This question comes from Stella, who was at my West Dean course in September:
What exactly is the difference between transparent and opaque paints and how does it affect my paintings?
The answer is that transparent paints let the light through to the underlying paper while the opaque paints reflect the light, effectively blocking it and stopping it from reaching the paper. The effect is that transparent paints have a more glowing, three-dimensional finish thanks to the resulting layering, while the opaque paints have a flatter, matt appearance.
Some media such as gouache, chalks and pastels will always be opaque, because the medium itself is opaque.
Other media such as watercolours, oils and acrylics are transparent, so the transparency/opacity of the paint will depend on another factor, which is the pigment used in each colour.
When it comes to transparency, there are 4 categories of pigments:
- Transparent, which let all the light through
- Semi-transparent, which let most of the light through but reflect a small part
- Semi-opaque, which reflect most of the light but let a small amount through
- Opaque, which reflect all the light and let nothing through
Here are some examples of what this means in practice.
Case A – A single wash of transparent blue over white paper
The light goes through the paint, bounces off the paper and comes back through the layer of paint. The eye sees the blue colour, with a bright finish thanks to the brightness of the white paper underneath.
Case B – A single wash of opaque red covers the paper
The light bounces off the paint without allowing it to travel through to the white paper. The eye sees the red colour, with a flatter finish because of the lack of depth.
Case C – Three layers of transparent paint over white paper
The light travels through all the layers, bounces off the white paper and comes back through all the layers. The eye sees all the colours at once, with a lot of depth created by the layering.
Case D – One opaque wash of green between two layers of transparent colours
The light goes through the yellow layer to the green opaque layer but cannot go any further. The eye will see the green through the yellow, giving a yellowy green colour with some depth, but the grey layer and the white paper will disappear entirely, limiting that depth and annihilating the white paper-given glow.
Now it’s up to you to play with all the above, combining your pigments to reach your desired effects. Remember that this will only work in a transparent medium. If the medium is opaque, only the top layer will be visible no matter what pigments are used.
Examples of transparent colours: all the Quinacridones and Phthalo colours, Permanent Rose, Gamboge and Indian Yellows, Perylenes and most blacks.
Examples of opaque colours: all the Cadmiums, Cerulean Blue, Naples Yellow and all whites.
Lemon Yellow and Sap Green are the troublemakers. Depending on the brand, some are transparent and some are opaque. I will write about them in the Pigment Spotlight section in different posts.
I made a video about transparency/opacity and why transparent pigments can turn into opaque paints. To see the video on my free YouTube channel, please click here.
There will also be follow-up posts about Optical Mixing and Harmonic Shadows, which are two techniques deriving directly from the transparency/opaque dichotomy.
Keep the questions coming; I will answer them, whether directly or with a blog post or video.