Glass surface
The condition of the glass surface is of primary importance for UV printing. Glass lacks the behaviour of most existing materials that can be decorated with UV inks without any special cleaning and pre-treatment. Glass has a non porous surface offering less mechanical anchor points for any coating that is applied to it, in this case, an UV ink. Glass has a non absorbent surface so any liquid that comes on top of the glass, if not cured instantly will generate considerably larger ink drops resulting into a visible blurriness of the print itself. Glass surfaces always differ from the bulk material because of the changes that occur while the surfaces are interacting with the environment. The storage factors: humidity, ventilation and temperature affect the surface of the glass and when not respected the so called glass corrosion appears with an almost irreversible effect. This effect is visible as rainbows of milky colour on the glass surface and is due to the calcium present in water that is precipitating and deposited at the level of the surface of the glass, causing stains that are hardly ever removable. As the glass surface atoms and molecules are not in an equilibrium state, since they are neither solid, nor liquid, the surface tension helping the ink drops take their form and volume on contact, will be different for different atoms orientation. Most glass used for exterior and interior applications nowadays are produced as developed by Sir Pilkington after the second world war by pouring molten glass onto a tin bath on which it flows and this is why it is referred to as float glass. Therefore one side of the glass always has more or less metal contamination. Then lucids (separation agents) are used to keep the sheets of glass separated and the sheets are cut using oils to lubricate the cutting process. Next stages include lamination and or heat treatment for toughened or heat strengthened glass.
The surface of a sheet of glass which has been exposed to 650° C may vary from that of an untreated piece of float glass. Also handling with sweaty hands and application of stickers adds to alterations of the glass surface which may be permanent.
Finally the cooling process can cause the glass to have induced tensions that may cause it to break when reaching temperatures of even 190°C. This was the case in many ceramic ink ovens where the tensions cause the glass to break during the oven cycle melting the inks inside the glass surface.
As a consequence of the above the glass quality is critically important.