Texture medium (or gel or paste) is, as the name suggests, is a material that you mix with paint to change its consistency, in particular, to add surface texture to a painting. It's stiffer than paint straight from the tube, so will hold a form or shape more readily. It's also cheaper than paint, so adding a texture medium is an economical way to build up thick layers of impasto. You can mix it with a color, or put a layer of the gel down and paint over it.
A tub of texture gel has the consistency of cooking lard: when you scoop out a lump with a palette knife, the medium holds its shape. It doesn't drip or droop but stays put. You can create peaks and grooves with a palette knife, brush marks with a coarse-haired brush, press patterns into it, use it as glue to add collage items. It's extremely versatile!
Texture mediums come in white and clear, so if you have a preference, pay attention to the label.
By the way, the word medium can also mean the type of paint, for instance, acrylic or watercolor. (You can usually judge what is meant by the context in which the word is used.)
Different brands of acrylic texture medium are formulated differently and variously labeled as pastes, gels, and mediums. They all do the same job of adding texture, but some will be glossy when dry and others matte; some will dry completely transparent, others will be slightly opaque or stay white. The medium may also act as a drying retarder to give you more time to work with it.
How do you know what it'll be like? Read the label on the container, which should give you this information. If it doesn't, see if there's an information sheet available from the manufacturer, or test it before you use it on a canvas. Be aware that there are differences so that if a new tub of texture medium doesn't act exactly as you expected, don't panic that you're doing something wrong.
Whether it's glossy or matte isn't absolutely critical since you can change something from glossy to matte (or matte to glossy) when you varnish a painting relatively easily. Just add a varnish to give the finish you want.
The opacity of the medium is important if you're mixing it with color as it will have an impact on what the color looks like when it's dry. Don't get caught out by a medium making your colors appear lighter than you'd intended. It's something you learn from a bit of trial and error until you get a feel for it. Remember, you can always paint over the texture medium, so if something isn't the right color when it's dried, it's not a disaster.
How long texture paste takes to dry depends on how thickly you've laid it down. Very thick layers will be dry to the touch in a few minutes but not dry all the way through, so if you apply lots of pressure it may flatten. Again, a little experimentation will soon teach you what to expect.
Different types of texture medium will give you different results. Texture medium is rather like toothpaste: a paste has a fairly flat profile out of the jar, while a gel has softer, rounder, more liquid edges. Some mediums dry to an opaque white and some are transparent. Read the label carefully, and experiment until you're satisfied you understand what to expect, so you don't get an unwanted surprise in an important painting.
You can use any type of tool to apply texture paste onto a canvas or sheet of paper. Different tools will produce different textures. A coarse or stiff-haired brush will create more marks in the paint than a soft brush. A painting knife is useful because it's easy to get the paste out of the tub, it's easy to spread out and to scratch patterns into the paste.
Spreading texture paste with a painting knife is akin to buttering a slice of bread with a springy knife. The action is the same, and if you don't like what you've done you can scrape it all up and start again.
In some brands, you can use texture paste straight from the container, without mixing any paint into it. Some brands look very white when you apply them but won't after its dried. You can also apply the paste right on top of some dried paint—as with all acrylic mediums, you can use it at any stage in a painting's development.
If you press a painting knife into the texture medium and then lift it off, the result is a ridged texture. It's very different from the smooth result you get when you spread the paste sideways. Be warned that texture medium a bit unpredictable, since the way it behaves depends on how much medium you are applying, how dry it is, and the size/shape of your painting knife.
Texture medium is great for adding textures in skies, seashores, grasses, rusted surfaces, windswept hair. Don't focus on getting a perfect end result when you first use texture paste, but play around and experiment to see what happens.
Once the texture medium has dried, you can paint over it without disturbing it. You can press a knife into the texture paste, let it dry, then apply paint over it with a brush or by spattering across it.
By running a brush over the surface lightly, the paint hits only the top ridges of the texture. By pressing a brush firmly against the surface, it'll go in between the ridges too. Another option is to use very fluid paint, which will flow off the ridges and puddle between them.
While it's still wet, it's easy to fix mistakes in the texture medium level or to remove it completely. Simply scrape it off with a painting knife or a cloth. How much time you've got before it dries depends on what brand you're using and how warm it is in your studio. A draft of air across your painting will also increase drying time. Again, it's something you'll get a feeling for through experience.
If in doubt, remove the medium while it's still wet and then think about what you're doing with it. When it's dry, however, you'll have to take some sandpaper to smooth down the surface.