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 4. RE: Gilder's bole 5. RE: Yellow-Green Staining on Polyethylene Bags 6. RE: Yellow-Green Staining on Polyethylene Bags ------------------------------------------------------------------------------------------------------------ 1.From: Yvonne Shashoua Posted: Friday July 17, 2026 7:34 AM Subject: RE: Yellow-Green Staining on Polyethylene Bags Message: 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 ------------------------------------------- Original Message: Sent: 7/16/2026 12:30:00 PM From: Marybeth Tomka Subject: Yellow-Green Staining on Polyethylene Bags I am inquiring if anyone has had the experience of finding staining or discoloration of polyethylene bags containing archaeological materials that can't seem to be explained by mold or other natural causes. I have found it in several collections at two repositories in Texas and it has recently been reported to me at one in North Dakota by a former student. However, this case is more rampant and more extensive. In my previous work we had linked it to the plasticizers reacting to UV light, possible vehicle exhaust fumes in warehouses, and oxygen. One article we read linked it to the breakdown of phenolic antioxidants to quinone which appears yellow green. I am an archaeologist with training in collections management, so chemistry is not a strength, but I had a volunteer with a biochemistry background that helped with the research. We are really looking for some answers to find out if we have a chemical reaction that is innocuous and unsightly or something of danger to the objects. The first time I saw the reactions it was with organic materials and ferrous metals; the second time the discoloration was appearing where the bags were stressed from the objects being too large for bag size. Unfortunately even if I could share pictures, the images do not illustrate the likeness of the discoloration. I would appreciate a conservator's opinion on the chemistry of what might be happening and any anecdotal comments on having seen this strange occurrence. We want to share the results so that people are forewarned. It seems objects may have been discarded because they were thought to be contaminated -- in my experience there was no smell and recent testing in ND did not indicate mold or spores of any kind. Ideas? Thank you. Marybeth Tomka MA RPA Prof Spec CMArgyle Archaeological Services 2.From: Robin Hodgson Posted: Friday July 17, 2026 7:34 AM Subject: RE: Gilder's bole Message: Hi Caroline, What colours are you looking for? I have a few I very much doubt I'll ever get to use again I'll check in the morning what I actually have ------------------------------ Robin Hodgson Owner RH Conservation Engineering Flinders [email protected] ------------------------------ ------------------------------------------- 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: Dermot Cox-Kearns Posted: Friday July 17, 2026 7:38 AM Subject: RE: Yellow-Green Staining on Polyethylene Bags Message: When they are making the bags, some of the producers use something to prevent static and sticking of the plastic material as it comes off the roll into the bag forming machine, when the material is going through the machine that formats the bags, static and sticking can slow down the speed the machine can work at ... unfortunately I have no real idea what the materials are that they use to prevent static and sticking on the machines, possible it is similar to the coconut power that is used when they make disposal powder latex gloves, or some other similar material, similar materials are also used in the Litho printing industries. A possible solution to eliminate the staining is to use food or pharmaceutical grade plastic bags, which would have a much cleaner production than standard plastic bags. ------------------------------ Dermot Cox-Kearns Owner On The Edge Ashford - Wicklow Ireland ------------------------------ ------------------------------------------- Original Message: Sent: 07-16-2026 12:30 From: Marybeth Tomka Subject: Yellow-Green Staining on Polyethylene Bags I am inquiring if anyone has had the experience of finding staining or discoloration of polyethylene bags containing archaeological materials that can't seem to be explained by mold or other natural causes. I have found it in several collections at two repositories in Texas and it has recently been reported to me at one in North Dakota by a former student. However, this case is more rampant and more extensive. In my previous work we had linked it to the plasticizers reacting to UV light, possible vehicle exhaust fumes in warehouses, and oxygen. One article we read linked it to the breakdown of phenolic antioxidants to quinone which appears yellow green. I am an archaeologist with training in collections management, so chemistry is not a strength, but I had a volunteer with a biochemistry background that helped with the research. We are really looking for some answers to find out if we have a chemical reaction that is innocuous and unsightly or something of danger to the objects. The first time I saw the reactions it was with organic materials and ferrous metals; the second time the discoloration was appearing where the bags were stressed from the objects being too large for bag size. Unfortunately even if I could share pictures, the images do not illustrate the likeness of the discoloration. I would appreciate a conservator's opinion on the chemistry of what might be happening and any anecdotal comments on having seen this strange occurrence. We want to share the results so that people are forewarned. It seems objects may have been discarded because they were thought to be contaminated -- in my experience there was no smell and recent testing in ND did not indicate mold or spores of any kind. Ideas? Thank you. Marybeth Tomka MA RPA Prof Spec CM Argyle Archaeological Services 4.From: Ana Flynn-Young Posted: Friday July 17, 2026 1:07 PM Subject: RE: Gilder's bole Message: They still seem to have it in stock at Jaksons Bole, Grounds & Lacquer for Gilding | Jackson's Art <https://www.jacksonsart.com/techniques/gilding/bole-grounds-lacquer> Good luck Ana Flynn-Young Head of Painting Conservation. North East Museums (formerly Tyne and Wear Archives and Museums) Laing Art Gallery New Bridge Street Newcastle upon Tyne Newcastle NE1 8AG Tel: 0191 278 1531 Mobile: 0775 247 5473 Email: ana.flynn@ <[email protected]>northeastmuseums.org.uk https://www. <https://www.twmuseums.org.uk/collections/conservation>northeastmuseums.org.uk/collections/conservation <https://www.twmuseums.org.uk/collections/conservation> Help us ensure art, culture & heritage continue to make a positive impact in our region. https://www.northeastmuseums.org.uk/donation <https://www.northeastmuseums.org.uk/donation> Privacy notice <https://www.twmuseums.org.uk/privacy-policy> ------------------------------------------- Original Message: Sent: 7/16/2026 7:08:00 AM 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] 069165 www.coconservation.co.uk <http://www.coconservation.co.uk> 5.From: Marybeth Tomka Posted: Friday July 17, 2026 1:07 PM Subject: RE: Yellow-Green Staining on Polyethylene Bags Message: 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 6.From: Marybeth Tomka Posted: Friday July 17, 2026 1:07 PM Subject: RE: Yellow-Green Staining on Polyethylene Bags Message: Dermot, Do you know what grade of plastic that would be or the relative cost? Thanks, Marybeth ------------------------------ Marybeth Tomka Argyle Archaeological Services Converse Texas USA ------------------------------ ------------------------------------------- Original Message: Sent: 07-17-2026 05:27 From: Dermot Cox-Kearns Subject: Yellow-Green Staining on Polyethylene Bags When they are making the bags, some of the producers use something to prevent static and sticking of the plastic material as it comes off the roll into the bag forming machine, when the material is going through the machine that formats the bags, static and sticking can slow down the speed the machine can work at ... unfortunately I have no real idea what the materials are that they use to prevent static and sticking on the machines, possible it is similar to the coconut power that is used when they make disposal powder latex gloves, or some other similar material, similar materials are also used in the Litho printing industries. A possible solution to eliminate the staining is to use food or pharmaceutical grade plastic bags, which would have a much cleaner production than standard plastic bags. ------------------------------ Dermot Cox-Kearns Owner On The Edge Ashford - Wicklow Ireland ------------------------------ 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.
