Porcelain Insulator Markings
Porcelain insulator markings help identify both the manufacturer and the date of
- Most porcelain insulators are incuse marked, that is, a stamp was used to impress
the manufacturer's logo into the leather hard clay prior to firing. Shown
are some examples.
- Older dry process insulators were sometimes embossed. Like with glass insulators,
markings were engraved into the mold, and when pressed the letters would be raised on
the insulator surface. A number of earlier Ohio Brass styles are also
embossed. Some embossed samples are shown below.
- Recessed-embossed insulators are a combination of both types -- raised letters
against a depressed background. Ohio Brass was the primary user of this technique,
with their "B inside the O" mark. Some recess embossed examples
are shown including the Harlow patent.
- Underglaze ink marked insulators are stamped with an ink pad
prior to glazing. The glaze does not hide the mark completely, and so a permanent
identification is made. This method is commonly used at present. Some
examples are shown below.
- Ink stamped insulators are stamped on the surface of the glaze after
firing. This is a relatively unusual marking technique. The two
insulators made in Japan below are ink stamped.
- Sand-Blast markings have been used on some relatively modern multiparts and large
uniparts. This mark is made by etching the glaze with a sandblaster through a stencil.
The occupied Japan insulator shown has a sandblast marking.
- Wax resist is an additional marking technique where wax or oil is applied in a
pattern on the unglazed porcelain before glaze application. The wax keeps the glaze
from sticking, leaving bare porcelain exposed in the desired lettering or pattern. The
wax or oil evaporates during the glaze firing. This is a relatively uncommon marking
technique that has been used on some non-North American porcelain and on some factory
glaze tests. A Pittsburg glaze test marking is shown below.
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Updated Sunday August 31, 2003