In crafting with paper, glass or even paints and stains, opaque is a term used to describe the material's ability to block light. Opaque means light cannot travel through the object; in other words, it is neither translucent nor transparent. Opaque properties may apply to any color of crafting material.
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Opaque, Transparent and Translucent
To understand opaque qualities, compare red construction paper and red tissue paper. The construction paper is opaque -- you cannot see through it at all -- while the tissue paper is translucent, meaning light travels through it. Holding the object up to a light source allows you to test whether it is translucent or opaque. A transparent item is virtually clear, such as a clear gift-wrapping tape. Paint is generally opaque when applied to a surface in a thick coat. Adding a transparent glaze allows you to see the color underneath. This technique comes in handy when creating a color wash, for instance, by layering a color such as a blue glaze over an opaque color, such as yellow.