The environment of the unprotected painting
All the graphs in this article have the same pattern, with RH curves clustered at the top, temperatures in the middle (with scale on the right side) and mixing ratios in a band at the bottom of the graph. A thumbnail of the structure is shown at the bottom right. Two RH curves are given, the measured value close to the surface of the canvas and the value calculated from the surface temperature of the canvas. The two curves are linked by a coloured fill, which is pink for the canvas and, in all the other graphs, blue for the RH close to the back plate.
The graph in Fig. 6 imitates the normal situation of a painting with an unprotected back and ample circulation of room air behind it. In an art gallery this is usually because the top of the painting is tilted away from the wall, but in our experiment we made a uniform 10 mm gap between frame and wall.
The canvas temperature was approximately half way between ambient and the wall temperature. The vapour concentration at the canvas surface was the same as that in the room, as shown by the nearly identical and flat mixing ratio curves for the room and for the canvas. The RH varies strongly in opposite phase to the canvas temperature.
The RH between the canvas and the wooden cross beam was much more stable. The mixing ratio at this sensor deviated from the room value. This indicated vapour exchange with the wood, and with the adjacent canvas, in this poorly ventilated space.
Adding an impermeable back plate
An impermeable, moisture inert plate of aluminium was fastened to the back of the frame, but still separated from the wall by 10 mm. This gave a completely different microclimate within the sealed enclosure (Fig. 7). There was a nearly constant RH behind the canvas and the temperature cycle was reduced. The RH stability comes from humidity buffering by the painting’s canvas, as revealed by the varying mixing ratio, indicating repeating evaporation and absorption of vapour into the confined space. One might object that the infrared reflectivity of the aluminium plate is a factor, but this is only the case when there is a gap between back plate and wall. As can be seen in Fig. 17, the difference in temperature span between “10 mm gap, backed” and “external insulation” is only about 1 degree K (about 6 degrees K and about 5 degrees K respectively), so there is a difference but it is not the significant effect for this experiment.
In this experiment new canvas was used. Degraded canvas, different grounding etc. may have slightly different sorption properties. The data used for sorption calculations is based on a study by Gregers-Høegh et al. [6] The calculations can be found at [7]. The RH close to the cross beam was still very stable. However, the RH at the back plate surface varied strongly over the temperature cycle.
Humidity variation through a temperature gradient
The varying RH at the moisture-inert metal back plate can be explained by reference to two well established diagrams, the psychrometric chart and the sorption isotherms of cellulose [8].
The starting point for the graph in Fig. 7 is a uniform environment throughout the assembly at 26 °C and 60% RH. When the temperature at the canvas surface declines to 24 °C at the low point of the temperature cycle, the RH in equilibrium with the canvas surface falls only about 1%, as defined by interpolation on the graph of sorption isotherms in Fig. 8. So the system moves on the psychrometric chart diagonally down and to the left, following close to the curve for 60% RH to the point marked ‘canvas’. This corresponds to a lower mixing ratio.
The mixing ratio does not vary with temperature change. A falling temperature will increase the density of the mixture but will not change the ratio between water molecules and air molecules. A changing mixing ratio indicates that vapour is entering or leaving the air through humidity buffering by materials. Also, its variation in space indicates the degree of mixing of the air within the enclosure. In this context, one can regard air as entraining water molecules and helping to distribute them evenly throughout the space. There is no interaction between water vapour and air other than random elastic collisions; one should not regard air as a solvent for water vapour, only as a transport medium.
As the air close to the canvas disperses out into the space behind it, the mixing ratio does not change, because the canvas, which could potentially be a source of water, is in equilibrium with the RH at its surface. When this air reaches the surface of the back plate it is at a point on the diagram which is horizontally at the same level as the ‘canvas’ point, because the mixing ratio has not changed, but it is at a lower temperature. This point intercepts the curve for 72% RH.
The mixing ratio alone does not govern the moisture sorption of the materials with which the air is in contact, but in combination with temperature it determines RH. The potency of water vapour for chemical reaction and sorption into solid materials is expressed by the relative humidity. One has to switch focus between mixing ratio, as one possible cause of vapour movement in space, and vapour potential, RH, as a cause of change to artefacts.
In a confined space subjected to a sudden temperature change the exchange of vapour between solid and air is negligible. In this particular picture enclosure of 0.006 cubic metres, a five degree temperature change around 25 °C will change the water content of the painting canvas by 0.01% as it maintains a constant RH in the enclosure. Contrast this modest disturbance with the situation of the unprotected painting, exposed to a varying temperature and the constant mixing ratio of the room air, as in Fig. 6. It seems from the sorption data [7] that the painting canvas will exchange much more water, 1.8%, because its surface RH will change substantially, while the mixing ratio will not change (this is a simplified explanation; the situation is complex and should be investigated in future work), being held constant by flushing with room air. Good stability of RH is achieved by very little water exchange by the canvas within a small enclosed volume.
Evidently, a tight enclosure around the painting allows it to control its canvas surface RH with hardly any moisture exchange. It is the surface RH which matters; how it is achieved is irrelevant. The addition of an impermeable, non-absorbent back plate is beneficial to a painting on canvas.
Modelling the air flow within the enclosure
Figure 9 summarises results from computational fluid dynamic (CFD) calculations to support sensor measurements which indicated rapid air mixing within the enclosed space, except in the narrow channel between the painting canvas and the wooden cross beam.
With a temperature difference between back plate and the painting canvas of four degrees, the modelling shows vigorous convective circulation within the vertical space, with a central zone of slow mixing between the rising and falling air streams. It also shows the steeper temperature gradient close to the surfaces with a flatter gradient in the middle. This emphasises both the difficulty of knowing exactly the RH at the surfaces and the importance of realising how rapidly the RH changes as one approaches in millimetre detail a surface at a different temperature to that measured in the room.
The left column in Fig. 9 shows the situation in the clear void, with vigorous vertical air movement but with mixing horizontally across the section. Movement in and out of the plane of the section is not displayed. The horizontal movement shown on the graph below the arrows image seems to be very slow, only a hundredth of the vertical speed, but this is misleading because the speed is the net residual velocity of many molecules in the horizontal direction, while the vertical movement is of molecules streaming mostly in the same direction as a relatively uniform mob.
The centre column of Fig. 9 shows the effect of the beam. It considerably diminishes the flow on both sides, causing air to circulate mostly in two separate cells above and below the beam. The air flow is one way and sluggish in the 3 mm gap between beam and canvas, just one sixth of the unimpeded flow in the 29 mm space between canvas and back plate.
The blocking action of the beam is illustrated in a different way in the column to the right. This shows the stream line of the airflow, which can be used to approximate the possible flow paths of a single particle released within the air volume. It spends a lot of time trapped, slow moving, in a vortex just above the cross beam.
The simulation shows that the stream line of air flow is an elliptical circulation with laminar flow parallel to the two vertical surfaces. It gives no information on the rate of release and absorption of water molecules through the canvas surface.
Insulation between wall and back plate
The large amplitude of the RH variation at the back plate shown in Fig. 7 means that if the wall is very cold, there is a possibility of condensation on the inside surface of the back plate. The water can then run down and collect at the bottom of the frame, causing damage to the lower edge of the painting.
The 10 mm gap between painting and wall is therefore important because it reduces the temperature swing at the back plate. It also allows room air to warm the wall surface where air circulation is restricted by the proximity of the painting and thus inhibit condensation on the wall surface.
When the back plate was set directly against the wall, its RH exceeded 80% when the wall temperature reached 16 °C. For this reason, fitting a back protection should be combined with thermal separation from the wall, by a space or by insulation.
Protection against condensation on the interior of the back plate can be provided by putting thermal insulation outside the enclosure, between the wall and the aluminium back plate (Fig. 10) acknowledging that this would change the reflective property of the aluminum. The reflective properties, however, are only relevant when there is a gap between the wall and the back plate, which is not the case in this situation. In this case the back plate is placed directly against the wall.
The RH variation with external insulation of the back protected painting was quite moderate, because the temperature variation at the back of the frame was half of that when the back plate touched the wall. However, the point of condensation danger was shifted to the wall surface behind the insulation, which was now much colder than the exposed wall. This is a particular danger in humidified exhibition rooms in cool climates.
Many conservators use insulating back plate materials, such as multi-wall polycarbonate plates. These will behave in principle just like the combination of aluminium plate and polystyrene backing behind.
Hygroscopic back
Historically, many backboards have been moisture absorbent materials such as wood and card. To simulate these materials, a piece of washed, dense cotton canvas (632 g/m2) was stuck to the inside surface of the aluminium plate.
The RH cycle amplitude behind the painting canvas was now rather large (Fig. 11), but the RH cycle measured close to the cotton on the back plate was smaller than when the back plate was bare.
The process here is a more complicated version of that proposed for Fig. 7. In that construction, the painting canvas was the main provider of water vapour. When the back plate also is hygroscopic it will compete for influence over the humidity within the enclosure.
As the temperature descends to a minimum, the RH at the back plate will tend to rise, but the back plate is hygroscopic and will absorb water vapour to move its surface RH towards equilibrium with its moisture content. This is not always the case, but here the effect of RH is dominant. The surface air of reduced mixing ratio will migrate to the painting canvas, which is about three degrees warmer. The simplified explanation is that during the journey through the temperature gradient to a warmer surface its mixing ratio will not change, so its RH will decrease. At the painting canvas surface, vapour will be released into the unexpectedly dry air, in an attempt to keep the surface RH in equilibrium with the painting canvas moisture content. Its moisture content will therefore decrease.
The two hygroscopic surfaces were competing as humidity buffers, which is why the observed RH change at the back plate was less than it was in the absence of the buffer surface. The painting canvas RH span was however of greater amplitude than when there was no buffer at the back.
The mixing ratios at canvas and back plate were changing in the same direction but were not identical, indicating that the air mixing is not entirely effective in ensuring a homogenous atmosphere at this rapid temperature cycle and with energetic exchange of water molecules between air and materials. Nevertheless, the mixing ratios were moving in phase, though the RH values at the two hygroscopic surfaces were in opposite phase.
This graph is an example of an important and widespread phenomenon—the destabilisation of the RH around an artefact when its buffer ‘protector’ is transiently at a different temperature. Thus, it can be a risky decision to put a water absorbent back plate on a painting.
Buffering against air leakage
There is much discussion among conservators about using hygroscopic back protection. Concern over leakage into the painting enclosure of a hostile room RH has dominated the discussion [9, 14]. However, humidity buffers play two distinct, separate roles: they serve to maintain a constant RH during temperature change, as shown by the temperature insensitivity of the cellulose isotherms, and they replace vapour leaking between the enclosure and the room air, even when there is no temperature fluctuation at all. This is because of the huge quantity of exchangeable water in the buffer compared with the tiny amount of water in the small air space. Thomson [10] and many subsequent authors have described how to buffer against leakage, with calculations to predict how much buffer is needed to give a desired time delay for the move towards equilibrium with the room climate.
If water vapour exchange with the room is an important consideration, due to a persistently unsuitable environment, one can add extra humidity buffer, so long as it is always at the same temperature as the painting. If the buffer material is close to the painting it is more likely to have close to the same temperature as the painting. If it is mounted on the back plate as it is sometimes done, it is more likely to have a temperature closer to that of the wall.
Putting an extra humidity buffer close to the canvas, and thus at a similar temperature, will not cause competition between the absorbent surfaces and it will slow down the RH change caused by leakage. For this to succeed, there must be thermal insulation to shield the buffer from the back plate temperature.
In order to show this, the same canvas that was used as the back buffer was instead fastened to the front of a 20 mm thick slab of expanded polystyrene insulation which fitted between the back plate and the painting canvas. It was spaced 3 mm behind the painting canvas to avoid spoiling the taut contours of the painting. The twenty four hour cycle is shown in Fig. 12.
The RH stability of the painting canvas was now very good, as was the RH between canvas and cross beam.
This combination of sealed back, impermeable insulation and a buffer layer close in temperature to the painting canvas, gave stability to both temperature and relative humidity at the back surface of the painting, as well as providing a water reserve to compensate for leakage of room air into the enclosure.
The disadvantage of this arrangement is a large temperature difference between buffer and back plate and a correspondingly high risk of condensation on the inside surface of the back plate.
The mixing ratio in the narrow unbuffered void between the insulation and the back plate was varying in the same way as in the buffered space behind the painting canvas. This shows good air mixing throughout the space, even with reasonably close fitting insulation panels within the frame.
In winter, the wall may always be cooler than the room temperature and sometimes below the dew point of the air just behind the painting. In instances of very large T gradients and very slow air exchange rates, water can condense onto the back plate from vapour provided by the buffer at the warmer room temperature. However we consider this situation to be rare. Thus, it follows that insulation is not a universal good, just as buffer materials have to be positioned with understanding of the microclimatic implications of their presence.
This construction is best reserved for paintings with a relatively benign wall temperature but a seasonally variable room RH, where extra buffering will defend against RH change caused by air exchange through the painting, or around its frame.
Condensation behind paintings has often been observed on the wall behind paintings both with and without back boards. The painting provides enough thermal insulation to keep the wall behind it cooler than the wall directly exposed to the room air.
Conservators, aware of the risk of condensation on the cool inner surface of the back board, have tried to avert it by seemingly intuitive solutions, such as cutting the corners of the back board, or by drilling holes through it. This won’t help if the room air has a dew point above the back plate temperature.
The effect of glazing the painting
Nowadays many paintings have glass in front. This extra insulation layer will result in a greater temperature variation at the painting canvas as the wall temperature varies, but a smaller temperature difference between front and back of the enclosure. This results in a smaller RH cycle amplitude at the back plate and thus a diminished risk of condensation, however, glazing reduces temperature variation at the painting when the room air temperature changes rapidly. Figure 13 can be compared with the unglazed equivalent, Fig. 12.
The glass reduced leakage of air through the paint surface, so this construction is well suited to paintings which hang in a room with unsuitable RH.