A Short History of the Fusible Plug

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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

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
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
2) Failure of casing to meet chemical specifications – i.e.,
use of brass instead of bronze. Figure 5 shows a casing made of
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

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

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

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


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
Grade A: ‘High Purity’ 99.8 percent pure.
Used for food containers, foil and collapsible tubes (toothpaste
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

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