Safety

Laser

You have only one pair of eyes and you need them! Always wear appropriate safety goggles while working with a laser! Fumes and gasses produced during lasering might be toxic. Use an enclosure with a fan and a hose connected to an exhaust (chimney or window).

Chemicals

Take time to read corresponding Material Safety Data Sheets (MSDS) before working with chemicals referenced in this article. Although most of them are relatively low dangerous, wear gloves, safety goggles and a respirator mask while handling powders. Keep in mind that isopropanol is a flammable solvent.

Ceramic Tiles

These are widely available in construction stores, sometimes referred to as porcelain tiles. The glazed surface should be clean and free of grease, use isopropanol to clean it before using.

Titanium Dioxide

Available in pottery stores. Despite its white color, it is responsible for black color after lasering. This is mostly due to formation of crystalline defects on the surface of TiO2 particles, as a result of partial reduction of titanium ions at high temperature, especially in presence of carbon and organic substances. These structural irregularities do not reflect the visible light, making such surface-modified titanium dioxide look black.

The well-known and extensively documented Norton White Tile (NWT) method is based on this property of TiO2, as a main component of some common white paints sold in spray cans. This method is limited to black marking on white ceramics and is not covered in the present article.

Ultrox - Zircopax Plus (Zirconium Silicate)

Available in pottery stores. A white pigment widely used in pottery, it doesn't change its color at high temperature.

Chalk (Calcium Carbonate)

Available in pottery stores and elsewhere. I found it useful to add it to Zirconium Silicate in order to reduce the size of the white dot in the absence of organic binder. Most likely promotes faster cooling of the melted dot due to endothermic decomposition with release of carbon dioxide gas.

Frit 3124

Available in pottery stores, used mostly in glazes. A fine powder of glass composed of different oxides and having a low melting point, very useful to obtain color marking on ceramics.

Kaolin EPK

Available in pottery stores, general purpose aluminosilicate white clay powder.

Bentonite Western 325M

Available in pottery stores, extremely fine Sodium aluminosilicate clay powder with a high capacity to swell in water. In absence of organic binder, it plays a role of viscosity-increasing (thickening) agent to slow down the sedimentation of the slurry and to facilitate its even spreading over the surface of the ceramic tile.

Pottery Pigments

Available in pottery stores. These are key components for color marking of ceramic tiles, fine powders of specially formulated mixtures of inorganic oxides encapsulated in zirconia glass. Some of these oxides are highly toxic, but in such an encapsulated state they are less dangerous. Yet, please use them with due care, protect yourself!
I tried pigments produced by Mason and BASF. For more pure and vivid colors, avoid using organic binders: they produce carbon black while burning under the laser beam, which makes the color darker (unless this is your desired effect).

Polyvinyl Pyrrolidone (PVP) K-90

Available in stores of materials for home-made cosmetics. A water-soluble binder and thickening agent. Helps to decrease the dot size. Can be used as a 2% solution in isopropanol (attention: full solubilization may take up to 72 hours with occasional agitation).
Most likely, PVP could be replaced by Polyvinyl Alcohol, but I had no chance to test it.

Unfortunately, the carbon black formed during laser burning of PVP and other organic binders makes them incompatible with white marking of black ceramic tiles. Also, colors get darker if such binder is used, for example, red becomes brown.

Isopropyl Alcohol (Isopropanol)

Available in general hardware and construction stores as a paint or lacquer thinner. Flammable, but not very toxic solvent miscible with water. Yet, take all safety measures!

Avoid using isopropanol with high content of water from a pharmacy, if no other option, accordingly remove water addition from the slurry recipes below.

Process

In this article, I will limit myself to a description of processing raster pictures. Vector images are easier to process, if necessary, laser parameters can be adjusted to get best results.

The process consists in addition of material by melting it on the surface of the ceramic tile. This is not engraving, the glaze of the tile is not getting removed, but it slightly melts on the surface together with added material. The intensity of the laser beam does not affect the darkness of the resulting dot in a wide range of laser power. This means that a grayscale image cannot be rendered directly, through variation of the laser power, but only through picture pre-treatment while converting it to a black and white dotted image, a process called dithering.

Picture Pre-Treatment

Not all pictures would give suitable dithered images even if best algorithms are employed. This topic could be a subject of a whole separate article, I will only provide some general recommendations here.

Picture Quality

Your selected picture should have enough resolution, contrast and sharpness, the background should be blurry enough. The minimal resolution should be 10 dots/mm (254 DPI). Graphic pictures (sketches, drawings, engravings, pictures with enhanced contours) will be rendered better than soft halftone photos.

For soft halftone photos with smooth transitions, I would recommend the use of special software or plugins to transform it into an artistic sketch or drawing, often this gives very interesting final results.

External Photo Editor

In an external photo editor, you can not only adjust brightness and contrast of a picture with more precision and accuracy before dithering, but also improve sharpness, perform crop, adjust resolution, convert to grayscale, add vignetting and even proceed with dithering. This would give you full control of the process before importing the picture into Snapmaker Luban, even the possibility to delete or add individual dots after dithering.

Please note that it is normal if the photo before final levels adjustment looks oversharpened.

Also, the external photo editor would be useful to separate colors for multi-color applications.

For white on black process, the picture colors should be inverted before the final levels adjustment and dithering.
For pictures with smooth tone transitions, it would be appropriate to use some artistic filters like G'Mic Illustration Look available in GIMP or Krita software and maybe some minor manual dodge/burn adjustments.

For final curve adjustment (using, only for example, Adobe Photoshop Levels tool), I would recommend the following generic parameters (provided the picture looks well-balanced on the screen): 

• Black slider of Output levels: 130 to 200 (to prevent dots overlapping)
• Midtone Slider: 1.50 to 2.00, depending on the picture

After this adjustment the picture should look considerably underexposed, but this is required in order to get normal rendering as a result of the whole process. Similar Levels tools are available in other photo editors as well. This is the simplest, yet efficient way to prepare your photos for lasering.

I prefer a slightly more complex approach, using frequency separation and adjusting the levels only in the low-frequency background layer. This way the sharp contours are better preserved, yet no dot overlapping occurs in dark areas. Advanced photo editing skills are required in this case. 

If you prefer to perform dithering in an external editor, your picture resolution before that should match the expected laser Fill Interval processing parameter in Snapmaker Luban (for example, a resolution of 10 dots/mm or 254 DPI is equivalent to 0.1 mm fill interval). For dithering, I would recommend Stucki or Floyd–Steinberg algorithms. Note that Adobe Photoshop has a similar method in its conversion to Bitmap tool, under Image Mode menu line, it is called Diffusion Dither.

The dithered picture below is obtained in a different external editor (Photoline), which I prefer, using Stucki algorithm. Do not forget to save the dithered picture in bitmap (.bmp) format, otherwise the quality may suffer during import in Snapmaker Luban.

You can also use other laser-engraving softwares to control the whole process, but their descriptions are not the topic of the present article, in any case the basic principles stay the same.

Importing into Snapmaker Luban

Once imported into Snapmaker Luban, previously adjusted grayscale pictures may require scaling to match the predefined working area. Then proceed with dithering, better using Stucki or Floyd–Steinberg algorithms. Do not forget to switch to the GREYSCALE processing mode. If the picture was not adjusted well enough in an external photo editor, some limited tweaking can be performed using Contrast and Brightness sliders (the picture should look considerably underexposed on the preview), but I would recommend adjusting levels elsewhere before importing. It is better to use Snapmaker Luban sliders only for fine tuning.

If your picture has been dithered in an external photo editor (recommended), switch to B&W processing mode after importing the bitmap file and scaling. Please note that your picture resolution should match the expected laser Fill Interval processing parameter in Snapmaker Luban (for example, a resolution of 10 dots/mm or 254 DPI is equivalent to 0.1 mm fill interval), otherwise you will get unexpected results. The Threshold slider in this case affects only the preview of the dots on the screen, not the final result (except its extreme values of 0 and 255), so do not rely on the preview, it could be misleading.

Slurry Recipes

General considerations: the powders should be carefully mixed in a dry beaker before adding liquids, and even more carefully mixed after. Protect yourself, apply safety measures!
The proposed recipes are just examples, yet a few months of experimentation stand behind. You are free to unleash your creativity!

Black

This one has many similarities with classical Norton White Tile (NWT) process, with the following particularities:

• Full control over the final result
• Considerably cheaper
• No highly toxic and highly flammable solvents involved
• Deeper black color, higher contrast
• Slightly larger dot (254 DPI recommended)
• Better for drawings
• A bit less good for soft grayscale pictures with smooth transitions

In a dry polyethylene beaker, carefully mix the following components (by dry volume):

• Bentonite Western: 1 volume
• Kaolin EPK: 1 volume
• Titanium Dioxide: 1 volume

Add the following liquids, carefully mixing after each addition:

• Isopropanol: 2 volumes
• Water: 1 volume
• 2% PVP in isopropanol: until ready for application

The readiness for application is based on experience and desired effect: the thicker the final slurry, the thicker the layer on the tile. For better results, the layer should be thin enough, this would give smaller dots. A too thick layer may not work at all. Observe the way the slurry flows out of a bamboo stick: several drops a second should be OK.

White

In a dry polyethylene beaker, carefully mix the following components (by dry volume):

• Bentonite Western: 2 volumes
• Frit 3124: 1 volume
• Chalk: 1 volume
• Ultrox: 2 volumes

Add the following liquids, carefully mixing after each addition:

• Isopropanol: 2 volumes
• Water: 2 volumes
• Isopropanol: until ready for application

PVP as a binder and thickening agent is not suitable in this case, due to undesirable carbon black formation at high temperature.

The readiness for application is based on experience and desired effect: the thicker the final slurry, the thicker the layer on the tile. For better results, the layer should be thin enough, this would give smaller dots. A too thick layer may not work at all. Observe the way the slurry flows out of a bamboo stick: several drops a second should be OK.

Color

In a dry polyethylene beaker, carefully mix the following components (by dry volume):

• Bentonite Western: 1 volume
• Frit 3124: 1 volume
• Pigment of desired color: 1 volume

Add the following liquids, carefully mixing after each addition:

• Isopropanol: 2 volumes
• Water: 1 volume
• Isopropanol: until ready for application

PVP acts as a binder and thickening agent. It is not suitable if you want to get vivid colors, due to undesirable carbon black formation at high temperature.

Otherwise, for darker colors but smaller dots, 2% PVP in isopropanol can be used in final addition.

The readiness for application is based on experience and desired effect: the thicker the final slurry, the thicker the layer on the tile. For better results, the layer should be thin enough, this would give smaller dots. A too thick layer may not work at all. Observe the way the slurry flows out of a bamboo stick: several drops a second should be OK.

Ceramic Tile Pre-Treatment

Always clean the glazed surface of a ceramic tile with isopropanol before covering with slurry, lens cleaning wipes are well suited for that.

Always mix well the slurry immediately before application: it has a tendency for sedimentation, especially if it does not contain PVP.

To remove particulate matter, always filter the slurry through an old nylon stocking or sock before application.

Cover the tile by an uniform layer of the slurry with a gentle pouring on its surface, followed by tilting in all directions to spread it evenly (isopropanol is flammable, keep away from sources of ignition!). Although it sounds simple, this operation is tricky and requires considerable training to get the desired uniformity of the layer. At the last step, a lazy-susan can help as a centrifuge.

Also, the slurry can be sprayed on the surface using a pneumatic paint spray system. This would require a preliminary dilution of the slurry with more isopropanol (beware of sedimentation!). Personally, I prefer the pouring approach, since it creates less mess.

Once the tile is covered, let it dry for at least 2 hours at room temperature (isopropanol is flammable, keep away from sources of ignition!) . After that, it is ready for laser treatment.

Toolpath Parameters in Snapmaker Luban

With a 10W Snapmaker Laser module, the following parameters are applicable for this process:

• Movement mode: Line
• Fill interval: 
  • 0.1 mm for slurries with PVP as a binder (black or dark color)
  • 0.2 mm for slurries without PVP (white or vivid color)
• Work Speed: 3000 mm/min
• Jog Speed: 5000 mm/min
• Laser Power: 70%

Please note that if you preformed dithering before importing into Luban, your picture resolution should match the laser Fill Interval processing parameter in Snapmaker Luban (for example, a resolution of 10 dots/mm or 254 DPI is equivalent to 0.1 mm fill interval), otherwise you will get unexpected results.

Be careful with Fill Interval parameter, decreasing it below recommended values can lead to overlapping of dots, resulting in local overheating and melting of tile glaze. This may have a negative effect on the image quality, as these overlapped spots would look like less exposed ones (less deep blacks, or less bright white, or less saturated color, depending on the slurry recipe used). This may happen also as a result of improper preliminary photo processing.

The dot size is not only related to the focusing of the laser beam, there are physical reasons for fused material to spread like a donut due to surface tension forces because of the radial temperature gradient.
Smaller dot sizes could probably be obtained with a 1.6W laser due to overall lower temperatures and smaller focus point, but I have not performed such tests with these slurries. 

Please also note that real work speed would in fact be lower due to acceleration/deceleration curves. Therefore for working with vector images, additional optimization of parameters might be necessary.

After that you can export the toolpath to the Snapmaker Luban Workplace, transfer the file to your instrument over Wi-Fi (recommended), adjust the origin point and start the process. For this particular picture on a tile of 200 mm × 200 mm, it took around 7 hours.

Ceramic Tile Post-Treatment

Wipe the excess of dried slurry with a wet cloth or paper towel and discard it. Wash the tile with water and dish soap, then rinse it with water. Let the tile dry at room temperature from both sides.

And it is done!

Multiple Colors

An external photo editor can be used to separate color layers and those layers can be processed and dithered separately, giving individual pictures for each of colors in the whole project.

By using guiding rulers attached to the platform, the tile can be repositioned exactly the same way for each color and the same work origin can be set.

It is important to let the tile dry completely each time before applying a new layer of slurry to avoid the influence of the water absorbed in the ceramic tile during washing.