This post is Part II, so if you have not yet, read Part I first! There I discuss oil mediums, solvents, and the mediums that I DON'T use or recommend.
I demonstrate this oil painting technique in my online course Glazing and Scumbling Roses.
Solid, Particle-Based "Mediums"
Technically these are additives, not mediums, but exploring their properties leads me to the medium I currently use and recommend, so it's useful to describe them here:
Fumed Silica (heat-processed sand) Fumed silica is an airy, feathery, powder dust made from granite sand. The particles have a huge surface area and low mass, so when it's mixed with paint or oil it takes on "thixotropic" properties. This means when you mix it or apply pressure it behaves like a soft flowing liquid, but when you don't touch it, it holds its shape like a gel. I've used it by mixing it directly into my oil paint with a palette knife right on the easel, and along with a little oil, it's a great way to extend the paint while keeping it transparent to make glassy glazes. The proper way to mix it is with a muller, but I've enjoyed the paste I can get just with the knife. However, there is an easier way to use it which I'll cover below. To remember its properties, keep in mind: Silica is transparent! It's sand, and that's what glass is made of, so use fumed silica for transparent glazes.
Chalk (ground calcium) Chalk dust is the same stuff children for generations have clapped out of blackboard erasers, and it's just as messy! I've used it by mixing it directly into my paint, and it makes the paint "chunky", dry, and easy to pile up into craggy impastos. I feel certain it's likely the main ingredient in any true "secret medium of the Old Masters". Like fumed silica, you can also mix it more properly and thoroughly with a muller. To remember its properties, keep in mind: Chalk is OPAQUE. That's why we use it to write on chalkboards! So use chalk in your whites and light-paint mixtures, to build up chunky impastos, push 3D shapes forward into the light, and literally catch the light with bright peaks of texture.
My Preferred Mediums And now is where we get to the good part: The mediums I most highly recommend! It's actually very simple: They are just the dry solids I listed above, but conveniently mulled and tubed with linseed oil. Natural Pigments makes these mediums. They are very simple and cheap, and you could also make them easily at home, but Natural Pigments has done the work for me, and I prefer to just open the tubes and start painting.
Tubed FUMED SILICA Medium for Glazes: Oleogel medium by Natural Pigments I use Oleogel by mixing it into my paint right on the palette with my palette knife, and I also use it to oil out my working area of my painting with a makeup wedge (left image). Because it has solid particles mixed into the linseed oil, it's much more stable than using linseed oil alone, and it makes a really beautiful transparent glaze. Out of the tube it looks like a clear gel, you can see it in the middle of my palette in the middle image. (Natural pigments also makes fast-drying version called OleoRESgel, which I believe has alkyd added, so that might be a a great replacement for Liquin or Galkyd. And Natural Pigments lists all their ingredients on their labels and fact sheets.)
Tubed CHALK Mediums for Impastos:
Impasto putty medium by Natural Pigments Impasto medium by Natural Pigments Velazquez medium by Natural Pigments
These are 3 different proportions of the same ingredients: Chalk dust mixed with linseed oil. Impasto Putty has the most chalk, and it's really thick, almost like a dry peanut butter, and it forms short peaks when you "lift off" the palette knife. Impasto Medium is in the middle, the consistency is more like room-temperature butter, with medium peaks. Velazquez Medium is my favorite, it's a bit less chalk and more oil, and so you get long ropey peaks, and the consistency is more like a stretchy sour cream. All of them allow you to pile up your paint into thick impastos that look like old-master paint effects to me. These chalk-based mediums also allow you to stretch out the paint very thin, so I use it for my lead white under painting layer as well, where I am using the opacity and transparency of lead white paint to create a range of values over the brown raw umber underpainting....
I'll be sharing more about creating a lead white under painting when I release my new painting video course later this year: Glazing and Scumbling a Still Life with ROSES. My online video course Glazing and Scumbling is a great introduction to the techniques I'll be sharing in the more advanced Roses course. Sign up for my mailing list to be notified as soon as the new online video course is released!
I teach Alla Prima, Direct, and Indirect oil painting here online, offered as fully pre-recorded video courses you can watch any time, including my Intro to Oil Painting which is perfect for beginners. I also offer mentorship programs if you want help and support while working through the courses.
Your Questions about Mediums Answered: These are more questions people asked me about mediums on social media that I couldn't fit gracefully into the post:
Do you use different mediums for plein air vs studio work? Working en plein air or even alla prima in the studio, I find I'm racing against time so I use just one medium, a simple mixture of 50/50 linseed and odorless mineral spirits.
Do you use different mediums for different grounds or supports, like chalk primed panel, or oil primed linen? No, but I use different grounds for different kinds of paintings: I use a chalk gesso ground on a smooth hard panel for Indirect painting, and I love RayMar's oil primed linen panels for my direct and alla prima paintings. You can see my materials lists with links to my recommended products.
Why do some mediums make the paint remain tacky, and should you paint on a tacky layer? If the previous paint layer is tacky you are probably using too much oil - or maybe other ingredients that are not drying fast enough. A good way to gauge if your previous paint layer is dry enough to paint over is the "thumbnail test" - If you can make an indentation in your paint film with a firm press of your thumbnail, you should wait for it to dry more before painting on it.
Can you mix different mediums together? As long as they are simple mediums, probably yes, but you should be familiar with every ingredient in your paint. I like to keep transparent mediums and impasto mediums separate, since I use them for different purposes in different parts of the painting.
Are some mediums more harmful than others? Solvents (paint thinner or mineral spirits or turpentine) are far worse for your health than any other ingredient used for painting, so do everything you can to limit your exposure to fumes.
Is Galkyd fat or lean? Fast-drying or slow-drying is far more important principle than fat or lean. Alkyd mediums are fast-drying, so use it only in the lowest layers of a painting, or for wet-in-wet methods, as in alla prima or plein air painting.