Unless it is desized muslin. You will have to desize it with a) oxidizing bleach or b) alpha amylase enzyme or c) some other chemical treatment. Washing with detergent and rinsing will not dissolve starch size. The easiest method is to avoid the issue by purchasing desized cloth or cloth of a fiber type that is not sized with starch. 

Mary W. Ballard
Senior Textiles Conservator,
Museum Conservation Institute, Smithsonian Institution
4210 Silver Hill Road, Suitland MD 20746 U.S.A.
tel: 301-238-1210 fax: 301-238-3709 
email [log in to unmask]
-----Original Message-----
From: Textile Conservators [mailto:[log in to unmask]] On Behalf Of diana zlatanovski
Sent: Thursday, October 22, 2009 5:29 PM
To: [log in to unmask]
Subject: Muslin

I am guessing the answer is yes, but I wanted to  
doublecheck...unbleached and unsized muslin purchased from an archival  
supplier still needs to be washed before using it with objects, correct?

Thanks,
Diana