This usually is a very small amount that can't be measured. Measuring the sand, silt, and then surmising what the clay will be will Utah Agriculture in the Classroom help to determine the type of soil in the jar. Within 2 days the students should be able to see the different layers easily. The layers may be easy to see or difficult depending on the color of the minerals in a given soil sample. Soil color is determined by the minerals in the soil, not the texture.
Red soil can be sandy or made of clay.
When texturing soil samples by hand students should follow this criteria. Sand feels gritty. Silt feels smooth, soft, somewhat slick, like the smooth silkiness of baby powder. Clay feels sticky, and often stains the fingers. Clay soils stick together.