I have a heat treatment furnace which is very similar to furnaces used to make jewelry, such as to melt gold, silver and other precious metal alloys for casting into jewelry. In fact, the manufacturer's operational video actually shows the operator putting a crucible into the furnace, which has a front loading door. This type of furnace is sometimes called a "muffle furnace". It looks like this:

enter image description here

So, my problem is that the operating manual says not to open the furnace if the internal temperature is greater than 300C. However, if I wait for it to cool down, then my material will begin to solidify.

Now, I haven't really tested it, so maybe the chamber can be made to cool down rapidly so I can withdraw the material when it is still hot, I don't know. But I am asking here to see if anyone else has run into this problem and have a solution? According to the manual the risk to the furnace of opening it when it is hot is thermal shock which could cause the lining to crack, but that seems kind of unlikely. The lining is pressed ceramic fiber which as I understand it is pretty stable, but I don't want to second guess the manufacturer, though.

  • 3
    Easy. Operate the furnace in a 300C room, then no temperature difference when you open the door. How to do that would be the subject of another question. :-) You might need to use a different type of furnace.
    – fixer1234
    Commented Apr 18, 2021 at 21:55
  • 1
    Curious, why use a closed furnace? It doesn't seem like the right tool for the job; I never used anything but an open crucible for heating metal, because of the need to transfer it to a mold. My personal inclination is that the solution here is "use the right tool for the job."
    – Allison C
    Commented Apr 19, 2021 at 13:59
  • 2
    Send this question to the customer service of the manufacturer. Explain your motive. Ask for a method of workaround. If the manufacturer is hesitant to recommend unofficial (potentially risky, not guaranteed to work) workarounds, look up the repair agency in the warranty. They may feel less obliged to keep their mouths closed when it comes to empirical experience & solutions over the prescribed procedure. If they also don't help, look up other, similar companies who repair things like this, and send your question there too.
    – Levente
    Commented Apr 19, 2021 at 19:36
  • Does the manual actually state that not opening at >300°C is to avoid damage to the furnace? Could they mean for safety reasons?
    – Chris H
    Commented Apr 20, 2021 at 13:53
  • 1
    @everyone I'm just feeling nice today and gave away 10% of my rep. good luck getting those 50 points! ...remember: best answer = free 50 points
    – Isaac750
    Commented May 4, 2021 at 4:40

1 Answer 1


There are ovens made to be opened when hot and there are ovens not made to be opened when hot.

From what you mention what the manual says, yours can not be opened when hot.

As there are different parts that go wrong at strong heat loss or getting the full blast of the oven heat, it is impossible to predict what your oven can handle and whether you can adjust it.

Your best bet is to replace this oven with one made to be opened when hot. Your second best bet is to ask the maker or a community of oven users where someone may have more knowledge.


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