Tempering is required in order to get the cocoa butter into the correct crystal form for the chocolate to have a good snap and sheen. Cocoa butter is polymorphic and, if melted chocolate and left to cool without tempering, it will result in chocolate bloom (the fat will crystallise and appear on the surface).
Tempering requires practice and can be rather tedious. At work I have tempered chocolate using the water bath method (constant stirring of the chocolate in a bowl whilst heating and then cooling it slowly in a water bath). This method is slow and requires a lot of arm power! However, it is well controlled.
Another method for tempering chocolate is by seeding it; some pre-tempered chocolate in very small pieces (i.e. shavings) is added to the untempered chocolate (which must be at a specific temperature so that it doesn't melt the shavings) to "seed" the correct cocoa butter crystals. This is probably not a suitable method for a single origin bean to bar chocolate since one wouldn't want to taint the flavour.
So, I decided to try marble tempering, or granite tempering in my case (since I did not have access to marble). I read on Cherrapeno's blog that it is possible to temper using granite if marble isn't available.
My first attempt at tempering using a granite floor tile
I watched a Youtube video to guide me (unfortunately I cannot find the one I used to share with you). For some reason I was quite nervous during my first attempt at tempering. However, it is easy to re-melt the chocolate and try again if it has been unsuccessful (this is the convenience with making plain bars rather than individual chocolates with fillings or inclusions).
In short, one has to heat the chocolate to 42ºC in order to melt out all of the fat crystals. Then about three quarters of the chocolate is moved around on the slab with spatulas (cleanly, unlike myself, as one can see in the picture!) in order to cool it to below 27ºC. The temperature to which one needs to cool the chocolate does depend on the type (dark, milk or white) and other factors. Once the chocolate has cooled to the correct temperature, it is added to the remaining warm chocolate and stirred thoroughly. The warmth from the non-cooled chocolate should warm the cooled chocolate up to 31ºC which melts out all of the "bad" (i.e. unstable) fat crystals leaving only the "good" crystals (i.e. form 5, the most stable of the 6 polymorphic forms of cocoa butter). The chocolate should then set in a tempered state.
It takes a lot of practice and I am still trying to get my head around it, however, I am improving! Once the chocolate is tempered, however, one needs to mould it...