I already wrote about the difference between transparent and opaque colours in a blog post here. This time I am taking it one step further: why do transparent pigments sometimes make opaque paints?
To recap the previous post, transparent pigments let the light through their layer to hit the paper and bounce back through the layer of paint, making the colours more glowy and luminous. Opaque pigment cover the paper much more efficiently, meaning the light doesn’t go through them. They are the super efficient bouncers of the pigment world: nothing goes through them. The result is a flatter, matter colour. And a boring party.
But pigment is not the only thing that affects the transparency of paints.
2- The medium
Although pigment is the most important ingredient (and the life of the party) in a tube of paint, it is not the only one. The second VIP is the medium, which in the case of watercolours is gum Arabic. In the same way that maples exude maple syrup, acacia trees exude a transparent, sticky gum. High grade gum Arabic is clear and pure, but lower grade can be cloudy and slightly yellow. Low cost watercolour ranges use low grade, which affects the transparency and the colour of the pigment.
The other guest to this party is the one no-one wants there, but they found a way in anyway: the filler. Again to cut costs, cheap brands fill the tube with an inert filler, so that they can cut the amount of pigment they use. The problem is, the filler is usually chalk, and chalk is opaque.
Even the most transparent of pigment will not save a paint from poor quality gum Arabic and opaque fillers. Here is a video that explains all of this in greater detail:
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