Skip to main content

Historic Preservation - Technical Procedures

Spectitle:

Cleaning Historic Glass

Procedure code:

0880002P

Source:

Developed For Hspg (Nps - Sero)

Division:

Doors And Windows

Section:

Glass & Glazing

Last Modified:

02/24/2012

Details:

Cleaning Historic Glass



CLEANING HISTORIC GLASS


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on cleaning glass in
         existing windows.  It also includes methods of cleaning
         historic glass where the glass is dirty, discolored or
         etched because of time and negligence.

1.02 DEFINITIONS

    A.   Dirt accumulations on glass exposed to weather causes
         surface crazing and alkaline reaction opalescent films if
         not washed at least every several years.

    B.   Discoloration is an oily film on the surface of the
         glass. It is caused by oil, coal, and other fossil fuels
         existing in the atmosphere, by magnesia dioxide photo-
         oxidizing in the glass under strong ultra-violet light,
         or by the addition of excessive alkali salts to produce
         a colorless product.

    C.   Etching is scratching of the glass produced by vigorous
         cleaning, steel wool, abrasive papers, or kitchen
         scouring compounds.  Etching can occur on windows that
         have been exposed to wind blown grit, or left unprotected
         during sandblasting of masonry buildings.  Etching can
         also be caused by the hydrofluoric acid-based chemical
         cleaners for masonry.  Etched glass usually requires full
         replacement of the affected pane.

1.03 SYSTEM DESCRIPTION

    A.   A window glass in proper condition is free from streaks
         of dirt from rain water combined with atmospheric
         impurities.


PART 2---PRODUCTS

2.01 MATERIALS

    NOTE:  Chemical products are sometimes sold under a common
    name.  This usually means that the substance is not as pure as
    the same chemical sold under its chemical name.  The grade of
    purity of common name substances, however, is usually adequate
    for stain removal work, and these products should be purchased
    when available, as they tend to be less expensive.  Common
    names are indicated below by an asterisk (*).

    A.   Washing liquid - washing soda (Arm & Hammer, no ammonia
         if hardware is bronze).

    B.   Household Ammonia:

         CAUTION:  DO NOT MIX AMMONIA WITH CHLORINE BLEACHES, A
         POISONOUS GAS WILL RESULT!  DO NOT USE BLEACH ON BIRD
         DROPPINGS.

         1.   A weakly basic compound that is formed when ammonia
              dissolves in water and that exists only in
              solution.

         2.   Other chemical or common names include Ammonia
              water*; Ammonium Hydroxide; Aqua ammonia*.

         3.   Potential hazards:  TOXIC; MAY IRRITATE THE EYES.

         4.   Available from chemical supply house, grocery store
              or pharmaceutical supply distributor, or hardware
              store.

    C.   Household Vinegar

    D.   Commercial Window Cleaner - non-alkaline (without ammonia
         if hardware is bronze).

    E.   Fine pumice, commercial whiting or old hardwood sawdust
         used as a scouring powder to clean and polish glass in
         combination with a commercial liquid window cleaner - use
         this for stubborn dirt only.

    F.   Clean, potable water

2.03 EQUIPMENT

    A.   Heavy gloves and protective gear

    B.   Very fine 0000 steel wool

    C.   Pliers and chisels

    D.   Sponges, natural or artificial, window washer tools,
         squeegee

    E.   Soft bristle brush

    F.   Clean, soft cloths


PART 3---EXECUTION

3.01 ERECTION/INSTALLATION/APPLICATION

    A.   General Cleaning of Glass:

         1.   Brush surfaces with a soft bristle brush to remove
              abrasive dust and oil films which build up on the
              glass.  These contain sharp dust particles which
              can scratch and degrade the glass.

         2.   Wash the glass with a solution of nonsudsing
              household ammonia in water, -OR- vinegar in water.

         3.   If the above procedure does not sufficiently clean
              the glass, apply commercial window cleaner and wash
              (starting at the top of the window) with natural or
              artificial sponge, or with window washer tools.


              a.   Use straight overlapping strokes and wash from
                   side to side.  

              b.   Wet the window thoroughly and use a wooden
                   scraper with an up and down stroke to remove
                   stubborn spots.  Take care not to apply any
                   pressure to the glass.

         4.   If dirt is still stubborn, combine window cleaner
              with fine pumice, commercial whiting, or old
              sawdust to clean and polish glass.  

              NOTE:  DO NOT APPLY PRESSURE, AS THIS CAN ETCH THE
              GLASS.

         5.   Wipe down painted metal components with metal
              cleaner.  Rinse off immediately and dry.

         6.   To dry the glass:  

              a.   Wet the side of the squeegee and pull it
                   across the window.  

              b.   Wipe the squeegee blade with a wiping cloth
                   after each pull.  Wipe corners of each pane.  

              c.   Pick up water from the corners with the sponge
                   braced with one finger.  Wipe edges with a
                   wiping cloth if necessary.  

    B.   For discoloration, wash the glass with a non-ionic
         detergent and very fine 0000 steel wool.  DO NOT APPLY
         TOO MUCH PRESSURE - IT WILL BREAK THE GLASS OR SCRATCH
         THE SURFACE OF THE GLASS.

                         END OF SECTION