Petersen Tegl’s bricks are clinker-fired, of course.
Frost damage on clinker-fired bricks appeared only when, about 100 years ago, the worm press was invented. The worm presses the clay into the mould, forming a layered texture, which is clearly seen in the three, red bricks to the left. Due to the texture of the clay*, these layers may work as “flat” capillaries, sucking full of water. When the water freezes, these layers are pressed apart, and frost cracking may develop, which is clearly seen in the perforated brick to the left.
When the worm press produces the bricks correctly, the result is frost resistant bricks.