Kev sau ntawv zoo thiab sib faib cov xim ntawm cov crayons / xaum yog qhov tseem ceeb heev hauv kev kos duab thiab sau ntawv txhua hnub. Cov tshuaj ntxiv no feem ntau yog siv rau hauv crayons, xaum thiab lwm qhov chaw, tsom mus rau kev txhim kho qhov smoothness ntawm cov refill, txhawb cov xim dispersion, thiab txhim kho kev sau ntawv.