Most inksticks are made of soot from pine or oil combined with animal glue. An artist puts a few drops of water on an inkstone and grinds the inkstick in a circular motion until a smooth, black ink of the desired concentration is made. Prepared liquid inks vary in viscosity, solubility, concentration, etc., but are in general more suitable for practicing Chinese calligraphy than executing paintings. Inksticks themselves are sometimes ornately decorated with landscapes or flowers in bas-relief and some are highlighted with gold. Ink wash painting brushes are similar to the brushes used for calligraphy and are traditionally made from bamboo with goat, cattle, horse, sheep, rabbit, marten, badger, deer, boar and wolf hair. The brush hairs are tapered to a fine point, a feature vital to the style of wash paintings. Different brushes have different qualities. A small wolf-hair brush that is tapered to a fine point can deliver an even thin line of ink (much like a pen). A large wool brush (one variation called the "big cloud") can hold a large volume of water and ink. When the big cloud brush rains down upon the paper, it delivers a graded swath of ink encompassing myriad shades of gray to black. Once a stroke is painted, it cannot be changed or erased. This makes ink and wash painting a technically demanding art-form requiring great skill, concentration, and years of training.