36 - Sky Blue, or More Properly Turquoise, a Principal Colour in the Art of Glassmaking
Take the sea salt known as black salt, or rather coarse salt, since the ordinary white salt, that they make in Volterra would not be good. Put this salt in a frit kiln or oven to calcine, in order to release all moisture, and turn white. Next, grind it well into a fine white powder. This salt now calcined should be stored for the use of making sky blue or rather turquoise colour as described below.
You should have bollito frit also known as crystal in a large or small crucible. It should be tinted aquamarine colour, for which I have previously given many recipes. Let the colour be beautiful and well charged. This is of great importance in order to make a beautiful sky blue. So accordingly, you should start with a beautiful and optimal aquamarine. To this coloured glass then, add the above calcined sea salt stirring it well into the glass, as was done with the copper scale, add it little by little, until the aquamarine colour loses its transparency and diaphany becoming opaque.
The vitrified salt makes the glass lose its transparency and gives it a slight paleness, so that little by little you will make the colour that is called sky blue in the art of glassmaking. This is the colour of the stone called turquoise, a color foremost in the art. When the colour is good, you must work it quickly, because the salt will be lost and evaporated and the glass will become transparent again, with an ugly colour. Indeed, if you start to lose the color while working, add new burnt salt, as before, so that the colour returns and therefore you will retain the desired colour.
Furnace compositioners should take careful note here, when you add this salt, if it is not well calcined it always bursts. Therefore, you should be cautious and shield your eyes and vision, because there is a danger you could be hurt. Add the doses of salt little by little putting in abitat a time pausing from one time to the next until you see the desired color. With this, I do not rely on either dose or weight, but only on my eyes. When I see that the glass reaches the desired level of color, I stop adding salt. This all comes with experience; I
have made this color often, because it is very necessary in
beadmaking and is the most esteemed and prized color in the art. In order to make sky blue for beadmaking take aquamarine made with half bollito and half rocchetta glass, and it shall be a beautiful color, even though it is not all bollito.