Cyanometer revisited

In July 2008, after reading about Horace Bénédict de Saussure and his cyanometer, I decided to build a simple one. A cyanometer is an apparatus that allow you to determine what shade of blue is the sky. It turned out that the cyanometer had no scientific usefulness. But I thought it would be a good observation tool, as the sky is not just blue — it is made of many blue colours.

I even suggested that painters could use a model of cyanometer with more hues to analyze and mix accurately the blue colours of the sky they paint. You can read my two earlier posts: Blue sky research with a cyanometer and How to build a cyanometer.

I had forgotten everything about the cyanometer when Cathy FitzGerald who was preparing a radio program for BBC3 titled “Skylarking” (documentary to be broadcast shortly) contacted me. She wanted me to make the larger version of the cyanometer I had dreamed of (my original attempt only counted 12 shades of blue)… and I accepted the challenge.

Building the cyanometer

I started with an A4 size (21 cm x 29.5 cm) piece of plywood, which is a good size to carry around in a bag. I was looking for a light sturdy material I could paint on and plywood became an obvious choice.

I traced with a pencil the margins and then 6 rows of 9 squares (1.5 cm x 1.5 cm each). I then added all the diagonals to pinpoint the centre of each square. I drilled 54 holes with a power drill.

Why holes rather than square windows? I thought first about cutting out small square windows as I had done in the 2008 cardboard version of the cyanometer. However, cutting out these windows in plywood would have been complicated and time consuming; I also feared the wood would split in the end. This was when I thought about the Camera obscura and how a whole image could go through a pinhole. If did not need a large window after all. By drilling holes, I could have larger samples of each hue because the space taken by the window was minimal. But there was more to it — the result was aesthetically more pleasing: the colours formed an uninterrupted patchwork of 54 blue hues; the holes were evenly spaced; and I would have a nice contrast between the shape of the squares and the rounded windows.

As the power drill damaged the wood at the back of the board, around the holes, I had to mend the surface with wood paste. When the wood paste was dry, I sanded the whole board.

I applied a light coat of acrylic gesso on both sides of the board and its edges to prepare the surface for painting. I painted the margin with several coats of white acrylic paint.

Just for fun, I painted a blue sky with clouds on the back of the board.

For the cyanometer colours, I worked with a selection of blue hues from the Winsor&Newton Griffin range (plus a Cobalt Turquoise, which is only available in classic artist oil paint). The pigments are in oil modified alkyd resin and the paint dries faster compared to traditional oil colours. As I painted on wood with a thin layer, the paint was touch-dry in one day.

From left to right, the base colours at the top of each column are:

• Column 1: Prussian Blue
• Column 2: 1 part Prussian Blue + 1 part Phthalo Blue
• Column 3: Phthalo Blue
• Column 4: French Ultramarine
• Column 5: 1 part French Ultramarine + 1 part Cobalt Blue
• Column 6: Cobalt Blue
• Column 7: 1 part Cobalt Blue + 1 part Cerulean Blue Hue
• Column 8: Cerulean Blue Hue
• Column 9: 1 part Cerulean Blue Hue + 1 part Cobalt Turquoise

 Each colour or mix of colours is then lightened with Titanium White to get lighter and lighter shades down each of the 9 columns. The final result is a beautiful patchwork of blue hues in different tones.

How to use the cyanometer

To work out the shade of an area of blue sky, you hold the cyanometer in front of you and peek through the holes. Move it around until you find a close match between the bit of sky you see through the hole and the blue hue painted in the square around the hole.

Related articles

• Blue sky research with a cyanometer 
• How to build a cyanometer