The Hurricane shells you create may not be directly exposed to a flame but you should not underestimate the heat generated by a votive or tealight candle burning for hours in a confined space.
In other words, it is essential that the wax used to make the shells has a melting point higher than that of regular paraffin wax, in order to avoid that the shell walls melt and expose the - potentially flammable - items embedded inside them.
To achieve this goal, several options are available, many of them making use of different additives in small or large quantities.
If you are lucky enough to have access to the many products available to US candlemakers, you should be able to lay your hands on specially formulated Hurricane pre-blends. These blends already contain all the necessary additives and normally have a melting point higher than average (around or above 160°F). If they are available to you, do not hesitate !
But if you don't, as it's probably the case for many of us, don't fear, there are other options available.
One of these options is to add between 10 and 20% of microcrystalline hard (also called micro hard to the paraffin. Micro hard has a melting point ranging from 185°F to 200°F and mixing it with a drugstore-grade paraffin should raise the melting point of the blend enough to make it usable for a Hurricane shell.
Another option is to add clear crystals to the paraffin; their melting point is much higher than that of paraffin too and will also raise the melting point of the blend. Unfortunately, they are almost impossible to find outside of the United States.
Using stearic acid in a wax blend for Hurricane shells is not recommended because stearin opacifies the wax, which is not the effect we are looking for: the shell should be as translucent as possible or the embeds won't be visible.
Because microcrystalline hard has a melting point, it is not recommended to add it "as is" to the melted paraffin wax: it might stay afloat a long time, only partially melted, or will take hours to melt entirely.
To add micro hard to paraffin wax, melt it separately in a small pan placed on direct heat (keeping an eye on it at all times) and add it to the paraffin when entirely melted. Mix thoroughly.