Melting and casting of pitch

The pitch tool is an indispensable tool for polishing and parabolizing an a mirror telescope and is therefore useful to have a practical guide that tries to illustrate how to proceed to achieve it.
First, it is good to have in mind all the steps needed for its creation in order to proceed smoothly with the work, because once you start the process, you will not have much time to recover any forgotten passages.
The tool consists of 2 basic elements, a circular support, usually made of plywood or dental plaster and the dough in pitch.

First thing of all to do, is prepare, a container of water at a temperature as close as possible to the air of the room where you are working (It is therefore appropriate to prepare this vessel the day before in order to enable him to get to thermal equilibrium with the environment).
Another preliminary step is the realization of the support which should have a diameter appropriate depending on the type of processing that will be carried out (working with sub-diameter or full diameter) and a stiffness that will not deform during the adaptation phase of the patina or during machining (use plywood of adequate thickness in relation to the diameter of the tool).
Then you will have to cut a strip of cardboard, cardboard or other material having a length a little longer than the circumference of the support and height pariah the thick support, plus 3-4 cm. This strip of construction paper will be wrapped and fastened right around the circumference of the support so as to realize the banks to hold inside the pitch at the time of casting.

At this point we need to prepare the ingredients for the dough:

  • Rosin or Greek pitch
  • linseed oil
  • beeswax
  • turpentine

Pure Rosin, has a melting point of approximately 120-130 ° C and is too hard for optical processing. Then at the rosin are added other ingredients such as linseed oil, beeswax or turpentine to reduce the melting temperature of the dough and make it the right hardness.

As regards the amount of rosin to be used you can make a approximate count of this type:

formule colata pece

Where:
M is the amount of rosin to be used in grams
D is the tool diameter in cm
h is the average height of the layer of the pitch above the tool in cm (1-1,5 cm are the norm)
1,05 is the density of rosin in g/cm ³

While the amount of linseed oil and beeswax depend very much on the temperature of the working environment and must be sufficient to make the dough the right hardness. There is no exact recipe for these quantities, you go more for experience or trials, Although a good basis can be analyzing the data in this post: Testing the hardness of pitch

It is not necessary to start with absolutely exact quantities, as you will have the opportunity later to change the batter before pouring over the tool.

MELTING AND CASTING OF PITCH:

To proceed at this stage, you will need a saucepan (big enough to hold the pitch to be merged), a spoon to mix (a metal spoon is ideal) and a heat source. Ideal would be an electric hotplate with thermostat because at the fusion of rosin there are emitted vapors that are flammable, and that could catch fire if they came in contact with the flames of a gas burner (This does not mean that gas should not be used, but with them you have to keep a greater attention). In any case it is recommended to carry out always work in a well ventilated.

It starts pouring the Rosin inside the Pan and placing it above the stove over low heat. Rosin will start so to melt a little at a time. Stir frequently and take special care that the pitch never comes to a boil otherwise its properties degrade quickly. From my experience almost always happens that while some pieces of pitch you have yet to melt the remaining dough which has already melted boils. If this happens, remove the saucepan from the heat and continue stirring with spoon. Meanwhile the stored heat from the bottom of the pot will continue to melt the rest of pitch. Once the boil you will be toned down you will again put the saucepan on the stove.

In the next picture you can see that in this case the pitch has come to the point it begin to boil and it is therefore appropriate to take it off the stove.

pece bolle

While in this, you can see that, After mixing it well, the boil will be attenuated. Small bubbles that you see are air bubbles that were trapped between the flakes of rosin at the beginning of the merger and that does not represent a big problem because it will disappear during the process .

pece fusa

When the pitch is fully melted add linseed oil and beeswax previously prepared, stirring well to mix the dough.
At this point you have to take a bit 'of pitch from the pot with a spoon and dip it in the bowl with water prepared the day before. Shake well to speed up the cooling process until, Once the water temperature, You can test the hardness of the mixture through the fingernail test (or other methods).
If the test found that the pitch is too hard or too soft will add inside the pot more amount of linseed oil or beeswax or a few drops of turpentine if the dough is too hard, or other else rosin if the dough is too soft.
Once the hardness of the mix meets our expectations we will remove the pan from the stove and will proceed to the casting of the pitch on the tool, not before it had mixed for a few minutes the whole, in order to homogenise the mixture and get rid of all kinds of air bubble trapped.

(the following video shows the casting phase of pitch to complete 3 sub-tool diameter. These are propped up on a shelf that sits on an electronic scale in order to give everyone the right amount of material)

Now the dough in pitch will smoothly over the support, creating a beautiful flat surface free of bubbles and impurities. Otherwise blow bigger bubbles immediately and remove any impurities.

The following photo refers to the situation just after the casting. You can see immediately how the surface is pretty smooth and reflective consequence of the absence of bubbles and impurities.

pece colata

After leaving it to cool for 40-50 minutes you can already proceed to take the spherical surface at the pitch.
To do this you must protecting the mirror surface by stretching over a sheet of tissue paper, wetting it slightly to improve the contact. At this point leave the tool above the mirror and put on top of the weights until the pitch bend is perfectly adapted to the mirror surface.
Later, after 40-50 minutes from casting the pitch has not yet fully hardened, but is still very malleable and therefore this operation is relatively easy. If you let it cool the mixture until the following day the pitch will be much less malleable it is you will need to soften it by heating it or with a hair dryer or immersing them in warm water.
The last thing to do is to create channels on the surface of the pitch that are absolutely necessary for correct operation of the tool, to form blocks around 2-3 cm sides.
These channels can be made in various ways:
can be created for example by pressing upon a flat bar of metal, or by using an electric soldering (or directly with the tip, or by applying at the tip a U-shaped folded plate to use as "teaspoon" to etch the surface) with which to pour part of pitch to form the grid of channels (P.S: Never coincide with the Centre of the Central block with the tool Center), or by using a silicone or plastic grid on which the tool is pressed (see video below).

To create the micro facet instead, just put a grid or a piece of mosquito net between the mirror and the tool so that the wires that create the mosquito net is rotated 45° more or less than the directions of channels.

The casting of the pitch can also follow other procedures, like for example that creates "candy" of pitch that once solidified must be glued onto support automatically creating channels between an anchor and the other. This procedure is well described in the article written by Giulio and which you can find at the following link: Tool with "candy" pitch

CHANGING THE PATINA

In some processes, especially in those with subdiametro tools you need to change the shape of the patina making them take a star shape to get an abrasive action more nuanced to the edge.
The following video shows one of the many methods that you can use to accomplish this while also generating micro facet.
This has been achieved by interposing between the pitch and mirror a card stock with the shape of the star and a piece of mosquito net.

The tool is now finished and ready for polishing or parabolizing.
N:B: Always remember to run a good fit between pitch and mirror before each work session.
Depending on the type of work that you intend to undertake, You may have the need to build a set of tools of different sizes, such as is shown in the picture below.

pece set utensili

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