Ice White Glass Pencil Liner

Is this actually white glass throughout, or is it clear glass with a painted white bottom like the "white" pencil liners at Lowe's?'

This specific pencil liner is a clear glass liner with a white backing - this is common for many glass tiles regardless of color as it keeps the reflective properties that make glass so desirable. If you prefer a more 'solid-looking' white glass, you might prefer frosted glass. While frosted glass does have both a backing for color and a surface treatment, it is much more matte and does not reflect light the way polished/clear glass does.

Is this item available in longer lengths?

Our glass pencils come in 12" as the longest option- that is the optimal length for grout joins to allow long, thing glass tiles to flex slightly and avoid cracks and breaks over time.