The Four Treasures of the Study: brush, ink, paper, inkstone

Brush (笔)

A Chinese brush is a bundle of hair, and the hair decides almost everything: how much ink it holds, how quickly it springs back, how sharp a point it keeps.

Goat hair, Hu brush (湖笔·羊毫): soft and holds a lot of ink. The hairs spread visibly, good for full, rounded lines and long continuous strokes. Springs back slowly, so pressing and lifting need to be unhurried.
Langhao, Hu brush (湖笔·狼毫): springy and gathers to a good point. Starts and ends are crisp, turns are clean, but it usually holds less ink than goat hair. (The app titles it weasel hair; 狼毫 literally reads “wolf hair”.)
Mixed hair (兼毫): different hairs combined to balance spring, ink capacity and control. Change the proportions and the brush changes character noticeably.
Purple hair (紫毫): traditionally rabbit hair. A sharp, short, strong tip, often suited to small characters and fine lines.

One more thing about brushes: they lag. Hair that has been pressed or turned keeps its shape for a moment. In a quick turn the tip trails behind the handle, and after a heavy press the spread hairs only gather back gradually. Old writing about brushes calls a good one jian (健): it recovers its shape quickly after pressure.

Ink (墨)

Traditional Hui ink (徽墨) is soot bound with glue and pounded into a stick. Which soot matters:

Oil soot (油烟): finer particles; deep black with a sheen, even, with rich gradations.
Pine soot (松烟): a different soot altogether, with coarser particles and less shine; a quieter, more matte tone.
Water and glue: glue and pounding decide how evenly the soot disperses. Once ground, water controls flow and bleed.
Overnight ink (宿墨): ink left standing in the stone loses water, its glue concentrates and the soot settles. That changes it, but does not simply make it better or worse.

Paper (纸)

Xuan paper (宣纸) is graded by its fibre mix and by whether it has been sized with alum and glue. Raw paper lets ink run into the fibres; sized paper keeps it on the surface.

Cotton-grade raw xuan (棉料生宣): “cotton” is the name of a grade, not cotton paper. A higher share of rice straw; ink bleeds lively.
Clean-bark raw xuan (净皮生宣): more blue sandalwood bark: tougher fibres, ink spreads but the stroke keeps its bones.
Extra-clean-bark (特净皮): even more bark; fine gradations, good for layered ink.
Half-sized xuan (半生熟宣): lightly sized: keeps some bleed but slows uncontrolled spreading.
Sized xuan (熟宣): sized with alum and glue; absorbs little, edges stay crisp. Good for fine work and repeated colour. Ink sits on the surface.

Inkstone (砚)

The inkstone is where ink is ground with water, and its well holds the result while you adjust the strength. Four stones are traditionally named together as the famous four:

Duan (端砚): fine and smooth-grained; produces ink steadily.
She (歙砚): hard and dense; produces ink quickly with a crisp feel.
Taohe (洮河砚): long known for being smooth as jade and grinding fine ink; kind to brushes.
Chengni (澄泥砚): fired clay rather than stone; warm, retains ink a long time.

And a fifth: the seal

The “four treasures” are brush, ink, paper and inkstone, but no finished work is complete without a seal and the cinnabar paste it is pressed into. That has its own guide: how to read a Chinese seal.

Questions

What are the Four Treasures of the Study?

Brush, ink, paper and inkstone (笔墨纸砚), the four basic tools of Chinese calligraphy and painting.

What is the difference between raw and sized xuan paper?

Raw (生) xuan is unsized, so ink runs into the fibres and bleeds. Sized (熟) xuan is treated with alum and glue, absorbs little and keeps edges crisp.

What are the four famous inkstones?

Duan, She, Taohe and Chengni.