N. R. "Woody" Woodward is the author of THE GLASS INSULATOR IN AMERICA, 1988 Report and developed the Consolidated Design Numbers identification system for glass insulators. 

This month's questions were submitted by Robert Tucker of Fort Sheridan, Illinois.

QUESTION: What is known about the time and place of manufacture of the S.F. insulators?

We have no definite information on the S .F. insulators. Several theories have been advanced, but no reliable proof has been produced for them. Judging from their appearance and the type of equipment used in their manufacture, they would have been made during the early years of this century.

QUESTION: How does one tell a CD 280 from a CD 281? What are the subtle differences, particularly for the No.1 Triple Petticoat?

CD 280 and CD 281 are very similar. The basic difference (as shown in the scale drawings in my "Reports") is that the CD 281 is exactly one-half inch wider at the base than the CD 280. Quite commonly the skirts of the CD 280 are about of equal length while the inner ones on the CD 281 are slightly shorter. But this feature alone would not be basis for a different number since skirt length varies considerably in those early power pieces.

In my files I have CD 280 embossed HIGH VOLTAGE TRIPLE PETTICOAT No.1 (these are the most common); also No. 71 HIGH VOLTAGE; and No. 100 H.V. These I think are quite uncommon. The CD 281's I have are embossed No.1 HIGH VOLTAGE and No. 4 HIGH VOLTAGE. All of these were probably sold interchangeably since they would be essentially the same in service.

QUESTION: I have collected four insulators that are not described in the McDougald's books. These insulators are probably only slight mold variations but maybe they are precious oddities. In either case, it is an exciting experience to find an oddity.

The only description that fits is listed in purple only. I collected several of these insulators in California in the late 60' s.

Your CD 154 Whitall Tatum (mold #3) is from the original production at the old Upper Works, probably made in 1922. That "I-A Press", upon which these were made was built in 1921, but I'm not sure if it went into service in late 1921 or early 1922. The purple ones were made the following year (1923) at the Lower Works. Your light green sample from California was probably purchased by Postal Telegraph. They seem to have had the lion's share of those early pieces. There were thousands of them on Postal lines between Seattle and Los Angeles.

This insulator appears to be a Whitall Tatum product produced prior to Armstrong's buyout or at least before the ['A' in a circle] was added. They came from California.

The CD 155 Whitall Tatum was made second half 1938. You will note there is a wide space between WHITALL TATUM and No. 1. The letters CO were removed in mid-1938 when Armstrong Cork Co. bought the plant; but the Whitall Tatum name was used for many years thereafter. You will also find this same insulator with the CO.; those were made the first half of 1938. The CD 155 style was adopted that year, so these would be the earliest ones. Hemingray also made a mold set for the CD 155 but did not get into full production on them until 1939.

The book does not show the 'A' in a circle variation. I collected this insulator near Sparta, WI in 1991. This insulator appears to be a late version of the style mold manufactured after Armstrong took over. What do all the numbers and dots mean?

On your CD 197, that arc lettering arrangement was adopted November 13, 1939, after Armstrong bought the plant. It shows up on some insulators as early as 1940, but they did not re-engrave the molds, and some of the less common styles were made in the old molds with straight lettering and the triangle for many years.

In your example, 13 is the mold number, 44 is the year of manufacture, and the tiny 43 is the year the mold was made. On these, the dots indicate quarters within the production year. So this one was made during the period January March, 1944. During this same time period, where the old molds with the WT triangle were still being used, the quarter dots were arranged around the triangle. However, these quarter dots were used only during the 5-year period 1942-46.

This insulator has a corrugated base, not a smooth base. The description of this insulator comes in clear only. There is no vertical bar. I found this insulator in an antique mall in Denver, CO in 1991. What do all the numbers and dots mean?

The CD 128 in ice green I've seen only in that one production run. I'm sure they didn't intend that they have that much color, it happened due to an error or change in decolorizer formula. The markings mean mold 25, manufactured in 1942; and the insulator made in 1946. On Hemingray insulators, the dots mean years, regardless of their positioning. The knurled base rim was first used in 1945 and was added to most of their insulators thereafter except the styles that were still being made with drip points. It relates to equalizing pressure during annealing; it seems that a perfectly smooth base doesn't allow free air circulation. You will see the knurled base on modem beverage bottles.