I have a question about fusibleplugs commonly found in the crown of boilers say 5" gauge. What is the fusible material? Is it soft solder, comsol or something similar or is it another alloy. I was just wondering what the melting point would typically be relative to the temperature of the water over the crown. I ve got one fitted in my boiler and know nothing about it. Jon MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.