Another important aspect of acrylic paints is their coverage in terms of opacity and transparency, i.e. their ability to completely or partially cover or hide lower layers of paint or substrates.
Opacity can range from very strong (opaque) to very low (transparent/translucent). The term "opacity" is used to describe the ability of the paint (the pigment it contains) to completely cover or conceal the substrate. Similarly, the term "transparency" describes the ability of the paint to let light through and reveal the underlying content.
Why do acrylic paints have different degrees of opacity?
The opacity degree of acrylic paints depends on different factors such as the refractive indices of the pigments mixed in the acrylic binder, the granularity of the pigments and its concentration in the binder.
When a beam of light hits the surface of a body (in our case acrylic colour), part of it is repelled (reflection) and part can penetrate inside it (refraction). In other words, the light passes through the paint, arrives on the canvas and is partially reflected and absorbed by the pigments. Pigments normally have a refractive index > 1.8. The higher the index, the higher the degree of opacity of the paint.
The covering power is directly proportional to the refractive index and is related to the size of the particles. When these do not reach the wavelength of their own coloured light, the covering power suddenly drops, reaching complete transparency. Another influencing factor is the shape of the particles; all other characteristics being equal, the spherical shape gives the maximum covering power.
The opacity or transparency of an acrylic paint depends on the difference between the refractive index of the pigment and the refractive index of the binder in which the pigment is immersed. The greater the difference between the two, the greater the opacity of the paint. If the difference between the two indices is zero, then the paint will be transparent.
How to get information about opacity and transparency of acrylic paints
The information if an acrylic paint or a pigment is transparent, semi-transparent, semi-opaque or opaque it is usually provided on the label of each colour through a square which can be
full-coloured (opaque),
half-coloured (semi-opaque)
empty but with a crossing line (semi-transparent)
empty (transparent).
How to test the opacity of a colour
If there isn't any indication , you can do a simple test:
Draw or paint a black line and wait for it to dry.
Cover the black line with the colours you want to test.
After drying, check if the colours cover the black line underneath: the more the black disappears, the more opaque the colours are and vice versa, the more you see the black, the more transparent the colour is.
Knowing the opacity or transparency degree of a colour can be very important, because if the colours are transparent, they can "disappear" into the (opaque) colours underneath.
The impact of transparency and opacity: an overview
What medium makes acrylic paints less transparent?
In order to diminish the transparency of acrylic paints you can add a little bit of titanium white, which will slightly effect the original colour creating a tint of the original hue. In this case you can add more of the original colour to the mixture in order to go back to the original one.
What is the effect of opacity in acrylic pouring? An experiment.
To better understand the effects when pouring, I conducted an experiment using one opaque colour (Turquoise) and two transparent colours (Primary Magenta by Amsterdam Royal Talens and Oriental Violet by Pebeo).
I used the flip cup technique and prepared two different pouring cups:
In one cup I put the colours in the following order: Transparent/Opaque/Transparent, i.e. magenta, turquoise and Oriental Violet.
In the other the colours were in the following order: Opaque/Transparent/Opaque, i.e. Turquoise, Magenta, Turquoise, Oriental Violet.
These were my results:
- Cup No. 1: Transparent/Opaque/Transparent: The two transparent colours predominate in the final result. Also, the more layers of transparent colours applied, the darker the colour becomes.
- Cup no.2: Opaque/Transparent/Opaque: Transparent colours only slightly changed the final result. The opaque colour predominates.
In the end, we can learn from this experiment that knowing the transparency or opacity of our colours can be really important and gives us the possibility to control the final result of our pictures.
Recommendations for acrylic pouring techniques
Deciding which colours are better for pouring depends on what technique we will be using.
For example, in a ring pour where we want to have defined colours without them blending too much, opaque colours are better. On the other hand, transparent or semi-transparent colours were developed for blending techniques and are better for techniques such as flip cups or Dutch pouring, where we want to emphasize the interaction of the colours, for example, through colourful cell formation and beautiful colour gradients.
When preparing pouring cups for flip cups, it is therefore advisable to place opaque colours at the top and transparent colours at the bottom, as the formers tend to completely cover the latters. Conversely, for other techniques where the colours are poured normally (such as open cup or Dutch pours), it is therefore recommended to place opaque colours at the very bottom and transparent ones at the top.
Last but not least, remember that transparent colours tend to disappear on a dark background or a dark colour base.
I hope I could help you to better understand one of the most important feature of acrylic paints.
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Have a colourful and creative day!
Laura