by Tim Padfield

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Conservation of paintings in thermally unstable environments (September 2019)

A study of the effect of back protection on the RH behind paintings on canvas which are exposed to a varying wall temperature.



Introduction to this compendium of articles on preventive conservation and building physics applied to historic materials and structures.

How to keep for a while what you want to keep for ever

A light hearted introduction to preventive conservation

Basic concepts in climate

Fundamental microclimate concepts

An introduction to concepts and units concerning heat, moisture and light, which affect the interior environment.

Calculator for atmospheric moisture

A calculator for deriving moisture concentrations in air from dew point and psychrometer data

Why do tea bags swell in hot water?

Vapour pressure of water in air, explained through the swelling of a tea bag.

Mixing Ratio and Absolute Humidity

Definitions and explanations of the usefulness of mixing ratio and absolute humidity

Calculator for conservation heating

A calculator for deriving the temperature at which a certain RH is attained

Calculator for dehumidification energy load

Calculate the energy needed for dehumidification of storage spaces, given the interior temperature and target RH.

Calculator for energy use in museums

A calculator for estimating the energy consumption associated with the climate specification and the design of a building.

Light and photochemistry

The effect of light on museum objects

The effect of light on museum objects with particular attention to colours

The Candela - unit of luminous intensity

An explanation of the fundamental SI unit for light intensity, from which all other luminous units can be derived.

The light-fastness of the natural dyes

Information on the light-fastness of natural dyes is reviewed. New tests on the fastness of several dyes in fluorescent lamp light are reported.

The Deterioration of Cellulose in light and UV

This is a review of research on the deterioration of cellulose through exposure to light, ultraviolet and high energy radiation. The literature coverage stops at 1969, but the radiation sensitivity of cellulose was by then adequately researched because of the widespread use of cellulosic fabrics in aircraft construction before 1930.

The lux is a poor predictor of photochemical damage

The lux gives great weight to light at about 550 nm, which is far from the most damaging radiation at about 400nm

Properties of Materials

The absorption of water by materials

The sorption isotherm describes the reversible absorption of water by materials exposed to a changing relative humidity

Humidity exchange with absorbent materials

The exchange of moisture between materials and air

The reaction of wood to changing climate

The sorption of water by wood and its effects on the physical dimensions of wood.

Bending the evidence

Permanent set in wood caused by unsymmetrical moisture stress

Water vapour transmission in a temperature gradient

Water vapour transmission through a porous material in a temperature gradient seems to depend on the water vapour concentration gradient rather than on the vapour pressure gradient, as is often assumed.

Stress, strain and craquelure

A description of the concepts of stress and strain with particular application to paintings on canvas.

The art robbery at Skamkloster

A story which illustrates the efflorescence of inorganic salts as the relative humidity fluctuates


Museum microclimates

Proceedings of the conference 'Museum Microclimates' held in Copenhagen in 2007

Buffering relative humidity within a temperature gradient

There is a large risk of condensation when cooling or warming happens quickly. The thermal inertia of materials generates large temperature differences over small distances which can cause acute condensation.

Transporting a clock

A tutorial on packing an object for transport, with a paper wrap, then polyethylene, then insulation.

Evidence from thin air

A story about the effect of changing air pressure on the relative humidity around goods in transit

The second Seismograph of Chang Heng

On the difference between heat flow into an enclosure and the temperature which it reaches

A Himalayan Legend

A Himalayan legend describes the surprising consequences of moisture movement caused by temperature gradients in small enclosures.

The Hunt Ball

A glimpse into the microclimatic mysteries of glass cases.

How to protect glazed pictures from climatic insult

The climate in a glazed picture frame is surprisingly violently affected by exposure on an outer wall or to direct sunlight indoors.

Climate behind the pictures in Ledreborg chapel

The climate behind the oil paintings mounted on the outer walls of the Baroque chapel of Ledreborg house in Denmark is surprisingly benign, compared with the climate in the chapel.

The spontaneous transfer to glass of an image of Joan of Arc

A woven silk picture transferred its image to the glass against which it had been pressed for fourteen years. The image was formed from salt, which occupied the areas where the silk had not touched the glass. Salt impregnation increases the water absorption of silk at a relative humidity below the value at which pure salt deliquesces. A salt solution had formed in the fabric at a moderate relative humidity and then migrated to the glass where the salt precipitated because the relative humidity was below the deliquescence point of pure salt. The process has been replicated.

The response to changing temperature of a back-protected canvas

An experiment with a picture set against a variable temperature 'wall' and protected by back plates with varying hygroscopicity and placing.

A humidification chamber for relaxing works of art

The working principles of a humidity chamber for moistening art without risking condensation

Air exchange between an enclosure and its surroundings

A tutorial about the measurement and interpretation of the air exchange rate and introduction to the calculation of moisture buffer capacity in enclosures.

Control of RH and air pollution in showcases

The effect of absorbent materials and restricted air exchange on the course of the RH in showcases and picture frames.

Humidity buffering by porous, absorbent insulation

The influence on the room climate of moisture absorbent insulation in the exterior walls of houses

Trouble in Store - indoor air pollution

As air pollution in cities diminishes, or at least changes its nature, the air pollution generated within buildings becomes a relatively more serious cause of deterioration. The low rainfall indoors allows hygroscopic salts to develop and remain, without ever being washed away. Nearly airtight enclosure brings the threat of damage by modern materials outgassing into the air around museum objects, as well as chemicals given off by self-destructive objects. The commonest indoor pollutants are acid gases from the decomposition of wood, cellulose acetate and cellulose nitrate. Acid attack requires a thin watery film on the surface of the object to allow ionic processes to occur. Within showcases, there is competition between absorption on the object, absorption on a sacrificial absorber and ventilation out through imperfections in the seal.


A lecture to the lime forum in 2011 about the concept of breathability applied to buildings

The physics of drying cloth

Heat and ventilation are both necessary to drying. An experiment shows the relative importance of these two influences and how to dry without over drying to equilibrium with a low relative humidity

Correlation without causality keeps alive the myth of ventilation as good

The persistent emphasis on the need for ventilation to suppress mould growth is wrong and forces unnecessary complexity and cost on environmental control.

A workshop cool plate

A cool plate for conservation studio use, as well as research. A temperature range of 10 ° to 40 ° C can be programmed and also remote controlled.

Film cooler for small collections

A cold store for small collections of film which works by slightly warming a box within an ordinary food freezer.

The interaction of water vapour with paper in small spaces

The interaction of water vapour with paper, with particular reference to humidity buffering of leaky spaces where the kinetics of water movement through paper is significant.

The effect of moisture on the performance of woodwind instruments

The player's breath causes condensation within the joints of woodwind instruments causing the joint to swell against the outer band of wood, metal or ivory. On drying, the joint contracts to a smaller diameter, throwing the instrument out of tune.

The microclimate in a Neolithic passage tomb

Climate measurements show the performance of sealed versus open grave chambers and the advantage of an impermeable floor covering

A cooled case for George Washington's commission

A cooled case using thermoelectric cooling

Air conditioning and building physics

Mechanical air conditioning

The principles of mechanical air conditioning in large buildings

The Mollier diagram and the Psychrometric Chart

An explanation of the construction principles of the Mollier diagram and its almost identical alternative - the psychrometric chart.

Physics of low energy storage

A comprehensive account of the physics of the microclimate in stores without explicit temperature control

Low energy museum storage

Allowing a moderate annual temperature cycle in a museum store allows a much simplified and energy efficient control of RH.

Humidity buffering by absorbent materials in walls and objects

A review of the moisture buffering performance of construction materials and an introduction to the B-value concept for estimating the moisture buffering by artefacts.

Requiem for the Cologne City Archive

The idea that a massive building gives a good archive climate is not as clear cut as one might think.

Course material - Museum storage design

Material for a course in the design of museum storage: lectures, articles and exercises.

The role of absorbent materials in moderating changes of relative humidity

A phd thesis on moisture buffering by construction materials. Chapters on the fundamentals of the interchange of moisture between air and materials

Condensation in the walls of humidified buildings

Examples of museum buildings which have suffered condensation in the structure caused by leakage of humidified indoor air through the outer walls and roof.

Drifting temperature air conditioning

Drifting temperature with dehumidification provides a cool storage environment, but will environmental standards allow this?

Climate control in the archive of the Arnamagnaean Institute

Semi passive relative humidity control by mixing heat flow from the outside and heat from the building interior

Humidity buffering by clay walls

How to use absorbent materials in walls to buffer the relative humidity in archives and stores.

Casting mud in the debate on museum environmental standards

A suitable choice of building materials would give museums a stable climate without air conditioning.

Designing museums with a naturally stable climate

A review of trends in modern museum architecture which reduce the natural stability of the indoor climate but paradoxically increase the possibility for stabilising it without massive air conditioning installations.

The breath of Arrhenius: air conditioning in photographic archives

The air conditioning needed for film archives, analysed in relation to Arrhenius plots of the rate of deterioration.

Low energy storage

Low energy storage - article in ICOM-CC 2011

The influence of building techniques on the indoor climate

A review of the influence of building construction techniques and materials on the inner climate, with a section on biological growth in houses.

The climate of Gundsoemagle Church

Humidity buffering by porous walls has a considerable moderating influence on the indoor climate.

Climate control without complexity

Museum environment standards inhibit the use of simple climate control, particularly in storage, through their insistence on fixed temperature limits and expensive storage protocols.

Low energy climate control
for museum stores

An examination of five control strategies for controlling the temperature and relative humidity in stores containing an abundance of moisture absorbent materials.

How to design climatically stable museums

Illustrations of the interaction of the local climate with a building without mechanical air conditioning.

Ten years experience of energy efficient climate control in archives and museum stores

Museum stores which use the earth below as a temperature buffer hold a moderate temperature all through the year. Dehumidification is necessary in summer. It can be solar powered.

The Off-Grid Museum

A lecture about low energy museum design, delivered to the AIC meeting, Philadelphia 2011

Designing a Museum Store

An analysis of the north European climate and of the preservation requirements of typical museum objects suggests a suitable low energy method of air conditioning a museum store designed to hold relatively durable materials: outside air is sucked in when its water vapour content is unusually low while the temperature is raised slightly to give a relative humidity not far below the limit for biological growth. This combination gives a low degradation rate for both objects and the building that encloses them.

solar heated store

Solar heating plus temperature buffering by ground and humidity buffering by construction materials provide fully passive climate control for a museum store (pdf)

Sensors, measurement and data handling

Saturated salt solutions for controlling relative humidity

The theory and practice of using saturated salt solutions to define a relative humidity for sensor calibration or for treatment.

Measuring stress and strain

Measuring stress and strain in the context of conservation and building physics.

The thermocouple

The thermocouple: how it works and how to use it.

Data loggers for climate measurement

Data loggers for climate measurements, with hints on how to choose and use sensors.

A climate chamber that measures the moisture buffer capacity of materials

A description of a climate chamber designed to measure the moisture absorption and release by materials and constructions exposed to a changing moisture flux.

UV detector for museum use

An instrument is described which allows the eye to detect sources of ultraviolet radiation. Coincident ultraviolet and visible images of the scene are viewed through an eyepiece. The suspected source of radiation is centered in the field of view. The visible image is then extinguished by operating a shutter. Ultraviolet radiation is revealed by yellow-green fluorescence from a small target in the centre of the field.

Using a camera as a lux meter

How to use a camera as a lux meter, particularly for estimating lux within an inaccessible showcase.

Manipulating and plotting climate data

How to use awk and other unix utility programs to convert and merge climate data files ready for plotting with gnuplot

Environmental standards

The Preservation Index and the Time Weighted Preservation Index

An explanation of the science behind the Image Permanence Institute's Preservation Index and Time Weighted Preservation Index

The environmental specifications in BS5454-2000

A criticism of the environmental advice in British Standard 5454-2000 for archives.

Does a standard temperature need to be constant?

The present museum standards for temperature all demand constancy to a degree or two. There is no evidence that this is necessary. It is surely just the assertion of what is thought possible with air conditioning. This article is in 'Going Green', British Museum, April 2009

Environmental standards for museums

Several articles and publications challenging the strictness of temperature standards for museums and archives and proposing a more open development process.

Proposal for a museum environmental standard

A simple and direct and un-pompous proposal for a museum environmental standard.


A history of conservation in Denmark

The 1996 exhibition made for the IIC congress in Copenhagen "Archaeological conservation and its consequences"

Bologna symposium 1989, review

A review of the Bologna Symposium on Science, Technology and European cultural heritage, 1989

A review of Langwell's Postlip lamination process

A book review in Studies in Conservation, 1965

Publications by Tim Padfield

List of publications by Tim Padfield

External links

Sustainable climate management strategies. A workshop organised by the Getty Conservation Institute on Tenerife in April 2007

Passive design,mechanical systems and doing nothing: a telephone discussion about environmental management. in the Getty Conservation Institute newsletter 22.1, Spring 2007


Creative Commons License
All articles in this Conservation Physics compendium, except the external links, are licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.