41. Operations depending on Grinding: Ground-in Joints.

The process will be perfectly understood by reference to a simple
case. Suppose it is desired to grind the end of a tube into the neck
of a bottle. If a stoppered bottle is available, the stopper must be
taken out and measured as to its diameter at the top and bottom.
Select a bit of tube as nearly as possible of the same diameter as the
stopper at its thickest part. Draw down the glass in the blow-pipe
flame rather by allowing it to sink than by pulling it out. After a
few trials no difficulty will be experienced in making its taper
nearly equal to that of the stopper, though there will in all
probability be several ridges and inequalities. When this stage is
reached anneal the work carefully and see that the glass is not too
thin. Afterwards use emery and water, and grind the stopper into the
bottle.

There are six special directions to be note

(1 )Turn the stopper through at least one revolution in each
direction.

(2) Lift it out often so as to give the fresh emery a chance of
getting into the joint.

(3) Rotate the bottle as well as the stopper in case there is any
irregularity in the force brought to bear, which might cause one side
of the neck to be more ground than another, or would cause the tube to
set rather to one side or the other.

(4) Use emery passing a 50 sieve, i.e. a sieve with fifty threads to
the inch run (see Sec. 144) to begin with, and when the stopper nearly
fits, wash this thoroughly away, and finish with flour emery,
previously washed to get rid of particles of excessive size; the
process of washing will be fully discussed in the chapter on
glass-grinding, which see.

(5) Any degree of fineness of surface may be obtained by using graded
emery, as will be explained, but, in general, it is unnecessary to
attempt a finer surface than can be got with washed flour emery. A
superficial and imperfect polish may be given by grinding for a short
time with powdered pumice stone.

(6) If the proper taper is not attained by blowing, or if ridges are
left on the tapered part, the process may be both hastened and
improved by giving the taper a preliminary filing with a
three-cornered file and kerosene, just as one would proceed with iron
or brass. A little filing will often save a good deal of grinding and
make a better job.

If a bottle without a tapered neck is to be employed, it is as well to
do the preliminary grinding by