Customer: 

Can I use yarn or thread to hold negative space in a pot and expect that it burns up or disintegrate in the kiln bisque firing? Would that be safe to try?

L&L: I do know of some potters using string or twine to get a certain glazing effect and the string does burn up.  If it is acrylic yarn, it should melt around 300F.  Wool yarn will not burn until around 1,100F.  Cotton and polyester should burn between 300-500F, depending on the mix.  I would suggest running the kiln vent when firing this way.