What is 'medium' and why is it different to water?

[Valkyrion]

As the title suggests - what's the difference between thinning with water and mixing with medium - and how do I know if I should do it?

[Closet Skeleton]

Medium is colourless paint. Its what paint is made of and what makes different kinds of paint different. If you're using oil paints, the medium is an oil, enamel paint its liquid enamel, water colours it is literally just water and with acrylic paint its a kind of plastic.

 

If you had some pure pigment like weathering powder or some water colours you could mix it into some lahmian medium to make a homemade acylic paint. You could even just grind up some mud and mix it with acrylic medium to make a brown paint or smash up a piece of charcoal to make a black one, this is what everyone used to do before commercial paint became available in the 1800s.

 

If you dilute an acrylic paint with water you get a thin, runny paint. . If you dilute it with medium you get an acrylic paint with less pigment in it so it won't be any runnier than the undiluted paint, Medium is used to create glazes which lets you apply thin layers of paint where the layers of colour are easier to see through. Glazes let you tint colours on top of each other while water thinned paints just flow differently.

 

Add too much water and you'll turn your acrylic paints into water colours and they'll be useless on non-absorbant surfaces except for leaving dirty looking streaks, add too much medium and you'll be painting a slightly tinted layer of plastic on top of your model.

 

Lahmian medium is the normal acrylic you get in regular citadel paints and probably the same as non-citadel branded acrylic medium. Contrast medium is the proprietary stuff contrast paints are made out of and can be used to turn any paint into a contrast paint, which you'll have to experiment with to find out what that means unless you're following a guide where someone tells you to do it.

Edited February 27, 2023 by Closet Skeleton

[Firedrake Cordova]

It basically as @Closet Skeleton says. If you add water, you're adding a solvent - if you add too much, you inhibit the ability of the paint to bind, so you get weak adhesion and other issues (e.g. altering the properties/behaviour of the paint). The medium is essentially acrylic binder, so you don't end up with this problem.

 

Vince Venturella had a very good video on the subject: