Seth Thomas Adamantine Mantel Clock, 6 Half-Pillar, Black and Marbleized

I overhauled the movement and polished the case of this beautiful Seth Thomas Adamantine mantel clock. The case is 18 5/16 inches wide at the feet, and is 11 5/16 inches tall. The dial’s minute track is 4 1/8 inches diameter, and the minute hand  is 2 1/8 inches long.

The movement is the No. 89, but has “4 1/2” stamped on it instead of 89. The movement model numbering system began later. Seth Thomas made this reliable and efficient movement for almost 40 years. It has thinner than average mainsprings to reduce wear.


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The case was the big challenge in this repair job. Adamantine cases usually look good after a bit of polishing, but this one looked terrible! Apparently some one varnished it or added some other type of coating. I used 0000 steel wool with “Mothers Mag and Aluminum Polish” to remove the coating from the adamantine. Then I polished it with  “Mothers Mag and Aluminum Polish” on a soft cloth. I also polished the celluloid half-pillars (polish only the outside).

The feet, lion’s heads, column bases and capitals, and the bezel, had silver paint on them. I stripped the paint (the silver paint was hard to dissolve) with Zip Strip to reveal the original finish. I soaked the bezel in clock cleaning fluid and used a soft brass bristle brush to make it look good, but not too shiny. Then I sprayed these parts with Deft lacquer to protect the finish.

Movement repair: I polished the pivots, installed 14 bushings, and replaced the click rivet on the strike main wheel. The time mainspring is the original 3/4 by 0.0168 by 108 inch mainspring. The original strike mainspring is 0.0162 inch thick. It had a squeaky action when nearly wound, so I replaced it with the following mainspring: Timesavers no. 15959 3/4 by 0.0162 by 120 inches.

Repair job 6081.

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  1. Your clock is gorgeous. I have one identical to it, that my grandfather bought at a farm sale. The only difference I can see is that mine doesn’t have the round door in the back panel. Mine is in pretty good shape; but, I wish I had it polished up as beautifully as yours. Thank you for sharing this. I really enjoyed these pictures.

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