Simply put, there are only two reasons a properly stretcher artists canvas goes floppy:
1. An older canvas can deteriorate to the extent that the yarn elongates over time and as a result of cyclic stressing – for the purpose of this article, we are not particularly concerned with this type of issue.
2. Moisture. (or more specifically, the lack of it)
A typical scenario.
You the artist order or simply walk in to a shop and purchase a pre-stretched canvas. You take it home or to your studio, unwrap it and start to work on it. At this stage the canvas is nice and tight.
However, over the course of a week or so you might notice the canvas is getting a bit loose or floppy, maybe the canvas fabric is making contact with the bracing.
An alternate scenario that occurs just as frequently.
You have stretcher frames available to you along with a nice roll of canvas. You stretch up your own canvas, apply your talent and produce a painting that sells quite rapidly to an avid art collector.
A few weeks after the new owner of your painting takes it home and hangs it on the living room wall, you get a phone call complaining that the nice but expensively priced painting has gone all floppy. Understandably, the client is not too happy about it and wants the situation resolved.
What is going on?
The cause of the issue is quite simple. All cellulosic fibers like cotton, linen, hemp etc are hydroscopic, that is they have a strong affinity for moisture and absorbs it readily. So when a piece of canvas is in the presence of moisture, it will absorb some of that moisture.
This where the relative environment that the canvas is in has an effect. We all know that the amount of moisture contained in the air around is constantly changing. The amount of moisture in the air is expressed as Relative Humidity (Rh) which is the amount of water vapor present in the air as a percentage of the greatest amount of moisture the air can possible hold at a given temperature (its saturation point). For example: at sea level if the air temperature is 20 degrees C, the maximum amount of moisture it can hold is 14.68 grams per Kg of air (0.83 M3 of air roughly, there are a few variables). So if the relative humidity is 60% then the air is holding 8.81 grams of water (14.68 X 60%).
In our workshop we constantly monitor the humidity of the air as it tells us what is soon to happen with the weather. A very dry day may have humidity that starts at 65% when we get to work but may go as low as 47% Rh by the end of the day, and when it is wet may be 99% Rh (or equipment only measures to 99% even though we know the air is fully saturated).
From our example above: The artist who stretches their own canvas in their studio, has when they have completed stretching the canvas, made a nice job of it, it is nice and tight and sounds like a drum when you flick the canvas surface.
The canvas cloth, and stretcher bars have typically been sitting around in the studio for a little while and will have reached a moisture equilibrium within that environment relative to the temperature and moisture content of the air. The canvas itself may have a moisture content of say 13% if it has been in an environment that may be relatively consistant 75% Rh. (these are arbitrary numbers as many factors affect the moisture content of cloth). Essentially the more moisture that is in the air, the more that the canvas cloth will absorb. Conversly when the moisture content of the air is low, the cloth will release moisture.
What has the moisture content of the cloth got to do with it?
The behaviour of the yarn that is woven to make the canvas is the more important consideration (there are other elements that are involved). As a natural fiber (cellulosic or plant cased fiber) it will absorb moisture from the air when the humidity is higher. When it does this it becomes fatter in cross section, and as a result of that becomes shorter in length, conversely when the yarn looses moisture it becomes skinnier and the length becomes longer. (see for example: GIANFRANCO POCOBENE, & IAN HODKINSON article Therefore the above mentioned artist stretched canvas will remain nice and tight while it remains in its original environment (assuming that the relative humidity is reasonably constant).
However, once the painting moves to a new environment – say the clients lounge wall there may be a significant change of ambient relative humidity. For example, the new environment may have a heat pump going for a good portion of the day, let us imagine that the painting is now in a environment that has a reasonably constant relative humidity of say 55%. Over a period of a few weeks, the moisture content of the canvas will decrease, when the canvas becomes drier, the yarn making up the woven canvas will become longer thus there will be a loss of tension - A floppy canvas.
The process of gaining and loosing moisture is a natural and completely normal characteristic of plant based fibers – THE CANVAS IS NOT FAULTY and it will constantly increasing or decreasing tension throughout the day as the ambient humidity changes.
What can be done about it?
1. Try to ensure the canvas cloth has a reasonably low moisture content before stretching it.
2. Do not stretch canvas once the relative humidity exceeds 75%.
3. Utilise high polyester content canvas.
4. If necessary, tighten up the canvas.
Pre-conditioning the canvas.
Over the years that we have been producing artists canvas there has always been the occasional complaint from customers about canvases that have gone floppy. Indeed, 2023 was a specifically poor year for making canvases, it was constantly wet (remember the flooding in Auckland, Hawkes Bay etc.) and the incidences of canvases going floppy increased as a consequence of a lot of moisture in the air prior to the canvas being stretched. We eventually put our theory into practice and built a 'pre-conditioning chamber’ which has a constant relative humidity of 25%. When an order is placed with us, we cut the canvas for the order, place it in the pre-conditioning chamber for a minimum of 24 hours and only take it out immediately before we need to stretch it. The typical moisture content of canvas before it goes into pre-conditioning is about 13% once it has been dehydrated the moisture content is less than 6% (it should be noted that moisture testing meters cannot give a reading that is less than 6% . I understand it has something to do with the electrical resistance of low moisture content cellulosic materials).
Keep an eye on the humidity.
We have several moisture gauges in our factory, and we are constantly aware of the ambient relative humidity and how it is trending. As a general rule, we do not stretch canvas when the humidity is above 75% - even though the canvas has been pre-conditioned. Our thinking behind this is that we have noticed that canvas absorbs moisture very quickily (but releases it more slowly) and we are unsure how much will be absorbed in the 10 – 20 minutes it takes to stretch a canvas. An observant customer will note that on all of our Professional Grade canvases there is a stamp on the back of one of the longer stretcher bars that exhibits the month and year of manufacture, and also to the right is the humidity at the time it was stretched.
High polyester content canvas
Our customers are aware that we strongly encourage the use of high polyester canvases as opposed to 100% linen or cotton. There are several reasons for this: Polyester is extremely strong – i.e. it is less likely to be ripped, it is highly durable, most importantly, polyester does not absorb moisture like cellulosic fiber does. Our RT12 polyester/cotton blend canvas has a 52% polyester content and as a result has a lower level of moisture related movement.
Tighten up the Floppy canvas
If you have nothing to do to keep yourself entertained have a bit of a search through the US Patent data base – it is full of wonderful ideas for devices to tighten floppy/loose canvases – none of them work very well, and in almost all instance put unbalanced stresses onto the canvas fabric. Wedges or keys are supplied by some canvases producers – we actively advocate against their use as they do more harm than good. We have repaired a large amount of quality art work that have been damaged through the use of wedges. They may damage the canvas and may cause a rapid deteriation of the paint film - these devices should be avoided.
In our opinion, the only proper way to tighten a canvas is to pop some staples, pull the canvas up tight and re-staple. The process is quick (usually only 10-15 minutes - most of that time is pulling staples out). We do this process for free for any of the canvases we produce but do charge if the canvas has been produced by someone else.
