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A cork ring is a simple cylindrical ring made of cork or a similar material, often used in labs to support round-bottom flasks and similar glassware or items that cannot be safely placed on a flat surface due to their round shape.
Cork rings have a cylindrical ring shape, with the inner corner being cut or rounded to allow holding spherical objects. As such, the cross section of the cork ring is that of a right trapezoid-like pentagon, which gives the spherical flask a better stability than if a simple circular disk with rectangular section was used. See this image for a better idea. Cork rings, as their name suggests, are made of cork, though in order to keep price low, molded agglomerated cork is more often used than a single block of cork. Cork is commonly used as it is soft and wood is not attacked by organic solvents, though strong acids and bases will slowly damage it. It can also resist temperatures up until 200 °C, and therefore it can be dried in a lab oven or autoclave.
Plastic round-bottom flask supports, the plastic version of the rings, have a different shape than the cork ones, they have a vague ashtray-like appearance, a disk with concave recess and beveled edge cone cavity. They are more expensive than the classical cork rings and in general they're not worth the price.
Cork rings can be purchased from lab suppliers and online.
DIY cork ring
If you have access to thick cork boards, you can make a very simple cork ring by cutting a circular ring, then trim the inner edge to the desired size. Be warned however, that cork is somewhat tricky to properly cut, as it tends to crumble.
By drilling holes through a series of carefully cut cork stoppers, you can make a very cheap and effective cork ring stand. Here's a good example of such cork ring.
Used cardboard rings from wide rolls of scotch tape are also good replacements for cork ring stands.
Subwoofer speakers can be used as good supports for holding round-bottom-flasks.
For plastic supports, you can use PVC flush bushings as an excellent (and cheaper) RBF support.
As a last resort, you can also use various cup-shaped items, such as jars, plastic cups, cup holders, ball stands, beakers, cutout boxes, deep circular ashtrays, egg cups, to hold RBFs, if they are the right size.