Lecture title: Artifact deterioration and environmental standards

Headings in blue refer to the illustrations in the pdf file

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grossoil title page

First I introduce the team, based, unusually for building physicists, in a museum, the National Museum of Denmark, though I participate from my retirement paradise in the hills of Devon, in southern England. We design climate control and pollution prevention in museum stores and archives.

Most buildings are climatised for the benefit of humans. Archives and museum stores can be climatised for the benefit of the artifacts. I will review the climatic needs of artifacts and describe how to satisfy them with simple climate controls.

rotting artifacts

Some threats are more immediate than others and risk derailing the careers of curators if ignored, while others are slow acting over centuries. The risk of biological growth is very immediate and has been quantified by Martin Krus and colleagues at the Fraunhofer Institut fur Bauphysik.

krus isofungus lines

So rule one for climate control in museums is to keep the RH below 70%. The annual average RH in Copenhagen is about 78%, but in some other places one can justify doing nothing, relying instead on buffering the collection so that the RH seldom rises above 70%. Paper is not very susceptible to fungal growth, as is demonstrated by the fine condition of the Spanish military archive in the citadel of Segovia

alcazar facade

The archive is behind these windows, just above the bushes.

alcazar climate year

The microclimate inside is very stable, though perilously close to the RH for fungal growth. This archive introduces two requirements for an archive - massive construction with high heat capacity and a lot of stored absorbent material to give good moisture buffering.

st catherines monastery

A more reliable way of doing nothing, from the point of view of stopping biological decay, is to put the archive in a naturally dry place. This is St Catherine's monastery high in the desert mountains of the Sinai peninsula in Egypt. The library is the Mussolini style building at the back, built in 1947, but the library dates from the sixth century.

st cath climate

The indoor climate is warm but dry - very dry - down to 15% in summer. This alarms curators familiar with conservation standards advocating a lower RH limit around 45%, but conservators say that the paper and parchment in this 1500 year old library are in good condition.

Lop nur excavators

The ultimate in negligent storage is surely the ruined house in Lop Nur, western China. Unwanted paper records had been torn and cast into a corner of one room, to be covered eventually by half a metre of blown sand. These fragments were thrown away in the third century. They were excavated by the Swedish explorer Sven Hedin in 1901. Their good condition is attested by the numerous fakes that were rapidly prepared by the local excavators from modern paper, still made in the identical traditional technique. The fakers didn't need to waste time artificially ageing the paper.

Dunhuang climate

The climate in Lop nur, at half a metre down, is approximately as shown here, the data coming from the nearest modern measuring point at Dunhuang. Notice the large temperature cycle.

sebera isoperms

All these examples show a safe RH from the point of view of biological attack, but what of the other causes of decay? Archival materials suffer most from slow hydrolysis - breaking of the polymer chain of cellulose or proteins by the addition of water. The reaction rate is accelerated by both temperature and RH, which is identical to the chemical activity of water. In this diagram the grey curves are lines of constant hydrolysis reaction rate. The orange line is arbitrarily given a rate of one, since it passes through the 50% RH and 20C typical of the human indoor climate. Consider the Sinai library: the summer temperature, about 30C, raises the reaction rate by four times, but the low RH, about 15%, reduces the reaction rate by an equal amount, so the final rate is not too bad. For really good durability it is necessary to keep the archive cool.

dogma cold store

Vulnerable material such as cellulose acetate colour film is kept very cold - down to -20C. Parchment and old paper is however quite durable at human comfortable temperature, so the mechanical complexity and consequent vulnerability of cold storage is not necessary, or at least one can put it off for a hundred years or so without being called irresponsible.

arnemagnaean facade

Back here in Copenhagen we choose to keep our archives close rather than export them to more suitable climates. We are concerned about the immediate threat of biological attack in the average 78% RH of Copenhagen. The city temperature is quite cool on average, so it is reasonable to use a little heat to reduce the indoor RH: we don't even need to raise it to human comfort temperature to reduce the RH enough. This is the Arnemagnaean archive in Copenhagen University, just across the water from here. The blank piece of facade hides the archive, which backs onto the corridors of a permanently inhabited building which is kept warm in winter.

arnemag cutaway

This is a cross section of the archive room. Notice that it is thermally insulated (the blue stuff) also on its internal walls, so its temperature hovers midway between the constant indoor temperature and the variable outside weather.

arnemag year

The indoor RH is moderated by the heat leaking from the building. The average archive temperature is sufficiently above the average outdoors that the RH is lowered to around 55%. The tendency to rising RH in summer is countered by the moisture buffer capacity of the archive contents.

arnemag climate detail

The RH is fine tuned by pumping in outside air when it is, by chance, of suitable water vapour content to send the RH towards the set point. Such a moment is marked by the green arrows. A sudden influx of Siberian air with low water vapour content made it advantageous to pump in outside air. Notice that the pumping, at 2 air changes per hour, hardly affected the temperature of the archive. A careful balance between thermal capacity and thermal insulation is needed to allow this process to function well. We hit the sweet point by accident: the thermal inertia comes from massive concrete construction forced by the client's demand that the archive should survive falling to the ground if a truck knocks away the supporting columns. The insulation thicknesses were calculated using a well known computer model, but the observed annual temperature cycle is not close to that predicted by the software. Fortunately, an exact temperature cycle is not essential since the RH does not have to conform to an exact value - it has to be steady and safe.

vejle storage building

It is possible to keep the temperature low and dehumidify to reduce the RH. If the room is airtight, this takes little energy. This is the Vejle storage depot for a consortium of museums in the middle of Jutland. It wasn't designed to be cool, but it is, so dehumidifiers have been installed. I don't know if that was an error in software, or if predictive modelling was used at all.

vejle climate

The RH control is perfect, except when the equipment fails (here) or when the doors are left open (here). The temperature follows an annual cycle with considerable buffering by the uninsulated concrete floor. The deterioration rate, based on the plot I showed earlier, is about half that of the archives which are heated to reduce the RH. So, although unintended, dehumidification has proved both cheap and effective, and reasonably fail safe, giving some inertial protection against the RH rising to a dangerous value. The outdoor temperature, the red dots, seems improbably high in summer, reaching 40C, suggesting radiant heating of the sensor. One of the difficulties of this research is the extreme rarity of reliable climate data, inside or out.

BS5454 temperature

The rarity of climate data is matched by the rarity of surveys or experiments which describe the effect on durability of materials of the annual cycles of RH and temperature prevailing in these example buildings. This uncertainty was maybe what influenced the anonymous expert committee of the British Standard for archives to be cautious. They give the curator a narrow choice of a constant set point, with no regard to the annual climate cycle and then insist on a maximum deviation of =/-1°C. This is a clear example of a twentieth century doctrine in building management: Use the "best available technology". There is not a shred of evidence that a gentle annual cycle of temperature has any deleterious effect. There is no evidence that rapid, but moderate, change of RH or temperature has any bad effect. But this standard has to be obeyed by British archives for them to be approved for storing state documents.

BS5454 RH and vent

The RH specification is also too restrictive to allow totally passive climate control in most regions on earth. Finally, the admonition to ventilate is illogical, since most archived documents are bundled into boxes or between book covers, which are unventilated.

candide

The instruction to ventilate is phrased as though air movement is good for archives but it is really just a method of equalising temperature throughout a building, and consequently equalizing the RH. As you can see, the unventilated interior of the book is doing better than the edge. In spite of innumerable assertions in the literature that ventilation inhibits mould growth, I know of no peer reviewed article that presents quantitative evidence for the influence of air movement on fungal growth. Passive or low energy climate control in archives requires an air leakage rate around 0.1 per hour or less. So this is another unjustified hindrance to sensible design of archives.

ipswich archive

The Sufolk county records in Ipswich, England, is heated in winter to 16°C but runs loose in summer, relying on thermal inertia to buffer the daily temperature cycle and moisture inertia to buffer the annual RH cycle. This was one of the pioneer buildings of passive climate control, far ahead of its time.

Ipswich archive climate

Now, however, the British Standard for archives, written ten years after the completion of this building, has judged its temperature cycle excessive, so the building has now been air conditioned.

Building committee

The reason that all archives are not built with variations on this theme of temperature and humidity buffering combined with a slow air exchange rate is arbitrary rules, unencumbered by evidence of merit.

Here you see the usual experts, architect, engineer, builder, accountant, and in our case a conservator or curator. Behind the conservator is the standards committee, whose anonymity is preserved by invocation of the British data protection act. Most building standards have recently been adapted to encourage minimal use of energy but this standard for British archives, enforces the feckless use of energy for unproved benefit. The specification that wastes energy is enshrined in the group smiley - a constant temperature set point and a permitted variation of plus-minus one degree, combined with forced ventilation.

One has to conclude that the standard is based on the engineers' passion for exact control rather than on concern for the durability of the stored documents. This is an influential doctrine - The British Standard is an export commodity - used over here in the Danish Royal Library, and suggested for St Catherine's monastery but subsequently abandoned.

mountain archive

I don't want to end on a negative note. What is the optimal archive design? A cool upland location where semi burial provides thermal inertia with good drainage, a sloping site allows convective ventilation according to the prevailing weather, the thin air slows the oxidation rate of the collection and reduces the danger of fire. The thin atmosphere allows radiative heat loss to the night sky, Deciduous trees on the south side reduce summer heating. A mountain stream provides the little power used in the building. Air pollution is minimised by curators and guests arriving by bicycle. If the worst happens and fire breaks out the water from the limestone mountains automatically deacidifies the saturated documents. I'm getting rather fanciful now, but please, those who convene to make standards that bind a whole industry, expand your knowledge of deterioration processes and abandon the illusion of climate constancy as the key to eternal preservation.

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