Spalting can be achieved by storing hydrated lumber or green wood conditions that allow mushroom spores or mushroom mycelium to contact wood that has a sufficient moisture level to allow growth. However, in the last two decades, many scientific papers on this topic have been published, indicating an increasing understanding of the conditions necessary to create spalted wood.
Once a poorly understand natural process, today there are companies producing fungi-based spalting kits for the home wood worker to induce spalting as a controlled process. Applying pure fungal isolates instead of storing wood in the presence of spores and picked mushrooms opens new possibilities for controlled spalting applications.
To begin proper induced spalting, you must start with green wood or wood that has been rehydrated to 30% or higher moisture content. Moisture is critical for allowing the fungi to run their course.
There are two major types of desirable spalting effects that may occur in wood.
Color Spalting occurs due to a range of fungi that have a tendency to create pigment in wood, creating and pushing pigment into the wood, starting at the leading edge of the fungi mycelium. This type of spalting occurs without the need for antagonism from other fungi species. Known color spalting fungi currently include several scytalidium and chlorociboria species.
- The scytalidium species do not form mushrooms and are only obtained by coaxing the fungi out of the wood using laboratory clone techniques. Color range: pink/red with interactions that lead to blue, purple, yellow, other colors possible.
- The chlorociboria species are extremely slow-growing. This fungus produces the blue elf cup fungus. Blue-green stains are associated with this species.
- Color spalting is also seen with the Ophiostoma family of fungi that are carried by the pine beetle, creating blue-gray colorations in wood. Spore-producing fungus bodies are difficult to identify and do not resemble mushrooms.
Due to the potential for damage from environmental release, the Ophiostoma family of fungi is tightly regulated by the USDA and are not commonly selected for spalting purposes. The pine beetle and the group of fungi that it spreads naturally have proven capable of destroying the potential value of forest products, this devastation also shows the importance of careful regulation of the movement and sale of spalting fungi. Even the fungus alone without the presence of the beetle has the potential to cause tree disease which may create preconditions enabling insect infestation.
"The beetles introduce blue stain fungus into the sapwood that prevents the tree from repelling and killing the attacking beetles with tree pitch flow. The fungus also blocks water and nutrient transport within the tree."
Zone Line Spalting occurs due to what is essentially a war inside the wood. The spalting effects that create zone lines are typically battle scars from a white rot fungus defending its food source from competing fungi. Zone line fungi are found far more easily in a wild setting, because they produce identifiable mushroom bodies. While colors may also form as a response to antagonism with other fungi (notably in trichaptum biforme or monascus ruber) the coloration will typically follow the patterning of zone lines.
Brown rot fungi are generally not favored species choices for inducing spalted wood effects. Brown rot colonization typically results in a long-term result of cubical cracking patterns, appearing across the wood grain. However, a notable exception is seen in brown oak. Brown oak is caused via colonization from the brown rot fungus Fistulina hepatica, which produces the beefsteak mushroom.
There is a commonly-cited "traditional technique" for creating spalted wood that typically involves applying various "stimulants" for the fungi, including beer, urine, coffee, and other substances. However, several recent researchers including Dr. Sara Robinson from Oregon State University have largely debunked the methods behind this oral tradition.
Careful observation and repetition of the spalting process has revealed to spalting researchers that the more easily-accessible nutritional sources there are outside of the wood, the less likely it is for the fungi to make the leap from colonizing the external nutrients to begin colonizing the lower-nutrition wood instead. The fungi are quite content to drink your beer while basking lazily in the slime layer created on the surface of your wood by the addition of beer.
What is desired for spalting are fungi that are incentivized to leap off of your inoculant media and attack the wood itself. Mushroom spores and mycelium from spalting fungi without an alternative desirable food source will readily colonize wood under compatible conditions. Wood colonization can also be achieved by the use of a colonized solid agar media with a low nutritive content. No beer is used, although a malt extract is commonly used in the creation of these spalting starters.
Here are links to published research on induced spalting techniques:
Inducing zone lines and melanin formation for decorative purposes on North American western wood species, with emphasis on conifers, Robinson, Weber, & Hinsch 2014
The influence of moisture content variation on fungal pigment formation in spalted wood, Tudor, Robinson, Cooper 2012
Feasibility of using red pigment producing fungi to stain wood for decorative applications, Robinson, Tudor, & Cooper 2011
Wood Species and Culture Age Affect Zone Line Production of Xylaria polymorpha, Robinson & Laks 2010
Methods of inoculating Acer spp., Populus tremuloides, and Fagus grandifolia logs for commercial spalting applications, Robinson, Tudor, Hipson, Snider, Ng, Korshikov, Cooper 2012