Since posting the pictures of Gaudi, I’ve had several people ask what the trick was to the roaning on his coat. Unfortunately there isn’t any real trick – no special tool or technique. The pattern was created using a #11 Xacto blade, or rather dozens of those blades. (I set the blade aside as soon as it begins to dull, and that happens pretty quickly when it is being scraped against ceramic bisque.) The process isn’t really all that different from the etching some use to customize factory finish plastic horses, though potters usually use the term “scraffito”.
So there isn’t really a trick, other than training yourself to maintain a really light touch with the blade. It also helps if you work in small areas for short periods of time. The temptation to rush and the mind’s tendency to create a regimented pattern are the biggest obstacles to achieving a realistic roaning pattern. That means they take a lot longer than it seems they should, but the end result is pretty cool.