岩絵具の番手を比べる:アラビアゴムとアクリルエマルション

Particle Sizes of Mineral Pigments: Gum Arabic and Acrylic Emulsion

Unlike pigments used for watercolors and oil paints, mineral pigments vary their color and texture according to their particle size.

For example, the smaller the labeling number, the larger the particles, and the heavier the specific gravity.  The larger the labeling number, the smaller the particles, and the brighter and whiter the color because more light reflects on particles. When comparing No. 5 and no byaku (white), the difference is obvious. There is such a difference in brightness and saturation that it is hard to believe that they are made from the same material.


The following is a difference between No. 6 and No.13 of the artificial mineral pigment Gunjo color  manufactured by Nakagawa Gofun company.



The first comparison is the weight and volume of the mineral pigments.

I filled the same jars with Gunjo No. 6 and No. 13 to the same height and weighed them.






Then, as you can see in the image above, No. 6 is heavier.

In other words, No. 13, which has a lighter specific gravity, has a larger volume for the same weight.

At PIGMENT TOKYO, pigments are sold in units of 1 ryo  (15g), and even for the same 15g, the larger the labeling number of mineral pigment, the larger the volume.

Therefore, when Gunjo No. 6 and No. 13 are mixed with the same volume of painting medium, the No.13 will become a larger amount of paint than the No.6.


The next is a difference in texture.

As mentioned above, the larger the number, the smaller the particles, and thus the opacity also differs. For example, No. 6 reflects light like sparkling sand, while No. 13 has a relatively matte texture like powder.




These are when mixed with gum arabic which is the medium used to make transparent watercolors. No.13 has enough opacity to cover the paper palette underneath, while No.6 seems silky and translucent.



 Let's try these mineral pigments mixed with the watercolor medium on paper. They are different when painted next to each other: No. 6 is applied on the top, and No. 13 is on the bottom.



【Art Materials Used】

Color Material: Nakagawa Gofun, Shin-iwaenogu (artificial mineral pigment) Gunjo No. 6, No. 13

Medium: Gum arabic watercolor medium

Substrate: Bamboo Washi paper for watercolor



Mineral pigments are color materials whose particle size is controlled for Japanese-style paintings, and so on. Therefore, these are mixed with animal glue to make paints, which are suitable for substrates such as paper and silk. Since glue has moderate water resistance, it dries with slight bleeding when applied in layers.




On the other hand, gum arabic, the medium of transparent watercolor paints, is not water-resistant, so the paints move and cannot be applied neatly in layers.


If you would like to use a medium other than animal glue for larger numbered mineral pigments, acrylic emulsion is the way to go.

Acrylic emulsion is an original solution of acrylics, which is the binder of acrylic paints.

This medium can be mixed with pigment pastes, Mizuneri (Japanese pigment pastes), and pigments to create original acrylic paints.


Because it becomes water-resistant when it dries, colors can be applied in more layers than when animal glue is used for the medium.


Another characteristic of acrylic emulsion is its strong adhesiveness.

For example, in the case of coarse mineral pigments, if you knead them with gum arabic, the paints will come off like sand paintings when you touch the base material with your hand.

By using animal glue or acrylic emulsion, this kind of peeling problem is less likely to occur.


Let's see the difference between No.6 and No.13. Above is how it looks after about 3 coats of paint are applied. The same as using gum arabic, one coat does not produce a good color. By applying three coats by drying each coat completely, the rough texture of solid blue appeared.



【Art Materials Used】

Color Material: Nakagawa Gofun, Shin-iwaenogu (artificial mineral pgiment) Gunjp No. 6

Medium: Acrylic emulsion

 

Substrate: Bamboo Washi paper for watercolor



When No. 13 is kneaded and painted with acrylic emulsion, it looks like this.

Compared to natural mineral pigments, artificial mineral pigments have more matte texture and opacity, so even when kneaded with acrylic emulsions, their characteristics are fully shown.

Of course, three or more coats can be applied to obtain a more matte color.

However, if you want to create a subtle bleeding effect by applying multiple coats, using animal glue instead of water-resistant acrylic emulsion allows you to achieve the color bleeding, paint puddles, and movement unique to the semi-water-resistant paints.




Furthermore, acrylic emulsions can be combined with a thickener to adjust viscosity, making it possible to obtain the desired hardness of the paints. However, the thickener alone has no adhesive strength and can slow the drying speed, so please adjust the viscosity by adding it in small amounts.


Another medium that dries water-resistant is oil mediums for oil paints, but as mentioned above, it requires multiple coats if the pigment is too coarse.

Since oil-based mediums take a long time to dry, they are unsuitable for layering mineral pigments and are not recommended unless you have a specific expression with a concept.


In light of the above, understanding the relationship between particle size, pigment color, and medium properties will open the possibility of your expression using mineral pigments.



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大矢 享

Art Materials Expert at PIGMENT TOKYO

AKIRA OYA

Born in 1989 in Tokyo. Master of Fine Art and Design at Nihon University College of Art. While working at PIGMENT TOKYO as an Art Materials Expert, he also continues his career as a visual artist.

Born in 1989 in Tokyo. Master of Fine Art and Design at Nihon University College of Art. While working at PIGMENT TOKYO as an Art Materials Expert, he also continues his career as a visual artist.