make much of a difference except that the low density should degrade faster, correct? In a nutshell, what this means for collections managers, is that we must take the assessment of our collections much more seriously in archaeology and dedicate more funds toward the repackaging of our collections on ten-year cycles. Everything that the managers of archaeological budgets do not want or can do. Again, thank you for answering my question, your wonderful resource book, and any additional thoughts you may share. Marybeth ------------------------------ Marybeth Tomka Owner-Operator Converse, Texas United States ------------------------------ ------------------------------------------- Original Message: Sent: 07-17-2026 05:25 From: Yvonne Shashoua Subject: Yellow-Green Staining on Polyethylene Bags Yellowing of polyethylene bags, such as Ziplock or freezer bags is most frequently due to degradation by oxidation. The reaction between oxygen in air and polyethylene results in the formation of carbonyl chemical groups that result in darkening and yellowing. New polyethylene bags contain UV absorbers, like sun cream, and antioxidants that protect the plastic from light and air. When these are exhausted, yellowing and weakening are the result. No plastic lasts forever and, in my experience, yellowing is common in polyethylene bags after 10 years. Because oxidation cannot be reversed, replacing these bags is the solution. You mention plasticizer as a possible cause of yellowing. Polyethylene does not require plasticizer to be flexible, it is an inbuilt property, whereas PVC does and it too yellows with time. Yvonne Shashoua Research Professor, National Museum of Denmark You are subscribed to "Global Conservation Forum (ConsDistList)" as [email protected]. To change your subscriptions, go to http://community.culturalheritage.org/preferences?section=Subscriptions. To unsubscribe from this community discussion, go to https://community.culturalheritage.org/HigherLogic/eGroups/Unsubscribe.aspx?UserKey=d16eaa87-0f69-494b-9f2f-303dbc1222e1&sKey=fab9aa4f27a04c5d876e&GroupKey=757a8f16-505f-4323-8e74-e376757aa9f7.
