number in the wheel grading following the grade letter is reality a number
indicating a density chart (in a 0 to 14 range a low structure number indicates
a close compaction, a high one less compaction). Generally a change in structure
has little effect on grinding.
In some coarse grain surface grinding wheels it is desirable that the wheel have more porosity than normal. This is done by using a granular materials as part of the abrasive mix.
After molding, this granular material will leave the mix as a gas before or during firing. The additional voids left in the mix through the use of the granular material are known as induced porosity and may be designated by high structure number and additional bond symbol or both.