Replacement Glass Dilemmas

At Historic New England, we attempt to preserve architectural elements as we find them.  Most often, basic repairs followed by routine monitoring and maintenance are effective.  But sometimes the element is beyond repair and replacement is necessary.  In the case of glass, this can present a dilemma.


When a lite is broken in a window and the break is deemed unfixable (or an existing piece does not properly fit the opening), a replacement piece of glass must be identified.  Prior to the glass being glazed in to place, “HNE New Glass 20XX” is etched into the bottom below the glazing line.  Any future work on that piece of glass would reveal this replacement.  Additionally, the repair intervention is documented via the appropriate completion report process.


But, when a piece of glass must be replaced, there is a dilemma as to what type of glass and what source of glass is appropriate.


Do we ….

 Replace with clear, flat, 21st century glass and accept the fact that this is new work?

    Pro:   This method is in keeping with Historic New England’s use of new material for wood repairs.

    Con:  The more pieces of a window that are replaced in this fashion, the more the “look and feel” of the window is altered.  Clear, unblemished glass wasn’t reliably available until 1960.


Replace with reproduction glass that was made in the 21st century but made to have the imperfections of earlier glass?

There are commerically available options in this category.  Some suppliers make handblown reproduction window glass that has the appearance of 18th and 19th century glass.  However, since this product is truly handblown some of this glass has more character than would be typical of late 19th/early 20th century glass, such as is the case with the glass found at Beauport.  Other suppliers melt modern float glass allowing imperfections such as waves and pits to settle in the piece.  This glass can be graded into different options depending on the degree and type of imperfections imparted to the piece.     
      Pro:   The general look and feel of the window is not significantly altered. 

Con:  This could technically be considered “faking it” despite the etching and associated documentation.


Replace with glass salvaged from windows of a similar period?

Pro:   The general look and feel of the window is not significantly altered.

The subtlety of the imperfections and glass thicknesses may be more closely approximated than with the reproduction options – assuming a large inventory of salvage options.

Con:  This would be considered “faking it” despite the etching and associated documentation.  We would not sanction using salvaged wood for a wood repair in any case.


At Historic New England, we will pursue the middle option – replacement with a modern material treated to appear as the original material.  But for homeowners, the use of salvage glass might be very appropriate – it is a green option and may prove to be more affordable.  Visually, we’ll very likely have the same result.


The Secretary of Interior Standards guidelines on replacement follow:


Following repair in the hierarchy, guidance is provided for replacing an entire character-defining feature with new material because the level of deterioration or damage of materials precludes repair (for example, an exterior cornice; an interior staircase; or a complete porch or storefront). If the essential form and detailing are still evident so that the physical evidence can be used to re-establish the feature as an integral part of the rehabilitation project, then its replacement is appropriate. Like the guidance for repair, the preferred option is always replacement of the entire feature in kind, that is, with the same material. Because this approach may not always be technically or economically feasible, provisions are made to consider the use of a compatible substitute material.


It should be noted that, while the National Park Service guidelines recommend the replacement of an entire character-defining feature under certain well-defined circumstances, they never recommend removal and replacement with new material of a feature that–although damaged or deteriorated–could reasonably be repaired and thus preserved.



1 Comment

Filed under Window Restoration

One response to “Replacement Glass Dilemmas

  1. I feel better to replace with reproduction glass that was made in the 21st century..

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