Screen printing: Burning the screen

Screen printing is a printing technique that allows us to make multiple prints of the same image . These prints can be made on different media, such as paper, textiles, clay, etc. Here we will discuss screen printing applied to ceramics.

Below is the technical sheet for this process, which I made available to the students I taught this course. After several months of research and experimentation, I came to my conclusions and results that seemed satisfactory. This means that this technical sheet will always be subject to improvements. If you feel like you should test any part of the process in another way, do so and try to do things your way.

Brief summary of the process

We start by editing the image that we want to print according to the final result we want. This image is printed on an acetate sheet; This sheet we call a photolith .

To engrave our image on a screen or mesh for screen printing, we must first apply a photosensitive emulsion to the mesh . After this emulsion dries, we will place our photolith over the mesh and expose it to UV light . The emulsion will react to UV rays and will “solidify” and become definitive by the action of light. It turns out that where the light did not reach the emulsion, that is, where the black paint on the photolith prevented the light from passing through, the emulsion did not react and will come off when we wash the mesh with water. This way we obtain the recording of our image on the screen printing screen.

From here on, we can use this screen as many times as we want and we can print our image directly onto the clay, with an appropriate paint for ceramics, or through other intermediate supports such as tracing paper or a gelli plate.

About the meshes

- There are a wide variety of meshes, with larger or smaller "holes". The smaller the holes (i.e. the greater the number) the greater the definition of our image; on the other hand, the bigger the holes, the easier it is to pass the paint without "clogging" the mesh.

- I got used to working with two types of mesh:

120M (US) / 48T (threads per cm, UK) + open

160M / 64T + closed (limit size, more difficult to apply paint)

- There are already engraved meshes for sale

- Choose the mesh opening depending on the result you want (160M for halftones, everything else 120M)

- Iron the fabrics to avoid creases (when they are folded; the creases will create defects in the print)

- Hoops (for cross stitch) are good for holding and stretching the meshes, to be able to apply the emulsion

About the emulsion

- Prepare the day before; store in the cold and dark

- There are two types of emulsion: emulsion with diazo sensitizer (shelf life of about 2 months, faster exposure) and "one step" emulsion (shelf life of up to one year)

- Apply the emulsion in a place without UV light; use red lamp

- Before applying the emulsion, simulate the place where we are going to place the frames, in a dark place, for the emulsion to dry

- Emulsion should be applied with the thinnest layer possible

- Drying takes a few hours, or half an hour with a hot air fan on

- Don't forget to place the fabrics on top to dry to prevent drips from forming.

- When it is dry, we remove it from the frame, always without using UV light, and it is ready to be recorded.

Edit the image

- Choose the image or images to be edited, think about the size of the frame and space that the image will occupy

- The image can be edited to have a more cartoon-like, photography-like appearance, etc. We can also use handmade drawings, texts, montages, etc.

Step by step to edit an image:

A. Open the image in Photoshop

B. Crop (if necessary)

C. Image -> Mode -> Grayscale

D. Image -> Adjustments -> Brightness/Contrast (repeat if necessary)

E. Image -> Adjustments -> Levels (pull blacks and whites to the center)

- text, drawings, simple images, could be just like that; move to point G.

F. (OPTIONAL) To explore other results:

Filter -> Filter Gallery -> Sketch (Torn edges / Stamp / Photocopy / others)

G. Image ->ImageSize -> 300 dpi

Set the right height and width

H. (OPTIONAL) To create halftone images

Image -> Mode -> Bitmap:

300 dpi/Halftone screen

Frequency: 40 Lpi (lines/ inch) (this applies to 160M mesh)

Angle: 45 degrees

Shape: Round/Diamond/other

I. Save as .PNG; To then insert (File -> Place) on a new A4 sheet, which when full we also save as .png to print on acetate

About halftones

- Meshcount 160 = 40 Lpi / This is because the mesh must be 4x larger than the Lpi (160M:4=40Lpi)

- It is interesting to explore and play with other effects, for example: Resolution 50 dpi; Frequency 5 Pencil; Shape: Line

- “Moiré” effect: Random overlapping phenomenon

Photolith

- Acetate where we print the image, already edited, that we want to record

- We must buy transparencies suitable for inkjet or laser printing, depending on the case (or print in a print shop)

- After editing the image, print on acetate with maximum print quality

Writing the image to the mesh

- UV light will fix the emulsion. Where light passes, emulsion solidifies; where it doesn't pass, it will leave.

A. SUNLIGHT: Only works in summer. Exposure for 25 seconds, more or less half a day, with a high level of UV (9 preferably).

B. LAMP: in this workshop we will use a 100W UV LED floodlight. The exposure time is 20 seconds, with the lamp approximately one palm high from the mesh. The lamp gives us results with greater definition than exposure to sunlight. Each lamp will have a different exposure time, as well as the type of emulsion, distance between the lamp and the mesh and thickness of the emulsion.

- The photolith and mesh must be secured between two glasses (or at least one glass on top) with springs. From bottom to top, in this workshop we will use: Black felt, glass, black cardboard, mesh with dry emulsion, photolith, glass.

- We turn on the lamp at the same time as we mark the 20 seconds (in this case) and turn it off after that time.

- We take the mesh to a tap and apply water under pressure (depending on the emulsion, more or less pressure may be needed; if the pressure is too much, it may ruin the image). The emulsion that you didn't set will be lighter than when you burned it and should all come out with the water.

- Be careful not to scratch the mesh, it will damage it.

- Hang the mesh to dry without it touching anything. The emulsion will always be a little sticky when it is wet, if it sticks to something and dries like this the mesh will be ruined.

Some defects to watch out for:

- Moiré effect

- Spoiled / out of date emulsion

- Creases in the mesh

- Crumbled mesh (too much pressure or friction when washing the emulsion)

- Mesh stuck together (be careful to dry without touching anything)

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