Thank you for posting your experiences and for raising this subject. As you mention, the rates of acidification have a huge dependency upon the surrounding environmental conditions. As such this means that unless we can compile a significant number of experiences from different environments it will be difficult to draw reliable conclusions. But I think that there are several things that can be said with a high level of confidence.
- Unless the Post Office used very costly true archival (rag based, no lignin) paper**, acidification is going to be an issue over time. The paper they used is not rag paper and contains lignin; it will begin to turn acidic over time.
- The rate of acidification will increase significantly if the environmental conditions are less than ideal. To minimize the acidification process, a cool, dry and stable storage environment is paramount; temperatures should be held at a constant 70°F with a relative humidity held between 30% and 50% (or as close as possible).
- Some types of 'plastic wrapping' can be harmful by themselves. But any non-porous enclosure can be a double-edged sword; either offering some protection from a poor external environment or withholding a poor internal environment. If paper is stored in a sheet protector, plastic wrapping or any other non-porous enclosure it adds another environmental variable that will need to be controlled and monitored.
- For further information, the document "NBSIR 83-2795 Air Quality Criteria for Storage of Paper-Based Archival Records" can be downloaded here https://stampsmarter.org/learning/H...General.html
- The level of acidification in any paper can be better understood if the paper is tested. There are inexpensive pens which use reagents to visually indicate the pH of paper. Note that these are permanent markers so may not be suitable for testing on collectable paper, but sacrificing and maintaining one Commemorative Panel as a test subject might be wise. https://www.amazon.com/Lineco-Testi...p/B000KNJCSS
In terms of sulphurization and hydrogen peroxide treatments…
This is another area which is sometimes misunderstood and includes many personal observations. One of the problems with the observations that I have seen is that there exists little or no control over two of the most important related variables;
- Strength of the hydrogen peroxide
- Length of time of the exposure
Hydrogen peroxide is not a stable solution and can quickly lose strength after opening. A new, just opened 3% bottle of hydrogen peroxide is not the same strength as a one year old 3% bottle. So if using an existing, older open bottle of peroxide makes it anyone's guess at what its actual strength it might be. And of course this ties back into how long the exposure might need to be to develop a meaningful sulphurization remediation treatment.
** True archival paper is typically defined as meeting or exceeding the following specs;
- High alpha cellulose pulp from purified wood fiber with 25% Cotton
- Buffered with calcium carbonate 2-3% alkaline reserve, pH 9 ±0.5, acid–free.
- Board passes the PAT (Photographic Activity Test) ISO IT 18916 (formerly ISO 14523 ANSI IT9.16).
- Sulfur content is less than 0.0008% reducible sulfur
- Lignin content is tested by TAPPI 236 cm-85 and has a Kappa number of less than five.