Masks and Stencils

• When we paint a window we often put tape on the edges of the glass, so we won't end up painting the glass
  • this is often referred to as a mask
  • it protects selected areas

• There are a number of different forms of masking
  • you can buy or cut a stencil in the desired pattern
    • you can either hold it over the surface or tape it down
      • you can apply paint with a brush or a spray can
      • you can usually reuse a stencil
  • you can cover the surface with a protective tape or film (in art circle, this is a frisket)
    • use a fine knife and cut out and remove sections of the film
      • again you might paint the exposed area
      • on glass you could etch the exposed area with a fluoride paste
        • you could also sandblast the exposed areas
      • you could electroplate a metal surface, selectively placing metal where you want
  • metal etchings can be done in a similar manner
    • a metal plate is coated with a varnish
    • small tools are used to remove the varnish from selected areas
    • the plate is then etched in acid
      • the exposed areas become small grooves that can retain ink
      • the plate is used to print
  • a silk-screen is another form of a mask
    • this is a cloth of finely woven silk (more likely nylon or rayon now)
    • you can paint on a layer of wax or other coating in some areas of the cloth
    • then you sandwich the screen (stencil )with paper and apply ink
      • ink oozes through the exposed areas
      • ink is blocked by the waxy areas
      • the paper picks up a copy of your image
    • often several different screens are made, each for a different color ink
      • one stencil can make hundreds or thousands of prints
• It is often desirable to be able to create masks in a more controlled manner (or with less artistic skill)
  • it's obvious we are about to explore ways to create the masks photographically
• The material of choice is a photoresist
  • a typical photoresist is a solid polymer (large molecule)
  • the resist is used to coat the entire surface we want to treat
  • the material is light sensitive (usually only in the ultraviolet)
  • it undergoes changes on exposure to light
• a negative photoresist generally becomes tougher on exposure to light
  • the unexposed area is easily removed with warm water or with a solvent
    • the result is a surface that has a mask
      • some areas are exposed
      • some areas are protected
      • the pattern is a copy of the negative we used when we exposed the surface
• now we can modify the underlying surface
  • we might etch it with acid
    • we could simply etch the surface to a moderate depth
      • this is the way photo-offset printing plates are made
    • we could completely dissolve away the underlying surface
    • we might electroplate and build up a new metal layer
    • we might sand blast
    • we would simply paint it if we wished
    • we could expose it to chemicals that alter the surface
    • (you can also coat silk-screen with a resist and make a photographic stencil for silk screening)
• usually we finish by using a stronger solvent and stripping away the remaining photoresist

• There are positive photoresists
  • materials that are easily damaged by light
    • the areas that receive light end up unprotected
    • the areas that were dark retain their protective plastic properties

Remark-

• Photoresists are capable of producing patterns of extremely fine detail
  • this it the way all modern electronic chips are made
  • details as small as 1 micron are routine
  • We will work with photoresists, but we will not attempt that fine a resolution

• a. We will make a printed circuit board
  • http://www.bgsu.edu/departments/chem/faculty/endres/projects/photoresists.htm
• b. we'll try to etch a pattern into a bottle or a glass plate
• c. we will try to electroform a thin metal object

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