Fusible Plug Analysis

Why your old fusible plug may fail when you need it the most

The fusible plug from the author's 1911 45 HP Case

The fusible plug from the author's 1911 45 HP Case, cut in half lengthwise to show the fusible material and its apparent change in composition from one end to the other. Note that the fusible "tin" still has a metallic luster at the waterside (top), while the fire-side (bottom) of the plug is dull, suggesting heavy oxidization. This plug failed to melt in laboratory tests of up to 1000 degrees F.

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The spectrographic analysis of the fusible metal in two fusible plugs indicated a combination of lead and tin (possibly solder?) and gave no indication that they had become oxidized. Instead of melting at 360-460° F they could not be melted in a metallurgical laboratory even when heated to more than 1000° F Would the same thing have happened if the fusible plugs had been properly filled with only tin ? The tin used in fusible plugs melts at 449.5° F, but an oxide of tin, SnO2, doesn't melt until 2966° F. As the metallurgical report from one plug concluded: "Considering that the purpose of the fusible plug is to prevent excessive temperatures inside the steam engine (boiler), the analyzed plug would have most likely failed to perform the required safety task."

As we have discovered in the previous article, fusible plugs have had a reputation of not always melting at a predictable temperature, and the unpredictable fusible plugs seem to be those that have seen extended service. In 1924 the American Society of Mechanical Engineers (A.S.M.E.) Code reacted to this observation, dictating that fusible plugs should be replaced every year. And while it seems evident that old fusible plugs carry with them a higher potential of failure, it should be stressed that not all fusible plugs are unpredictable. I have heard of four instances where fusible plugs in traction engines have melted out reliably, with no damage to the engine and no injury to the operator.

My Investigation

In an attempt to understand why some fusible plugs that have been in use for many years don't melt at the predicted temperature, I removed the plugs from three of my boilers so that I could analyze them.

The first plug came from my 1911 45 HP J.I. Case traction engine. After removing it I carefully split it lengthwise with a hacksaw and smoothed the surfaces with a belt sander. After splitting the plug, the first thing I noticed was that the appearance of the fusible metal in the plug was very different at the opposite ends. The half of the plug toward the water end had the metallic luster that I would expect to see. However, the other half of the plug lacked that luster except in some isolated spots. At this point I am not referring to the contents of the plug as tin, for I had no way of knowing if it was still filled with tin or if it had been refilled "in the field" (as often occurred) with some other metal, such as solder, babbitt or lead. The square head of the plug had wrench marks on it, so this is a distinct possibility. It's also important to keep in mind that one of my limitations in evaluating this plug is that I know nothing of its history.

The second plug came from my Brownell vertical boiler (horse power rating unknown), which was built some time around 1916. As with my Case boiler, I don't know anything about the Brownell's boiler history. The plug in the Brownell is a water-side plug screwed directly into a tapped hole in one of the tubes. To remove the plug I simply had to remove a hand hole cover that is there specifically for access to the plug. I was surprised at the small size of the plug. It had a 3/8-inch NPT pipe thread and was only 15/16-inch long. The boiler is four feet in diameter and eight feet tall. When I split this plug, I found the metal in the plug had a bright metallic luster from one end to the other. The bore of the plug tapered from 5/16-inch at the fire end up to 7/16-inch at the water end.

The third plug that I attempted to remove was from the boiler of my portable engine, a circa 1899 Peerless Model G. This plug was the biggest surprise of them all – it was simply a 3/4-inch cast iron pipe plug! I don't want to over generalize from just three fusible plugs, but so far, every fusible plug holds a surprise.

Metallurgical Analysis of a Fusible Plug

After looking at the three plugs (the plug from the Peerless obviously didn't require much analysis), I decided to take the plug from my Case engine to a metallurgical laboratory to see if they could melt it and at what temperature. When I told the metallurgist at C.C. Technologies in Dublin, Ohio, that I wanted the metal in the plug melted to determine its melting point, he said there was no reason to melt it, he could simply determine its composition and then look up the melting point in a handbook. I emphasized to him that I did not want the data to come from a handbook - the only thing I really needed to know was, "At what temperature will the contents of the plug melt?"

But it wasn't quite that simple. Now that it appeared that I had two different materials to deal with in evaluating the plug, it was going to be necessary to have twice the number of tests performed. At $200 per hour I was becoming quite interested in just how many hours were going to go into this investigation. We finally agreed that the lab would perform the necessary tests and that I would dig almost twice as far into my pocket to cover the costs.

Results from the Metallurgical Lab

Using energy dispersive spectroscopy, the lab determined that the plug from the Case was filled with a combination of lead and tin, with lead constituting a greater part of the total. This analysis might indicate the plug had been re-poured with solder at some juncture. After finishing this part of the analysis, the lab technicians proceeded to attempt to melt the contents of the plug in two segments; one from the water side and the other from the fire side. As it turns out, they were unable to melt the metal from either end of the plug. The material from the fire side of the plug turned into a powder, and the material from the water side retained its shape. When I returned to the laboratory to get the results, the metallurgist took me into the lab to demonstrate what had happened. He placed the large chunk from the water side in a small metal crucible and held it over a propane torch until the bottom of the crucible turned red hot. He said this represented about 800° F. The heat had no visible effect on the material in the crucible.

The metallurgist offered the following comments:

  • Oxides of many metals melt at higher temperatures than the pure metals.
  • Even though the material had a metallic luster, it was obviously highly oxidized.
  • The spectrographic analysis did not distinguish between metals and oxides.

The following gives two examples of how melting points compare:

                  Melting Point of Metal                   Melting Point of Oxide 

Lead          622° F                                             1627° F 

Tin             449.5° F                                          2966° F 

Second Metallurgical Analysis

When Brian Vaughn of B&B Steam Restorations in Greensburg, Ind., learned of my research on fusible plugs he sent me a plug to be analyzed. He had chosen the plug he sent because when it had been heated to a red heat only a small amount of metal had flowed out of it. When I split this plug lengthwise I noticed that the entire metallic contents of the plug lacked the bright metallic luster I had seen on the water side of the plug from my Case engine. The appearance was more like the fire side of the Case plug.

This plug was taken to C.C. Technologies for analysis. The following are excerpts from their report:

"The primary elements making up the composition are lead (Pb) and tin (Sn). The sample also contains appreciable amounts of calcium. 

"To determine the melting temperature of the metal, the sample was heated up in a crucible, while the temperature was monitored. The test stopped after the temperature surpassed 1000 degrees F without any sign of melting. The appearance of the fusible metal suggests considerable oxidation; the melting temperature of the tin dioxide and lead mono-oxide are approximately 3000 degrees F and 1600 degrees F, respectively" 

I had hoped that the plug would have been filled with pure tin, making it similar to the common A.S.M.E. approved fusible plugs. It is ironic to note that the cast iron pipe plug in my portable engine might melt at about the same temperature as the oxidized metal (solder?) in the fusible plug from my Case engine. Cast iron melts at about 2200° F.


Not every boiler has a fusible plug in it. Indeed, some, as I have found, may have a pipe plug where the fusible plug should be.
It is not safe to assume that an old fusible plug is filled with tin.
In 1924 the ASME Boiler Code required that fusible plugs be replaced annually. I believe that these tests validate that requirement.
When a mixture of lead and tin (solder?) is used in a fusible plug, it may oxidize over time. If this occurs, the melting point will rise until the fusible plug is incapable of protecting the crown sheet.
Additional tests need to be performed to determine whether or not tin, alone, in fusible plugs can oxidize as
does a mixture of lead and tin. Anecdotal experiences indicate that it may.
New fusible plugs work! As noted earlier, I have heard four reliable reports of instances where fusible plugs melted out in traction engines, with the only consequences being inconvenience, embarrassment and/or frayed nerves. An engine owner who replaces his fusible plug every 12 to 18 months gave me a plug that had been in his engine for 18 months. The tin melted out almost instantly when I put the flame of a torch against it.

I would like to acknowledge and thank the following people who made significant contributions to the preparation of this article, including providing information, advice, encouragement, relating experiences, raising important questions, offering fusible plugs for my inspection and removing fusible plugs from fireboxes.

Dr. Robert T. Rhode of Cincinnati, Ohio
Robert Roll of Hallsville, Ohio
Robby Baughman of Nelsonville, Ohio
Bob Baughman of Nelsonville, Ohio
Dean Wilson of Grove City, Ohio
Brian Vaughn of B&B Steam Restorations, Greensburg, Ind.

Bruce E. Babcock is a regular contributor to Iron-Men Album. Contact him at: (740) 969-2096, 11155 Stout Rd., Amanda, OH 43102.  


Analysis of Soft Plug, Mark Yunovich, MS, NACE Corrosion Specialist Project Engineer, C.C. Technologies Laboratories, Dublin, Ohio.
Levin, Robbins, McMurdie, Phase Diagrams for Ceramists, American Ceramic Society, 1964.
A.S.M.E. Boiler Construction Code 1924
Machinery's Handbook, 25th Edition, 1996.