Collectibles--General (Modern)/Porcelain Insulators
QUESTION: Hi Bob:
I am an archaeologist working on documenting a historic transmission line in southern Utah. We have thousands of shards of broken brown insulators, but none are intact and there are no identifying names, numbers or symbols. They are quite large (estimated 12 inches high by 10 to 12 inches in diameter) and they probably weighted five to 10 pounds each. They are mostly dark brown (with occasional white bands)and are glazed. We are trying to place the transmission line in historical context, and any information you might provide would be most appreciated. I have more photos if needed.
ANSWER: Hi -- For sure I think I can help with your project.
First question is which transmission line is this? Can you tell me the termination points? What year was if first put in service? Sample one appears to be a portion of a three part multipart insulator (not enough there to identify much further). The second piece appears to be a more modern "post" insulator which was probably a later replacement piece. Sample-2 is the top of a multi-part insulator. The white unglazed area is the firing rest where it sat in the kiln at manufacture. Can you send pictures of more pieces that you have found? I'm sure I can identify the insulators with a little more physical evidence along with the measurements. I may have more information on the transmission line itself as well. More pictures will help with the general shape of the insulator as well as glaze properties. It is also likely that more than one types of insulators were used on the line.
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QUESTION: Thank you for your quick reply. I am still in the early stages of the historical research, but here is what I can tell you so far: The transmission line terminus on the south is St. George, Utah. It trends due north from St. George towards the Pine Mountains (and along the west side of T-Bone Hill). Where it ends is a mystery because there are no towns or communities anywhere in that neighborhood, but it is possible the line eventually turns west and runs to the town of Veyo, which is the closest community in the general area. We do not have dates on the transmission line (yet). Based on the weathering of the posts, I would estimate 1920s or 1930s. All of the posts have been cut with a saw in a V-shaped pattern, and the cut surface is highly eroded indicating exposure for a considerable time (it is an extremely arid environment with less than 10 inches of rain per year so weathering occurs at a much slower rate here). I am attaching some more photos for you.
ANSWER: Thanks Jerry --
Your most recent pictures are actually from very recent insulators. The glaze color is called "sky glaze" designed to be less obvious up in the air. The darker glaze is partly conductive and designed to reduce corona discharge and AM radio noise. I've attached a couple pictures of period #1920's into 1930's# multi-part insulators for your review. More pictures of the earlier #brown# larger pieces you have found. Are there early insulators still in service along the line?
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QUESTION: Hi Bob:
The photos you sent sure look like what we are finding on the ground. Wish we had some bigger pieces, but it looks like they were used for target practice.
The transmission line in question is no longer in service, and it apparently was abandoned quite some time ago. The poles were probably harvested for firewood. Many are cut off at ground level and others are three to four feet above ground level. There is no "replacement" line in the area.
Based on the largest insulator shard, we project the insulator had a diameter of (ca.) 12 inches. Height could not be estimated with confidence, but it may be about the same measure high.
This program only allows me to attach 2 photos at a time. If you care to communicate more directly where I can attach multiple photos, you can reach me at
It appears the pieces you have will be too small to identify to a complete insulator. When researching these old lines one thing that can be very helpful is vintage photographs that show the line in-use. From those plus your shards I can probably identify the insulator style used. A number of the styles were made by several different porcelain makers so I cannot guarantee an exact ID unless you find a marked shard. If the power line dates from the 1920's into the 1930's it is actually just beyond the "early classic" period. Power lines from around 1898-1914 were the pioneer power lines, and most of the high voltage pin type development happened during that time. Most power lines were installed with one type of insulator, but as insulators were damaged they would replace them with newer replacements. Lines were also sometimes upgraded to a higher voltage and have all the insulators replaced. What is your goal with your research? Knowing what you are trying to accomplish would help. I have found that Google Earth sometimes allows you to track transmission lines (or their previous right of ways) -- a very cool tool.