The hard collar of the shirt is usually made of linen and resin cloth. When washing, first soak it in washing powder solution for 15 minutes and gently brush it with a soft brush. Do not wrung dry it or rub it hard.
Soak the collar of the shirt and use a brush to dip it in soap. After cleaning, use the washboard to scrub the part other than the hard collar. At this time, you can continue to use soap or use washing powder instead.