Sorry for getting your name wrong, Mikey!
The gum arabic in watercolour is the ‘medium’; it’s the thing that turns pigment into paint. It’s basically a ‘glue’. Without it you would be able to brush the pigment off a surface once dry.
When folk talk about using it to get different textural effects, it’s about the quantity of pigment to gum. For example, with more gum to pigment, you get watercolour (a transparent effect) with more pigment to gum you make gouache (opaque matt paint) same ingredients, different ratios, different effects.
Gum arabic can also be used to ‘glaze’ a dry work to offer areas that are a bit glossy, or used to ‘push’ paint around when working ‘wet in wet’ with watercolour.
But its primary function in paint is that of a medium (the ‘glue’ that makes pigment into paint). Some other examples of mediums include dissolved acrylic (in acrylic paints) and boiled linseed oil (in oil paint).
The reason I suggest gum arabic is because it’s not just used as the medium for watercolour, but also for the traditional enameling methods of painting stained glass (like in church windows) whereby frit powder (finely crushed glass) is mixed with gum arabic and water (and sometimes additional pigment) to form a paint that doesn’t bead up on the surface of glass, but instead spreads thinly and evenly (this is then often selectively scratched off to form the image before firing the frit onto the glass in a kiln).
The reason gum arabic is best for our purposes here in engraving tiles is because:
- The tiles are glazed (with glass) as the outer surface, that’s what we’re painting upon and many other paint mediums bead up on glass.
- It burns off cleanly (the firing in the kiln).
- It dries quickly, in my experience a couple of minutes, but I live in the subtropics!
- It’s removed with simple water
- There’s no waste, you can just let the paint dry out and then later reconstitute it with water.
- We know exactly what’s in it, we don’t have to risk the use of products (like PVA glue or commercial paint mediums) where the ingredients aren’t clear, and could potentially be dangerous when burned/inhaled (I’m most concerned with preservatives often added to such things).
This gentleman is mixing paint for painting glass with a brush. Seems like he’s been doing this for a long long time, so probably knows better than I! https://www.youtube.com/watch?v=T_Pfs-_Xa84
Hopefully he will be better at explaining quantities than I was. Sorry I couldn’t be more exact, it varies based on humidity and starting materials and such, and it really is one of those things that you need to just reach the right viscosity by ‘feel’ hence my comparisons to vegetable oil.
We may also want to experiment with paint mulling (grinding the paint smooth between glass) Mulling Paint: A beginner-ish guide — Scribal Work Shop for folk that are getting lumping issues or using sprayers. Glass mullers are expensive but charity stores often offer great alternatives, you just want something glass with a somewhat solid and very flat bottom, that doesn’t feel too clunky to hold. Candlesticks, vases, drinking glasses, paper weights, and old glass bottle stoppers are some options. 
Also be sure to clean your tile before painting. Especially make sure it’s free of oils or detergents. If you are getting little dots that the paint is shrinking away from, it’s becasue your tile isn’t clean.
I’ll try and get a proper guide together with the results and such!
