Obviously the string has a little stretch, but even so, the tension in the string is proportional to 1/sin(angle), which for very small angles makes very large amounts of tension, which will eventually bend or warp the frame.
Instead, you should deliberately leave a reasonable amount of slack, and the tension in the string will then stay under 3x the frames weight, which it will be able to handle forever.
I've seen it in a FB math group with the text: Applications of knot theory :)
The taut line hitch makes it easy to adjust the height without moving the nail and the cotton cord helps keep the taut line hitch from slipping (vs says monofilament).
The taughtline hitch is a much better knot for this.
I never could master the trucker's hitch though :-)
L over R, L over R would make a granny knot.
But maybe in the UK both are called a granny knot?
Interestingly, when a granny knot is collapsed (the cord on one side gets pulled taut), it becomes two half hitches. When a square knot is collapsed, it becomes a larks head or cow hitch knot. In the balanced state, the granny is a weaker knot and the square is stronger. In the collapsed state (then cleaned up a little bit), two half hitches is a stronger knot than a cow hitch.
Same in the UK.
Also, it's a taut-line hitch:
taut, meaning tight, not taught, the past tense of teach.
It's essentially a quick release Prussik tied with the working end.
You can use a toggle, or slip a second bight through the first to prevent accidental untying.
Big benefit is that it is faster to tie in longer lengths of cord and it's faster to undo, being quick release.
For any heavy or large art I tend to use a french cleat(s):
Get some 2 mm Dyneema rope, which has an average breaking strength of 300 kg, run it through some D-rings and tie it with an Alpine Butterfly. You will be able to do pullups off that artwork if you want.
But when it comes to UV and chemical resistance, I don't see how one would get any degradation in a museum setting. Dyneema is used in heavy industrial applications like mooring lines and tugboats for very large ships with lifespans over ten years. Unless you're hanging up something that you're not taking down for a century, it shouldn't be a worry.
To add to this; this is the vast majority of art.
For better or for worse steel is often used instead of synthetic rope for picture hanging.
Oil rigs are towed to location using steel cables. Steel wire rope is substantially stronger for its weight and for it size but generally harder to work with than plastic things of similar capacity. Both materials can hang pictures just fine. It's not exactly a dynamic load. People will argue over anything.
That is not true. The tensile strength of kevlar and dynema is higher than that of steel, while they have a density that is several times lower.
Platform anchoring cables are often a rather complex combination of synthetic cables and weights to obtain neutral buoyancy.
Steel cable will slice people in half and send wrenches through car windows and to stab the driver when it breaks.
Why I won’t drive over the cable car slot. Slice cars in half, that.
First, you can tie knots with steel wire, too.
Second, I would be much more comfortable hanging artwork on a thin twine than on steel wire. Steel wire tends to be sharp and scratch and punch holes in anything it gets into contact with. Rope that will be durable and can withstand the stress of hanging 30kg artwork is still very thin. Just google it and be surprised. (https://www.ingersollrand.com/pt-la/lifting-equipment-materi...)
If you are interested here is a non-affilate link:
We always just used wire.
A truckers hitch is as simple as it gets:
1. Tie a loop in the middle of the cord (I like the alpine butterfly but any loop, eg a figure 8 is fine)
2. Put the cord through your anchor point and the newly made loop.
3. Pull it tight and tie it off with a half-hitch.
You now have a 2:1 mechanical advantage, meaning you can tighten a load twice as much as you normally can. It also won’t loosen itself, and you only have to untie one knot to remove the entire system.