In the brown rough-coat, the spots of white, unslaked lime are quite easy to see, as they are often the size of a bean or pea. How ever, in the final white coat, these spots, being smaller and of the same color as the rest of the mortar, do not show.
After it has once begun to warm up, the lime should be worked or stirred thoroughly during the process of slaking, so that, after the action has been completed, it will be of the consistency of a pasty cream. After slaking, the lime should be run off through a fine sieve (No. 5 screen) put at the end of the slaking box, into the next lower compartment, or mortar-bed. The screen is intended to keep out any lime lumps too large to slake before the mortar is used, or any flinty settlement that may be found in the lime, and to allow only a pure and thoroughly mixed hydrate to be admitted to the bed.
When drawing or running off the lime, a large supply of sand already screened should be at hand to scatter in the bottom of the mortar-bed and to use for stopping leaks that may appear as the box gradually fills. This screened sand should be sufficient in amount to complete the mortar mixture. An ample supply of water, either in barrels or in hose piped from a hydrant, should also be ready at hand—to avoid any possibility of the lime burning.
For the putty or finish coat, the paste should be made even thinner before running off, and may be of the consistency of milk. The sieve through which it is strained should also be finer, of about the mesh of an ordinary flour or meal screen. The paste for this coat is often obtained by running off the lime a second time, as by this means a cooler working putty is secured.
The length of time that mortar for plastering should be mixed before being used, is a much-discussed question. It is generally stated in architectural specifications, that "the mortar should be mixed ten days or two weeks before using." As a matter of fact, this requirement is not always either wise or desirable. It is true that, in old English work, lime mortar was left covered over with earth to stand for long periods of time, often six months to three years elapsing before it was used. In this country, such slow-going methods are not to be expected. While lime does gain in strength by standing in this thin putty state before sand or other materials have been mixed with it, yet three or four weeks, at the least, are necessary before the increase becomes very apparent. It is also necessary that the paste should
remain moist, by being kept covered all the time. At the end of the fourth month its strength will have increased about one-fifth, and most of this gain has been made during that month. From then on the gain continues, but gradually decreases in amount.
It is more economical for the plasterer to use a lime that has been slaked for some weeks, as, when tempered down, it will work freely with the admixture of a much larger proportion of sand than is taken up by lime mixed as soon as it can be readily worked. This extra amount of sand does not add to the strength of the mortar; but, as it causes the lime to cover a greater surface, it is a considerable economy for the contractor, made, however, at the expense of the quality of his work.
Lime mortar need be left standing only long enough for all its particles to be thoroughly slaked, and, if properly mixed and wet down in the first case, a great deal of time need not be required to effect that result. This once secured, the quicker the mortar is mixed and put upon the building, the better and stronger will be the plastering that is obtained. It is further claimed that the accompanying loss of limewater is also very harmful, as this water—from the properties which it has already absorbed from the lime—is much better suited for carrying on the process of mixing than newly added clean water. Yet, if the lime has been long standing, it may be necessary to add clean water to replace the water lost by evaporation or seepage, although mortar mixed with clean water never becomes so hard as that mixed with the water obtained in slaking the lime.
The sand and hair are next added, the hair being put in before the mortar becomes too stiff to work readily. After the sand is mixed, the mortar should not be left to stand for any length of time, as it would become considerably set and a loss of strength would result. If the mortar does become set in the bed, reworking would be necessary before it could be put upon the walls. The strength then lost bears a direct relation to the length of time it has stood, and the solidity it has attained, before this final working up.