the surface constituting the label this facilitates marking by spaced apart 34 points 34 materializing signs constituted by straight or curved lines such points may be of various shapes as a function in particular of the mode of producing these points by mechanical physical or chemical means this variant of incorporation of the label using the filtering material itself both for making the label and as means for incorporation in the filtering bag complicates the production of the marking since it may be necessary to manipulate during marking the filtering bags themselves which may be of large dimensions in another preferred variant marking is simpler to effect the label is first made as a separate piece using the filtering material chosen for the filtering bag or other materials resistant to the temperature and corrosion conditions specified for the filtering bag these labels are then marked with a marking resistant to the temperature and corrosion conditions specified for the filtering bag for example with a marking as described hereinbefore the label thus marked is preferably incorporated in the filtering bag by sewing over a non filtering zone of the filtering bag and provided to that end with thread selected to make the filtering bag in another variant the sewing of the.