Screens

From PyroGuide

Screens (Sieves) are used to prepare and intimately mix finely-devided chemical substances, for size separation and to granulate damp masses (a process known as "ricing"). The process of dry-mixing materials - if done properly - ends up in uniform and well-performing compositions.

Screens are most essential blending tools and the simple truth is no pyro can get on without at least two sets of good mixing screens. Even if the process of ball milling is held in high esteem by the vast majority of pyros (and this with good reason, because milling being crucial to the performance of a number of end products e.g. gunpowder), it would be a serious (and dangerous) misconception if we concluded that every mixing operation necessarily involves ball milling or tumbling. While ball mills serve well for a) the preparation of individual chemicals before screening or b) the dense mixing of gunpowder type compositions without any metals, they must not be employed for mixing any compositions containing chlorates, perchlorates or metal powders. In all these cases the Screen method is the mixing method of choice.

A pyro who plans to mix compositions containing sulphur needs a set of screens. He who starts to use chlorates will need another one, followed by a third when using ammonium perchlorate. Screens exclusively used to prepare oxidizers should best form another separate set. A "set" consists of a number of screens of different opening sizes (mesh numbers) and comprises the most common mesh sizes used for mixing along with other sizes used mainly for size separation. A typical set will contain e.g. ten screens with mesh openings ranging from 10 to 300 mesh. Closely spaced partitioning gives us more possibility to accurately separate materials into different particle size. The necessity of tool separation comes from the fact that some chemicals form hazardous combinations and must be avoided for safety reasons. Hence, Lancaster states that "even if the operator did not use chlorate and sulphur in the same composition, s/he would not use the same sieve for the two materials either" (p.175). Stackable sieves form "nests" and are used to separate differently sized particles in only one operation.

Industrial hand sieves are made of copper, brass or - where special care is necessary - of hair and sometimes earthed against static electricity. Unfortunately suchlike wire mesh is hard to find and amateurs pratically use stainless steel or even plastic mesh.

The authors deviate from each other in the specifications they give for mixing screens and it seems to correspond to eastern practice to employ much finer mesh for mixing than the western industry does. However, in general mesh sizes ranging from 14 to 60 mesh are convenient for mixing. The actual process of screening is covered by a separate articles in the techniques section of this community (see Screen method). Individual materials are adequatedly prepared beforehand (via crushing with a roller, milling etc.) and blended by passing them through a the screen several times.

Making Screens:

Screens are basic tools and as such they are best built at the very beginning of a pyro career. Many screen designs exist and some of the more sophisticated units may be inserted into and locked on wooden screening stations specifically built for this purpose. This may be the scope of another article. The following lines describe how to build one´s own wooden square mixing screens following one of the most common designs. Manufacture is simple, fast (a set can be assembled in less than a day) and the results are sturdy and can last a pyro life.

We will build square 30cm*30cm (about 12 inch*12inch) screens as seen in the top picture. The actual size can be adjusted according to the available amount of screen cloth and according to your own concept. Any size is possible, but anything smaller than the 15cm*15cm becomes impracticable. Screens should show high walls to retain the material.

Materials for screens (30cm size):

Strips of wood (Lenght: 114cm/screen; Width: 4cm; Thickness: 1,5cm) used for frame

Strips of wood (Lenght: 114cm/screen; Width: 1,5cm; Thickness: 0,5cm) used for bottom bars

Wire mesh (at least 29*29cm in size)

Woodglue

Brass screws (about 30mm in lenght; 8 pieces/screen)

Brass nails (about 15mm in lenght with flat head)

Hammer

Hand or electric saw

Sanding paper

Pair of scissors

Awl and screwdriver or electric drill with auger and drill bit

Reamer

Square block of wood or other object showing at least one right angle

Pocket rule/ruler

Pen

Optional: hot melt glue gun

Optional: polyurethane

• Cutting the wood:

For each screen you will require eight parts of wood coming in the following sizes: cut off four parts from the thicker strip of wood, two of these being 27cm and two being 30cm in lenght. Cut off exactly the same sizes from the thinner strip of wood.

Comment: These pieces of wood will later be glued head to head and each connection will be supported with two screws. More sophisticated woodworkers may improve the design by adding brass corner braces or via using finger joints to connect the wood. In this case the dimensions of the individual parts have to be altered.

• Connecting:

Take the two 30cm lenghts of the thicker (the 1,5cm thick) wood and determine its harsher side (the other, smoother side should point inwards in the finished device - this makes cleaning easier). Then take the pen and mark the four points for the screws measuring 8mm inwards and 8mm upwards from its corners. This will ensure that the screws will fit centeredly into the next part of wood and will not come out sideways when we screw them in. Next pre-drill the holes for the screws with either the awl or by machine. Screw in the eight screws per screen to a point where they just protrude out of the strips of wood.

For the following steps we will make use of a rectangular block of wood or slt. giving us a perfect right angle to connect our pieces. Take one of the longer parts (the ones with the screws in) and one of the shorter (27cm size) parts each time and use the block to connect them in rectangular fashion (the gable end of the shorter piece will touch the screw-equipped edge of the longer piece of wood). Press them together for one second and remove them again (keeping their orientation). The screw´s pikes will leave two imprints in the gable end. Take the awl (or the electric drill) and form the holes for the screws. Doind so the wood won´t accidentially split apart when we screw the two pieces together.

Then apply wood glue to the gable end of the shorter piece and reconnect the two pieces using the block. Ensuring a perfect fit and pressing both upside down and inwards, connect the two pieces by bolting them together. Repeat the operation until you got two angled wooden parts and then connect these parts to finish the screen frame.

• Attaching the mesh to the frame:

Fix the mesh to the frame using brass nails at the edges and every inch. Start out completely fixing one side; then extert tension bending it over the brink at the opposite side of the frame (if we cut the mesh before attachment, we couldn´t exert the proper tension) and - while dragging - fix the mesh to the other side also using your nails. Then restart by fixing it at the third side - extering tension - fixing the last side.

At this point of assembly we use the scissors and cut off the protruding mesh at the sides. Next use a hammer and reamer to countersink all nails in the wood. You may also use your glue gun to apply a thin layer of hot melt glue to the area where the mesh touches the wood.

• Finishing:

After attaching the mesh, the four remaining thin pieces (5mm thickness) of wood are used to cover the lower narrow sides of the frame (where we fixed the mesh). They are fixed using brass nails about every two inches and in a fashion that each of the longer (30cm) remaining bars covers one of the smaller strips of the frame and vice versa. This makes the frame more sturdy and the bars make the devices look neater. With bars attached the mesh also doesn´t contact anything directly below the wires and will not scratch over a surface when removed. Lastly we finish the screens via countersinking the outer nails and via sanding all of the lower wooden edges. One can durably imprint an identification mark into the wood using hot metal. Any remaining undesired spaces between wooden parts can be filled applying some woodglue with your fingers. This is a good way to keep out any chemicals. It may also be a good idea to seal your frame with a few layers of polyurethane coating. This will alleviate cleaning and keeps any moisture out of the wood.

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