Tuesday, 21 January 2014

Environmental Considerations For Mixed Collections of Historical Artifacts - 2014

January 21, 2014

The following discussion focuses on artifacts that are comprised of metals, glass, ceramics, papers, textiles, organics or in-organics all kept in the same space.  This document outlines the environmental considerations and conditions for their storage and display in general terms for community museums.  I have provided several references at the end for further detailed information specific to each type of material.

Many historical artifacts are made up of several different materials attached and combined in various ways and would benefit from careful environmental considerations.  Temperature and moisture fluctuations put undue pressure on their structure and in some extreme conditions can cause fractures.  Different metals expand and contract with temperature changes to different degrees.  This is also true for different woods which will have different moisture content as well.  Particularly fragile are some older papers, wood, and porous ceramics that are hygroscopic – meaning they can absorb and give off moisture.  Higher and lower moisture levels can be particularly damaging to their cell structure.  Condensation can appear in some metals at temperatures below 10 degrees Celsius.  This promotes corrosion and can negatively affect other materials when they are stored in close proximity.  Storage materials can also play a role in deterioration since we know that materials such as some plastics can give off gases and over a lengthier period of time when combined with moisture form a mild acid.  Glass is more stable as is acid free wrapping paper, storage boxes and envelopes.

Some general information for artifacts kept in a community museum environment follows where it is assumed that special air conditioning or lighting controls are not available for storage or display of these artifacts.  This is true for most community museums in Nova Scotia, Canada and in general terms, elsewhere.  However, Nova Scotia is subject to a climate similar to United Kingdom in that we have on average 85% relative humidity over the course of a typical year but colder weather (and snow!) in a slightly longer winter.  The freeze and thaw cycle over the winter months is particularly challenging when some museums shut off the heating systems or set the thermostat too low while the museum is closed.  This is also a concern for those who have historical artifacts in their homes and leave for a few months with the thermostat turned down and the humidifier turned off while vacationing where it is warmer. 

In all cases it is best to keep things in a steady state.  That is, to avoid rapid fluctuations in humidity, temperature, light levels and so on.  Even moderate fluctuations are to be avoided but are not as damaging.  Where possible, monitor the temperature and humidity as a minimum watching for wild changes over a short time.  There is relatively inexpensive equipment available to do this but it requires effort on someone’s part to check the results.  If you must turn the temperature down or bring the relative humidity up or down (for example, when doing building repairs) do it over a longer period of time and return it back to normal settings over a longer period of time.  This allows the artifacts to slowly adjust without cracking, getting condensation, and so on.

Dust levels and vibrations are also a consideration if work is being done nearby or in your building.  Recently we became concerned at a community museum when roadwork outside caused serious vibrations inside our building when heavy machinery was working on the street outside our door.  We packed some things away to ensure they did not suffer damage from dust, falling over or off shelves. 

Basic Environmental Factors and Recommended Levels

Relative humidity, the recommended acceptable level day and night throughout the year is 50 or 55% +/- 5% with the acceptable range of 45 to 60% for mixed collections.

Light exposure, recommended generally acceptable level is 100 lux (lumens per metre squared), high levels of lux exposure are not usually harmful to metals, glass or ceramics but are detrimental to coloured objects since it can result in fading of colours; levels above 300 lux can cause eye adaptation difficulties for visitors in some facilities.  Light exposure can be measured with a light metre as used by photographers.

UV radiation, the general recommended level of Ultra Violet exposure is 75µw/l (microwatts per lumen).  Extended exposure to sunlight is the most damaging although quartz halogen lamps are also a concern.  Do not hang or store any painted artifact in an area that is subject to direct sunlight.  UV can be measured with broadband UV sensors, for example, which is specialized equipment usually outside the budget of most community museums.  Just ensure that all painted or coloured artifacts (examples: samplers, paintings, prints, papers, most textiles) are kept out of direct sunlight.

Temperature, the general acceptable level in winter is 19 C +/- 1 C and 24 C +/- 1 C in summer with a minimum temperature of 10C at any time to avoid condensation particularly in metals.

This document is a general discussion and overview of environmental factors.  Please refer to the references below for more detailed information.

1) This is an excellent overview of general preservation guidelines and recommendations for mixed collections thanks to the Minnesota Historical Society:

2) Canadian Museum of Nature – Collection Conservation:

3) Environmental Guidelines for Museums by the Canadian Conservation Institute: