Dear Mark

Having read your thoughts about digital media storage and a photo store being too humid and/or too cold, I looked again at ISO 11799, which we have just used in pulling together the review of BS5454.  The ISO standard proposes a range of options for magnetic media storage for example, where RH maxima are adjusted downwards as temperature parameters are adjusted upwards, with a standard base RH of 15% and 8degC.  This is so that those who cannot provide very low temperature storage can use warmer conditions and know which RH to use.

When Hugh and I established Dorset's photo store in the early 90s, it was a mixture of achieving what we could with the (in some cases very new) technology available at the time and with a view to storing historic material and our modern microfilm.  What you will have realized I think is that, if you continue to run it at c12-14degC and 35-40% RH, it does not fit precisely with the ISO optional parameters for Polyester base magnetic media.  It does however fit well with the acetate base magnetic media (listed as 'other' in the ISO standard) - i.e. old sound and video material - and most of the other photo media.  It is worth observing that the optimum environment for acetate plastic films and tapes is actually sub-zero - your store was a way of doing the best for a wide range of non-traditional media and accepting that some would still not be in the optimum environment but would last a bit longer in there than in the normal archive store.

The problem with the ISO standard and other recommendations emanating from it is that for polyester base media it jumps from a maximum of 50%RH at a maximum of 11degC, to a maximum of 30% at a maximum of 17degC.  It does not indicate what RH would be acceptable with a maximum of, say, 14degC or what temperature if you have 40%RH.  The simple truth is that material characteristics do not jump quite so dramatically.  Your storage of, say, 40%RH will be well suited to a 13degC environment for this media.  You are doing the right thing by keeping the material cool and dry - the right thing being: prolonging the life of the material using the resources you have.  It will not degrade faster because you cannot achieve 11C or lower or because you can't achieve 30% without having the temperature too high for the other items in the store.  If you were more concerned to meet a standard rather than to manage the ageing rate of this material, you can of course place your modern magnetic media into silica-gel buffered enclosures that reduce the RH internally to below 30% RH while keeping them in your 13C photo store, or even in your 15C archive store.  The excellent stability in those stores should ensure that this is a sustainable approach.

In reviewing BS5454 recently we have endeavoured to apply these more realistic principles than are apparent in the old ISO 11799.  I lean towards the approach used by Dr Michalski and co and the Canadian Conservation institute, which has undertaken a great deal of research on environmental effects on materials and which recommends, in effect, the ISO range from 8 to 23 C and 15 to 50%RH, without the artificial step changes in between.  I strongly recommend that, if you haven't done so already, you look at their Electronic Media Collections Care for Small Museums and Archives at

http://www.cci-icc.gc.ca/crc/articles/elecmediacare/index-eng.aspx

which also gives guidance on optical disks, and which takes the stance of leaving you to decide how long you want things to last and making your own parameter decisions based on that.

Chris Woods
Chair, BS IDT 2/9 (5454)
Director, National Conservation Service
www.ncs.org.uk
Visiting Research Fellow, UAL

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