Kiln Wash Application
You can apply kiln wash to your shelf (Barry has answered about the kiln bricks) with a haik brush (horizontal, vertical, and both diagonals – one coat each). You can spray it on – I use a pump up garden sprayer with as fine a mist as is allowed to coat the shelf. To get a smoother fininsh you can put hot water in your sprayer and spray over a perfectly level shelf, this will give a very smooth finish as the water evaporates. Anything more expensive than a garden sprayer is unnecessary, in my opinion of course.
More information is available here: http://glasstips.blogspot.co.uk/2009/08/applying-kiln-wash.html