I need jars that look full of an opaque liquid, and resin is too expensive — what else sets solid?

The other cheap material worth knowing is agar, which sets a firm clear gel from hot water and is genuinely edible, non-toxic and very inexpensive.

It sets like a firm jelly and, unlike gelatine, it does not melt back at warm room temperature — which matters under stage lights. Colour it with food colouring, or make it opaque with a little titanium dioxide or even milk powder.

The drawback is that it is water-based, so it will eventually dry out and shrink unless sealed. For a run of a few weeks that is fine; for a permanent display it is not.

A third option nobody expects: plaster or joint compound, thinned to a pourable consistency and tinted. Cheap, opaque, sets hard, heavy. Good where weight does not matter and excellent for anything meant to look like a thick paste.

Candle wax is the obvious answer for your use case and it is cheap by the kilogram.

It melts easily, takes pigment or ordinary candle dye, sets opaque and solid, and pours around an insert without any trouble. Paraffin is the cheapest; soy wax is softer and slightly less prone to shrinking away from the glass.

Two techniques worth knowing:

• Pour warm, not hot. Very hot wax poured into cold glass can crack it, and it shrinks more as it cools, leaving a dip in the middle. Warming the jars first and pouring in two stages fixes both.
• You may not need to fill it. For something viewed from a distance, swirling a thin layer of wax around the inside of the jar so it coats the glass reads exactly the same as a full jar and uses a tenth of the material. This is the standard trick and it also keeps the weight down enormously, which matters if they are being carried.

That second point is probably the whole answer for a stage prop.

For the tube down the centre, whichever material you choose, the practical concern is shrinkage pulling away from the insert.

Wax shrinks noticeably as it cools and will often leave a visible gap around anything rigid in the middle. Two fixes: pour in layers so each is thin, or top up the dip once it has set and cooled.

Also worth roughening the insert slightly where it will be embedded. A smooth tube gives nothing to grip and can rotate or slide; a scuffed one holds far better in any of these materials.

One more option if the jars will never be opened and only viewed: paint the inside of the glass.

Pour in a little acrylic paint, swirl to coat, drain the excess and let it dry. From more than a metre away that is indistinguishable from a full jar, it costs almost nothing, it adds no weight, and it dries in an hour rather than overnight.

It fails on close inspection and if anything opens the lid, so it depends entirely on how the props are used.