Knowledge of pencils
What is the degree and concentration of the core?
It is a symbol from 6B to 9H representing the thickness and hardness of the pencil core. The core of the pencil is from stiff to soft, depending on the proportion of graphite and clay, for example HB is 35% clay for 65% of graphite.
There are 17 types of core degree from 6B to 9h, the hard core uses the hard initials H, and the soft core is represented by the black initials B. There is an F in the middle, which uses the initials of the Firm.
In the Japan Industrial Standard (JIS), HB and the like are called the density symbol without saying the degree symbol. Historically, however, pencils were used as a benchmark for surface strength scratching. It is thought that the expression called degree is suitable generally from this.