I make a lot of rings but always explain that they are not as strong as sterling silver rings. As a pure metal Fine silver doesn't have the strength of an alloy like sterling silver.
So when designing a ring in PMC or Art clay silver you need to follow a few guidelines....
1) Make the ring thicker to add strength (most times at least 6 cards thick). 2) If you can, re-inforce the join by adding any decorative elements on top of it. 3) Fire it long & hot ideally in a kiln for at least an hour. I fire at 850-900 Centigrade for 2 hours to make extra sure...
I generally test their strength after firing, on a ring mandrel. Whacking them into shape is a great test, lol. Personally I wouldn't recommend hob firing for rings as it just doesn't get hot enough to fully sinter.
Torch firing can work but as the temperature is controlled by eye results can vary.
For those that dont have a Kiln
Maybe you could try these?http://www.metalclayfindings.com/rings.aspx. They're fine silver ring liners that you build your ring on top of - then you could torch fire it...they're an American company but are also available in the UK from their distrubutor Argentice.