About glass heaters:
- You are absolutely right about the common glass envelope heaters, like the ones used as aquarium heaters. They are usually made of soda-lime glass, which has a high coefficient of thermal expansion, making it very sensitive (fragile) to sudden changes in temperature. I have also experienced myself how a glass envelope literally "explodes" if taken out abruptly out of the water, while still hot.
- As an alternative to common glass envelope heaters, you may purchase a heater with borosilicate glass, less subject to thermal stress.
Other alledgelly known as "unbreakable" heaters:
- If the glass breaks, either because of thermal stress or by accident, why not simply substitye the envelope material?
- Metal (titanium) envelope heaters do exist, resisting several degrees of corrosion. For instance, check here.
- Plastic envelope heaters also do a good job by being "harder" to break than glass and also resistant to some types of corrosion. For instance, check here. The only drawback is that the thermal resistance of the plastic makes the system reach much more slowly to the target temperature, which I don't think is an issue for an etching application (the bath can be pre-heated to the target temperature before actually starting the etching process).
Now, delving specifically into your etchant (Sodium Persulfate),
- According to the Chemical Resistance Guide of Burkert, Sodium persulfate in aqueous solution is compatible with the following plastics, among others: PVC and PP.
I will not dare to use directly a ceramic heater without a proper enclosure. Except if you are willing to make it in a DIY fashion. For a professional (even for a hobbyst) application, I will use an appropriate plastic heater. As last resort, if you cannot find a commercial PVC enclosed heater, maybe you can create your own PCV enclosure, for an standard ceramic heater, then adding an external temperature PID controller with the appropriate configuration (trial and error).