Re: Medieval Wood Treatment
- I guess it's time for the BLO conversation again...
There are two products sold today that are labelled Boiled Linseed Oil. The modern product that you mentioned is regular linseed oil with heavy metals added to speed drying. The natural product is filtered and heated to polymerize the oil. Here a quote that discusses both, "cold pressed oil contains about 30% protein that is removed in a cleaning process. The removal of the protein is crucial for preventing mold and mildew. When the protein is removed, the oil can be boiled and sterilized. This is contrary to the linseed oil products available in most paint stores. These products are NOT actually boiled even though they are labeled "boiled". Linseed oil that has the protein cannot be boiled, it is technically impossible (the oil will become explosive when heated.) If the linseed oil is not boiled and sterilized it does not dry. Substantial amounts of chemical driers have to then be added to these "unclean" linseed oil products."
Alternatively, the natural product, polymerized or "stand" oil, is generated by heating linseed (or others like soybean or tung) oil near 300 °C for a few days in the complete absence of air. Under these conditions, the polyunsaturated fatty esters convert to conjugated dienes, which then undergo Diels-Alder reactions, leading to crosslinking. The product, which is highly viscous, gives highly uniform coatings that "dry" to more elastic coatings than linseed oil itself. Coatings prepared from stand oils are less prone to yellowing than are coatings derived from the parent oils.
Although both products are called Boiled Linseed Oil, they are very different animals. I believe that the natural product is much closer to the drying oils that were used in SCA period. A reasonable amount of information about original finishes can be determined though detailed analysis of extant objects, like types of oils and pigments. Analysis has shown that linseed oil was used throughout much of western Europe during SCA period.
There are various vendors that still sell the natural versions; one online source is SolventFreePaint.com (here's a link)
and Lee Valley sells the "Tried & True" finishes that are blends that include natural BLO. I use the T&T finishes and have been very satisfied with the results.
In Service to the Dream,
Ashgrove, Barony of Altavia, Caid
--- In firstname.lastname@example.org, "Ralph" <n7bsn@...> wrote:
> The problem with rare linseed oil is that it never truly gets "hard".
> "Boiled" linseed oil does. Of course today BLO isn't really boiled, but has chemical additives that provide the same result.
> As for period, when and where? Customs vary with date and time then as much as now.
> Trying to guess what something actually from period was finished with get hard, since paint flakes, oil dries and wood darkens.
> I tend to use paint, stain, oil finishes, what ever strikes my fancy at the time.
> An Tir