DON’T use an extension cord, ever.
There’s a reason that power cords on kilns are short, and that’s because they use large amounts of energy, usually 13-14 amps at a minimum.
Adding an extension cord adds two additional sets of contacts, not to mention the possibility of a smaller wire diameter.
One well-known kiln retailer (who happens to live and work in Canada) even states
You can run house wiring 100 ft from the service panel so there’s no reason you can’t use a 100 ft extension cord.
Gee, Dennis, that’s PRECISELY the reason you shouldn’t use an extension cord.
Move the kiln closer to the outlet. Have an electrician install an outlet closer to the kiln. These are all acceptable options. Using an extension cord is not. If a fire results from using an extension cord, your insurance company can use that extension cord as an excuse NOT to pay on the claim, and you get stuck having to pay for repairs. Extension cords are not designed to be used to power appliances. They are designed for short term usage of power equipment in places where there are no close-by outlets.