6.2.
PARLODION FILMS
197
be removed with the edge of a piece of filter paper [Fig. 22(e)]. As soon
as the slide has dried the screens are ready to use, each now being coated
with a thin film of Parlodion.
If high quality screens are being used which are very flat, or if screens
with unduly large holes are being employed, surface tension may pull the
Parlodion films down to the glass surface where they may eventually
stick as drying is completed. Thus the supporting films may be lost as the
screens dry upon the glass slide. This usually can be prevented if a piece
of paper towelling is used instead of the glass slide to pick up the screens
from the water surface. However, it is then almost necessary to transfer
the screens to a dry towel or to a glass slide to complete the drying process.
Paper towels having a fairly hard surface should be chosen for this since
they are less apt to shed strands of lint.
As the beginner's skill increases and he wishes to make high resolution
pictures, he will want thinner and thinner supporting films, even to the
point that the films are so thin that they usually break. Extra thin films
are made quite simply by using more dilute solutions of Parlodion to
cast them in the first place. Since "dry" Parlodion is apt to contain some
water it is not easy to weigh accurately. This author has had more
success preparing very thin films by diluting standard solutions of Parlodion rather than by trying to prepare reproducibly dilute solutions
directly. With each successive dilution the character of test films can be
assayed in the electron microscope.
In mounting sections, it is best to use freshly coated grids. Even an overnight storage is apt to result in many of the films becoming loosened from
the copper mesh, and ruptured. If one does plan to use coated grids that
have been stored, at least they should be individually checked to avoid
a later disappointment.
Parlodion solutions in amyl acetate are reasonably stable so that they
can be kept for a month or more without taking precautions. However,
there is a gradual decomposition so that eventually one realizes that the
films being prepared are not as strong as they should be, and then it is
time to replace the solution.
As one further precaution, the reader might be warned that this writer
has encountered bad lots of amyl acetate from which we could not get
satisfactory films until this was redistilled. We obtained substantial
quantities of more volatile contaminating substances, as well as a residuum with a substantially higher boiling point than amyl acetate.
Films of Parlodion also can be made by dropping 1 or 2 drops of
PARLODION FILMS
197
be removed with the edge of a piece of filter paper [Fig. 22(e)]. As soon
as the slide has dried the screens are ready to use, each now being coated
with a thin film of Parlodion.
If high quality screens are being used which are very flat, or if screens
with unduly large holes are being employed, surface tension may pull the
Parlodion films down to the glass surface where they may eventually
stick as drying is completed. Thus the supporting films may be lost as the
screens dry upon the glass slide. This usually can be prevented if a piece
of paper towelling is used instead of the glass slide to pick up the screens
from the water surface. However, it is then almost necessary to transfer
the screens to a dry towel or to a glass slide to complete the drying process.
Paper towels having a fairly hard surface should be chosen for this since
they are less apt to shed strands of lint.
As the beginner's skill increases and he wishes to make high resolution
pictures, he will want thinner and thinner supporting films, even to the
point that the films are so thin that they usually break. Extra thin films
are made quite simply by using more dilute solutions of Parlodion to
cast them in the first place. Since "dry" Parlodion is apt to contain some
water it is not easy to weigh accurately. This author has had more
success preparing very thin films by diluting standard solutions of Parlodion rather than by trying to prepare reproducibly dilute solutions
directly. With each successive dilution the character of test films can be
assayed in the electron microscope.
In mounting sections, it is best to use freshly coated grids. Even an overnight storage is apt to result in many of the films becoming loosened from
the copper mesh, and ruptured. If one does plan to use coated grids that
have been stored, at least they should be individually checked to avoid
a later disappointment.
Parlodion solutions in amyl acetate are reasonably stable so that they
can be kept for a month or more without taking precautions. However,
there is a gradual decomposition so that eventually one realizes that the
films being prepared are not as strong as they should be, and then it is
time to replace the solution.
As one further precaution, the reader might be warned that this writer
has encountered bad lots of amyl acetate from which we could not get
satisfactory films until this was redistilled. We obtained substantial
quantities of more volatile contaminating substances, as well as a residuum with a substantially higher boiling point than amyl acetate.
Films of Parlodion also can be made by dropping 1 or 2 drops of