I've been talking recently about some of the things home buyers should consider before taking on an older home, and I believe I've mentioned that a successful approach to restoration involves a considerable amount of detective work.
For example, the exterior of your house is made of stone, but some of the mortar between the stones has deteriorated to the point that water is beginning to find its way into the cracks, and causing the mortar to fall out. More water gets in behind the stones, deterioration occurs at a faster rate, and the cost of repair skyrockets.
Yet, just slapping some new mortar between those stones isn't usually the answer. Mortar fills in the gaps between stone and brick, helps distribute the weight of the stones and helps gravity keep the whole thing together. So, if the old mortar is falling out, simply running down to the home center, buying a bag of mortar, mixing it up and slapping it between the stones is not the right answer.
Mortar in older houses is either soft or hard, depending on when those houses were built. Changes in the building industry, especially in the days before mass marketing, came slowly, so you won't be able to determine the hardness of the mortar by knowing when the house was built. You'll actually need to conduct a simple test on a piece of the mortar to find out, but we'll deal with that later.
Let me first explain the difference between soft and hard mortar, since I learned it by making the common bag-of-mortar-from-the-home-center mistake on the basement walls of one of my past houses.
What makes a mortar soft or hard depends on what is used as a binder. For instance, in the 19th century, when my first house was built, I found that the mason had pointed the bricks with a mixture of sand, water and lime. Quicklime, or calcium oxide, is a soft binder, and is what results when calcium carbonate (limestone) is burned. The quicklime, which kind of looks like beaten egg whites when it is dumped in water, was combined with water and sand.
In my situation, it had been mixed with sand excavated from the building site and used to point -- fill the spaces between the stones -- the walls of the basement. In addition, lime was mixed with water and soil from the excavation and coated on those walls like plaster.
The advantages of using lime as the binder in this case were that it takes years for the mortar to harden, it doesn't expand all that much and does it so slowly that it doesn't crack and, best of all for basements, its porosity allows water vapor to escape.
In the 20th century, Portland cement replaced lime as the binder. Portland cement is what makes the mortar "hard." Its chief asset is that it cures quickly, which was a huge help when I re-pointed the base of a chimney into which the furnace was vented on a cold weekend in early December. The base of the chimney had fallen apart after years of moisture intrusion, and the pipe from the furnace fell out with it.
Unlike lime, Portland cement shrinks, won't let water vapor escape and won't permit any movement in the stone.
What you need to remember is that if your stone walls were pointed with lime mortar and you repair them with harder mortar, the new mortar may crack as the softer mortar underneath flexes or cause the softer mortar to fall apart because the harder mortar won't flex.
Water vapor will not pass and the interior walls will be damp. Think of moisture issues in the basements and you'll see that the price of ignorance, in this case mine, can be costly.
How can you tell whether the binder is lime or cement? Vinegar. A little dab will do you. Just put a little on the mortar and see if the vinegar bubbles a bit. If it does, lime is present. There is a little bit of burned lime in Portland cement, but, from my testing, it doesn't seem to react with the vinegar.
The proportion of ingredients in your mortar mix is as important as knowing what binder was used with the original.
Think of pancakes. Batter thick, thick cakes. Batter watery, crepes. It is all in the proportions. You may have to work with the ingredients to make your mortar exactly right, and don't expect to get it the first time.
One thing I've found is that Portland cement mortar is much easier to work with than lime mortar. A small amount of Portland cement added to lime mortar improves the situation, but you have to be careful not to overdo it.