The latest update, April 7 2005

 

Well, and then he threw the rubber into the fireplace...

As described earlier, one of the more difficult problems to be resolved was the sericin becoming a uniform wax-hard substance after a string being twisted and cooked. That necessitated rolling the string to crack the sericin. The resulting string had pretty nice mechanical qualities, but had an affinity to whistle in a very dry air (which had to be fixed by varnishing and such), and, well, was not a perfect string (if such is possible).

I had experimented with different substances and processes, with more or less success (some are mentioned in the last update, like gelatin, rabbit skin glue). The latest success, and this time I am truly happy with it, produced the strings, which are noticeably louder, clearer and are soft in handling. No rolling required. Also, one of the substances used, is very well researched, is used in industry in many plain and adulterated forms, and is very cheap and reliable. I will give my own process, in case it is a process sensitive.

The substance is dextrin, which is used extensively in making corrugated cardboard, for textile sizing, as a postal envelope glue etc. I make it from Tapioca starch (I liked its characteristics, but of course it is just one of hundreds starches). The starch is spread on a cooking sheet and baked at the temperature of 400F (204C) for TWO hours. In the process it turns light brown, I guess now it is called a "British Gum". This dextrin dissolves even in cold water, better in warm, with a bit of patience, as it tends to gather in clumps. I make a mixture of about 10-15% powder in the water, and then add glycerin, to the tune of about 10% to the whole. Just a touch of sugar (another humectant) and about 5% borax (makes the glue stronger, as I mentioned there is a plethora of information on dextrin glues) can be added. Just dextrin with glycerin works very well, too, but obviously, the mixture can be fine-tuned ad infinitum.

This glue appears to be so agreeable with the sericin and silk, that now i can cook the silk for a few minutes, just to have it penetrated by the glue, BEFORE twisting, pull it out, squeezing out extra glue, twist, and then cook again twisted string. That is all! The glue itself is very stable, can be brought up to about 85C repeatedly, cooled down, refrigerated and used again. Very handy.

As it is a kind of sugar, I would guess, watch for the flies and ants, but, besides of this, it is very easy to work with, and resolves the sericin dryness problem. It does appear that if the glue is heated repeatedly to 90C and above, it looses some of its flexibility. There are claims to that effect from older sources. The modern means of manipulation produce dextrin glues that are very stable to higher temperatures, however personally I did not experiment in this direction.

The second ingredient in the mixture, glycerin, is a worthy substance on its own. The strings can be made by cooking in the water-glycerin mixture both before and after twist. Proportions of the mix and the temperatures at which the silk is cooked, affect string diameter very dramatically in this case, and might be difficult to control. For example at the 50/50 mixture, with the soaking temperature of 60C and cooking temperature of 93C, increase in diameter (comparing with a water cooked string) is about 13% for the midsize strings. Thicker strings of course show more gain. This IS a very large increase. The string is of a very good quality, very flexible, but the inner adhesion decreases with added glycerin. This means that the string will fluff out in a knot. However, for example if the proportion is 25% glycerin to 75% water, and the soaking and cooking temperatures are both close to 90C, the diameter increase is only about 4%, and the inner adhesion is much better. Also, my experiments were made under pressure, in hermetically sealed containers, this could affect the end diameter considerably. As can be seen, this becomes a very careful play of many effects. Glycerin “sweats” out of such a string, good for the skin, but in the beginning it will feel like oil under a bow. However it mixes well with rosin and then offers no problems at all. Also glycerin reacts with the sericin, and keeps on doing so, so it might be a reasovable precaucion to make sure it is well removed off the string after cooking.