I'm reading from [Microchip's data sheet][1] regarding their MAXTouch microchip, which details on how to create a touchscreen compatible with their chip, and in discussing the "mechanical stackup" of a touchscreen it is noted that

> Front panel dielectric material has a direct bearing on sensitivity. Plastic front panels are usually suitable up to about 3 mm, and glass up to about 4 mm (dependent upon the screen size and layout). The thicker the front panel, the lower the signal-to-noise ratio of the measured capacitive changes and hence the lower the resolution of the touchscreen. **In general, glass front panels are near optimal because they conduct electric fields almost twice as easily as plastic panels**.

I'm unsure what this means that glass or plastic conducts electric fields. A typical mutual capacitive touchscreen consists of a set of x transmission lines underneath a set of y transmission lines (with space between them).

 I thought that that space and the transmission lines is what causes the electric fields, not the glass which is on top of it all.


There's also a note which says 

> Care should be taken using ultra-thin glass panels as retransmission effects can occur, which can significantly degrade performance

So I guess this means that you wouldn't want to try and use a touch screen with no glass/plastic overlay at all?


  [1]: http://ww1.microchip.com/downloads/en/DeviceDoc/mXT1189T%20Touchscreen%20Controller%20Product%20Datasheet.pdf