I like carving spalted birch. The obvious reason is that as the fungus works it’s way through the wood feeding on the more easily digestible materials it leaves some damn fine patterns and makes each carved object unique. However, there are some more intangible pleasures as well. Spalted birch is usually easier to carve as some of the strength of the wood has been compromised by the fungus. However what I enjoy most is navigating the point at which the wood has rotted too much and reached the soft, punky phase when it has lost all structural integrity. It’s a little like gambling- what will the next cut with the axe or knife reveal?
It is white rot that leaves the bleached and variously pigmented areas of the wood, feeding differentially on lignin and leaving the lighter coloured cellulose. Shiitake mushrooms are the fruiting body of a white rot fungus. The black or dark brown lines are so-called zone lines, made up of fungal mycelium where different fungal colonies come into contact, either with the same or different species and they create defensive barriers to protect their resources.
The spoon in the image above has a rather thick zone line running through the bowl and up the edge of the handle. I reckon this is about as rotted birch wood can get and still be used.
As the wood dries the fungal growth is halted. My experience is that once the carving is completed spalted wood soaks up oil more than regular hardwood which means that as it oxidises and dries it can help stabilise the weaker wood structure that has been left after the fungus has passed through. However, there is no doubt that in the normal rough and tumble of a kitchen spalted objects won’t have quite the same toughness as regular hardwood objects and will eventually wear out, though with any luck only after providing many years of service and developing the wonderful patina of age.
In the spoon above I have used toy safe tung oil, the subject of a previous post. It has left the spoon with a distinctly yellow hue, as with linseed oil. My experience of using tung oil on a few utensils now is that it becomes more viscous and sticky in the container as it ages and appears to have a much longer drying time than other oxidising oils such as grapeseed or walnut oil, having a slightly tacky feel some months after initial treatment. Given its higher cost than a bottle of supermarket speciality oil, I won’t buy any more once my supplies are exhausted. Grapeseed oil remains my favourite oil, having the least colour of the oxidising oils, relatively quick drying and easily available.
The spatula was carved from a different, larger log and the spalting patterns within it are more subtle. It has been treated with linseed oil, but hasn’t taken on anything like the same yellow hue as the tung oil-treated spoon.
Birch are not long-lived trees and any trip to a woodland with them growing is likely to yield a branch or two that have been on the woodland floor or some months, allowing the spalting fungus to invade. The white rot fungi need oxygen to grow, so if you find a nice piece of wood and can’t use it immediately, keeping it in a bucket of water once debarked will halt the growth of the salting fungi and the wood will remain stable in its waterlogged condition for many months, or even years. Look at bog oak!