Should you wet clay bricks before using them?
Well-made clay bricks should never require wetting before laying except (rarely) under extremely hot and windy conditions.
Pressed or extruded bricks of low porosity should never be wetted prior to bricklaying as they naturally have a significantly reduced water uptake (and almost zero with a Class A engineering brick) that, if wetted, would result in the brick retaining a thin film of water on all its surfaces and this would cause it to ‘swim’ on the bedding mortar; and that invariably leads to it both sliding out of face line and sinking out of level. If this happens adjust the water content of the mortar so that it is used as stiff as possible.
Clay bricks purchased from non-accredited sources could have unacceptably high porosity and water absorption rates and might need to be soaked on site with water before being used.
With highly porous bricks there is a danger that they might rapidly absorb moisture from the bedding mortar (particularly in warm weather) causing it to stiffen quickly. This would result in it losing the all-important characteristic of plasticity that would inhibit correct and accurate positioning to line and face-plane and the provision of a secure bedding, leading to poor adhesion with attendant negative consequences on aspects of compressive and flexural strengths of the overall walling.
How much water do I need on older or porous bricks?
The amount of water required to sufficiently dampen the bricks and reduce their absorbency to a level ideal for bricklaying comes with experience, but a brick that has been sufficiently dampened should not leave the hand wet when held.
There is a big difference between a brick that is ‘soaked’ and one that is ‘saturated’. A brick that has been soaked has a high percentage of moisture content, but retains sufficient available pore space to still provide the all-essential water uptake, or suction, necessary for it to be properly bedded into, and adhering onto, the fresh bedding mortar. A brick that has been saturated, however, has had all available pore space filled with water. In such a case there is no longer an ability for water uptake, with seriously reduced adhesion, or suction, so the brick ‘floats’ on the mortar; and it can even begin to shed its excess moisture into the bedding mortar that can result in it leaking out of the joint and staining the facework immediately below.
With some new bricks, a further possible problem is that saturation can liberate any integral soluble salts into solution resulting in disfiguring efflorescence (white salts) crystallising on the face of the bricks.