A Short History of the Fusible Plug

With References to Regulations and Standards

Fusible plug

Content Tools

The November/December 2002 issue of Iron-Men Album included an excerpt from The Book of Modern Marvels published by Modern Publishing Co. in 1917. Submitted by Ed Gladkowski, the excerpted quotation mentioned an exhaustive investigation of fusible plugs undertaken by the U.S. Bureau of Standards in 1914. As soon as I read this, I embarked upon a search for a copy of the Bureau's report. Not only did I find a copy of the report, I also discovered that the Bureau of Standards completed two major, additional investigations of fusible plugs; one in 1920 and the other in 1930. Knowing that the investigations had been conducted was only the first step, however. The second, and more difficult, step was obtaining copies of the reports. Had steam historian and author Robert T. Rhode not been willing to use his influence to extract copies of these and other documents from the Library of Congress and from university libraries, this project could not have been completed.

My current compilation of information is too extensive for publication as an article in Steam Traction, and the following consists of highlights from the larger document. The details of the American Society of Mechanical Engineers (ASME) standards were taken from copies of the 1914, 1924, 1933, 1943, 1946, 1952, 1956, 1971, 1992 and 2001 ASME Boiler and Pressure Vessel Code. There were other editions of the code produced at roughly one- to three-year intervals, so the dates of some changes are only approximate. Additional information was taken from various editions of General Rules and Regulations of the Steamboat Inspection Service and the Canadian Interprovincial Regulations for the Construction and Inspection of Boilers, Tanks and Appurtenances.

Figure 2: A -inch fusible plug showing the matrix type of oxidation at the top and the infusible crust at the bottom. The filling in this plug did not melt when it was heated to over 1,000 degree F at a metallurgical lab.

In addition to the documents found, I had, thanks to the generosity of many people in the steam community, close to 40 variations of fusible plugs for study. I'm alarmed at how easy it was to find examples of every defect identified in the government investigations, and I even found one type of defect that was not listed in the studies (the slug of scale at the left in Figure 3).

Figure 3: A -inch fusible plug showing three modes of failure: The plug of scale could possibly withstand the pressure of steam if the tin melted; the filling did not melt when heated to over 1,000 degrees F and is completely converted to the matrix type of oxidization; the lower portion of the plug was filled with the infusible crust, which crumbled when the plug was cut in two.


1803 - Richard Trevithick, Cornwall, England, invents the fusible plug.

1838 - The forerunner of the Steamboat Inspection Service is created, 'To provide better security of lives of passengers on board vessels propelled in whole or in part by steam.'

1852 The Steamboat Inspection Service requires fusible plugs be installed in boilers of all steam vessels of the American Merchant Marine.

1888 - The Steamboat Inspection Service requires all steamers have inserted in their boilers plugs of Banka tin. Banka (a.k.a., Banca) tin is a naturally pure form of tin that contains no zinc and only a trace of lead.

1914 - ASME publishes their Report of the Committee to Formulate Standard Specifications for the Construction of Steam Boilers and Other Pressure Vessels and for Their Care in Service. This was the first edition of what became the ASME Boiler Construction Code. The ASME boiler code was originally developed to eliminate the cost of large inventories for boiler manufacturers who had to stock materials and attachments to suit different state specifications. Recognizing that a standard code could result in safer boilers, most states eventually adopted the ASME code as law.

It should be noted that the specifications for fusible plugs have always been included in an appendix at the end of the Code. In the words of ASME, the appendix is, 'Explanatory of the Code and containing matter which is not mandatory unless specifically referred to in the rules of the Code.' It appears that ASME has never had a mandatory set of requirements for fusible plugs.

1914 A boiler explosion on the steamship Jefferson prompts the U.S. Bureau of Standards and the Steamboat Inspection Service to launch a major investigation of fusible plugs used in marine service. They conclude that small quantities of lead or zinc contribute to a 'matrix form of oxidation' in the tin in fusible plugs that raises a plug's melting point over 2,000 degrees F. They find this can happen in as little as four to 12 months. It is also determined that leakage between the tin and the casing contributes to the formation of an 'infusible crust' on the fireside of fusible plugs that has the same effect. Both of these forms of deterioration can be seen in Figure 2. As a result of this investigation, the Steamboat Inspection Service changes its specifications to limit zinc and lead contamination to 0.1 percent each, specifying the fillings must be at least 99.7 percent tin. They report that plugs not meeting these specifications can begin showing signs of deterioration after only 500 hours of use.

Within six weeks of the explosion on the steamship Jefferson the Steamboat Inspection Service orders all marine fusible plugs in stock melted out and repoured to meet the new requirements. They also require one in 20 of any new plugs produced after that date be sent to the Bureau of Standards for verification they meet the new requirements. There is reason to believe it might have been impossible to purchase new fusible plugs for marine service in the U.S. between June 30, 1914, and Aug. 5, 1914, while the specifications were being changed and manufacturers were being accredited.

1915 U.S. Bureau of Standards report categorizes the ASME specifications for tin in fusible plugs by saying, 'Manifestly such a specification is worthless.'

1918 or 1924 - ASME adds requirement that plugs be replaced annually.

1920 - The U.S. Bureau of Standards and the Steamboat Inspection Service conduct a second investigation of fusible plugs in marine service. This inquiry focuses exclusively on the condition of new fusible plugs that have never been installed in a boiler. The final report lists six reasons why new plugs might not comply with regulations:

1) Failure to meet mechanical specifications i.e., loose fillings. Figure 6 shows a plug that would not have met this requirement.
2) Failure of casing to meet chemical specifications - i.e., use of brass instead of bronze. Figure 5 shows a casing made of brass.
3)  Impurities in the original tin. Figures 2 and 3 show plugs that were severely contaminated with lead.
4)  Impurities introduced into the tin in the manufacturing process by overheating the casing.
5)  Impurities introduced into the tin in the manufacturing process by overheating the tin.
6) Impurities introduced into the tin in the manufacturing process by using dirty pots previously containing lead or zinc.

1922The Canadian Interprovincial Regulations for the Construction and Inspection of Boilers, Tanks and Appurtenances does not include any of the recommendations from the 1914 or 1920 U.S. government investigations.

1930 Following the explosion of a boiler on the steamship Mackinac, the Bureau of Standards and the Steamboat Inspection Service conduct another investigation of fusible plugs. One conclusion finds that 10 percent of the fusible plugs in service at the time cannot be expected to properly function when and if required, even though the plugs have been in use for less than a year and a half. The study focuses on plugs corrupted by chemicals in the boiler water that leak through the plugs between the casing and the tin. Because Banka tin is not always commercially available, the use of tin that is 99.3 percent pure is allowed as long as there is less than 0.1 percent zinc and 0.1 percent lead. A specification is added stating that a test shall be made to determine the fusible metal is not loose in the plug.

1937 For the first time, ASME specifies that casings must be made of bronze. Bronze does not contain zinc, which can contaminate the tin when plugs are poured. The ASME also changes the specifications of the fusible metal to limit lead to 0.1 percent, copper to 0.5 percent and total impurities to 0.7 percent. They do not place a specific limit on the contaminant zinc, even though the Bureau of Standards has determined that zinc is the most dangerous contaminant. The ASME also adds a requirement that the name of the manufacturer must be stamped on the casing and 'ASME STD.' must be stamped on the fusible metal.

1940 ASME narrows the allowed melting range of the tin from 400 to 500 degrees F to 445 to 450 degrees F. They do not place a specific limit on zinc, the element the Bureau of Standards had found to be the most damaging to the quality of fusible plugs. These specifications remain unchanged through 2001.

1943 - Sometime between 1930 and 1943 the Coast Guard eliminates the specific limit of 0.1 percent zinc contamination in the tin even though it has been determined to be the most damaging contaminant.

1943 ASME allows plugs that comply with the U.S. Bureau of Marine Inspection and Navigation (now the U.S. Coast Guard) Rules and Regulation to be considered as complying with ASME code. Such plugs have the letters 'ASME' stamped on the casings instead of on the fusible metal. A heat (batch) number is stamped on the fusible metal. The Coast Guard's standards were significantly more stringent than were ASME's.

1992 There is no mention of the Coast Guard or its rules in the 1992 ASME code. This was eliminated sometime after 1971.

Figure 4: A fusible plug that could not be melted with a propane torch, the upper portion of the tin contains the matrix type of oxidization and the lower portion the infusible crust.

2001;- According to the Medina County Sheriff's report (Section 7, letter from John F. Wallace), analysis of two fusible plugs, one from the Medina boiler and one obtained for comparison, indicate they were produced from 100 percent pure tin; 99.98 percent pure tin is classified by the American Society for Testing Materials (ASTM) as Grade AA 'Electrolytic Tin' (1961) and is used primarily for analytical standards and pharmaceuticals. According to the Bureau of Standards' investigations, when the tin is poured into a fusible plug it is likely to pick up a small amount of copper from the casing. It is interesting that no traces of copper or other contaminants are mentioned in the Medina report.

2002 The U.S. Coast Guard abandons all requirements developed as a result of the investigations of 1915, 1920 and 1930. Today, the U.S. Coast Guard accepts any plug with an ASME stamp. They also allow fusible plugs to be continued in service so long as, in a marine inspector's opinion, they appear satisfactory. It may be worth noting that in the early studies researchers at the Bureau of Standards were not always able to distinguish between good and bad fusible plugs prior to performing tests in the laboratory.

2002 - ASME says that to stamp 'ASME' on fusible plugs a manufacturer does not need to be accredited by ASME. The requirement on fusible plugs in Section I of the Code is a guideline for the users. They have no record of the manufacturers stamping 'ASME STD.' on fusible plugs.

This contrasts sharply with ASME's strict procedures for qualifying manufacturers of safety valves. The Crosby Valve and Gauge Co. summarizes the ASME procedures in their Crosby Relief Valve Engineering Handbook.

Figure 5: A -inch short pattern fireside plug. The middle section of the plug was filled with what appeared to be the infusible crust type of material. This crumbled when the plug was split. Heated red hot, no metal flowed out.

'A manufacturer, in order to comply with ASME Code requirements, must first prepare a Quality Assurance Program and submit to periodic on-site inspections by ASME. Completion of this task qualifies the manufacturer and allows him to apply an ASME Code stamp to approved products. Each product, however, must go through a specific qualification process.

'The product inspection agency for ASME is the national Board of Boiler and Pressure Vessel Inspectors commonly referred to as the National Board. Before a pressure relief valve can be sold with an ASME Code stamp, a group of valves, generally a quantity of nine, must be subjected to a flow test conducted in accordance with rules in the ASME Code. From this testing a flow coefficient is determined and submitted to the National Board. Once the results of the tests are approved, the flow coefficient is published by the National Board to be used for valve sizing. Thereafter, a sample of valves must be submitted to the National Board on a periodic basis for flow verification. Any major changes in the valve design required that the certification be repeated. All testing is conducted in laboratories, which are certified by the National Board.'


Figure 6: A 1-inch short pattern fireside plug that failed an adhesion test. Note the small counter bore at the bottom of the plug. According to U.S. Bureau of Standards, this was intended to keep the filling from falling out of the casing during shipping. The counter bore was not allowed after 1930.

Currently, the ASME specifies 99.3 percent pure tin for fusible plugs. It is disturbing to compare the grade of tin the ASME allows to be used in fusible plugs with other grades of tin that are available. The 1961 Book of ASTM Standards lists the following grades of tin: AA, A, B and C.

Grade AA: 'Electrolytic' 99.98 percent pure. Used for analytical standards and pharmaceuticals (the Medina County Sheriff's report claims that the tin in the two plugs tested as part of the investigation exceeded the purity of Grade AA tin).
Grade A: 'High Purity' 99.8 percent pure. Used for food containers, foil and collapsible tubes (toothpaste tubes?).
Grade B: 'Lower Grade' 99.7 percent pure. Used for less-exacting purposes.
Grade C: 'Common Tin' 99.0 percent. Used for alloying, solders and bronzes.

Remarkably, the ASME code allows fusible plugs to be filled with tin that does not meet the ASTM specifications for Grade B 'Lower Grade' tin.


In 1914, the Bureau of Standards found that plugs containing more than 0.1 percent zinc might begin to show signs of deterioration after only 500 hours of use.
The ASME specifications for tin in fusible plugs do not limit zinc to less than 0.1 percent. If there were no other contaminants, the zinc contamination could be as high as 0.7 percent and the tin would still satisfy ASME requirements.

Figure 7: A -inch short-pattern fusible plug that was melted out with a torch. The dark areas on the casing indicate where the filling was not tinned to the casing. The untinned areas extend from one end of the plug to the other, possibly causing the plug to leak and develop a layer of infusible crust on the fire side. The black area on the left side of the partially melted tin is oxide that formed between the tin and the casing.

A boiler fired for 72 hours during a show would take seven shows to accumulate 500 hours. A new plug after every seven shows would involve an investment of less than $6 per show.

I have four fusible plugs that cannot be melted at 450 degrees F (Figures 2, 3, 4 and 5). Two were heated to 1,000 degrees F at a metallurgical laboratory and two were heated with a torch. The only plugs I have been able to melt with a torch were ones in use in a traction engine for only 18 months and an old short-pattern fireside fusible plug that appeared to have had very little use. In the short-pattern plug the tin was not fully adhered to the casing and would not have been acceptable to the Steamboat Inspection Service (Figure 7). I also heated a waterside tube plug that melted out incompletely. I split the plug before I attempted to melt it, and I suspect that in doing so I broke up the matrix of oxidation that had formed. Had I not first cut the plug in two, the tin might not have been able to flow out of it.


The ASME stamp on a fusible plug provides me with very little assurance of the integrity of this critical safety device. I suspect that the number of hours a plug has been in use is the most reliable indicator of its potential usefulness.

I suggest the next time you change the fusible plug in your boiler, take a torch and heat the old plug after removing it. One of two things will happen: Either you will melt the tin out of it, thus preventing someone else from inadvertently using a plug that has already served its useful life, or you will prove to yourself just how dangerous an old fusible plug can be.

Steam enthusiast Bruce sits on the newly formed Ohio Historical Boilers Licensing Board. He is a regular contributor to Steam Traction. Contact him at: 11155 Stout Road, Amanda, OH 43102, or e-mail: babcock2@gte.net