Bricks, Re-Pointing & French Bread

 

My wife Pat and I were recently at the 1866 Italianate brick home of some new friends. Their home is in good shape but has three interior, exposed brick walls in the kitchen. They showed me where all the mortar is crumbling, which in turn leaves this nasty grit all over their counter tops. "We would like to keep this old look. What should we do?" they asked.

I mean, they really love these exposed brick walls. While I appreciate the fact that they enjoy the look, the exposed brick walls are definitely not the "old look."

The walls in their kitchen were stripped of their original plaster surface. They may have even been sandblasted. I suggested they consider having this brick surface re-plastered. This is the very best way eliminate the grit, have a healthier wall, and be exactly what the original wall were intended to be from day one.

In the preservation business we learned the hard way back in the 1970's that sandblasting or high pressure washing brick is a very bad idea. When the exterior, or interior, face of an old brick is sandblasted it's like taking a loaf of French bread, removing the hard crust and exposing the soft bread to rain. What happens? The softer inside of the loaf disintegrates from the moisture. It is no different with brick. Older bricks were not fired as well as modern bricks and so have a softer outer surface. If you remove this kiln fired surface you have essentially ruined the brick.

When the interior sides of exterior brick walls are exposed, it allows too much transfer of moisture through the wall. It may seem weird, but most of this moisture comes from the inside of a structure. Kitchens, bathrooms and restaurants generate a lot of interior moisture. This can cause serious failure of the mortar and brick. As excess moisture enters the wall, the freeze/thaw cycle in our area can cause the brick face to deteriorate and the excess moisture will soften up the old high-lime content mortar.

Sealing exposed brick is even more problematic. Silicon sealers used to be the treatment most used to seal exposed, sand blasted or high pressure washed brick. This was done because the brick was damaged by these so-called cleaning methods. We discovered very quickly that silicon did not allow the wall to breath which trapped moisture and caused even more damage. Brick or stone walls treated this way also looked dirty very fast because silicon is sticky. Any sealer used must be vapor permeable (able to let moisture through).

Bricks that are 100 years or older actually grow. These old bricks take in and let out moisture. This is normal and after a long period of time the brick will actually increase in size a very small amount. This is also why the mortar you use to re-point (tuck-point) old brick must always be softer than the brick.

Imagine the brick expanding as it takes in moisture. If a hard, Portland cement based, mason's mortar is used, the brick will expand and the mortar will not move. This will cause the brick face to break off and can even crack old brick.

Original mortar recipes are the best to use. You can take some original mortar out and send it into a lab for analysis. They will break down the old lime based mortar and give you the original mixture so you know you're getting it right. They will separate out the lime, sand and any other elements they find and send it back to you in several small baggies. It's well worth the $100 to $200 fee to get it right.

Most re-pointing mortars have very high lime contents. The original mortars before about 1905 were made with either hydrated/slaked lime or lime putty and sand, and that's it. Adding a very small amount of Portland cement to an original recipe can help the mortar cure a bit faster than the month it takes for pure lime and sand.

The color of all basic, lime based mortars is achieved with the sand, not mortar dyes/colorants. If you get a mortar analysis they will give you a baggie of the sand that was in your mortar. Most communities have sand suppliers that have been using the same sources since the mid-1800's. Take your sand sample with you and go look at all the different sand piles until you find one that looks like yours.

You can also buy what's call Type-N masons mortar. This is a high lime content mortar with that touch of Portland cement for faster curing. All you need to add to this is the right sand and water. A good mix is 3.5 parts sand to 1 part Type-N mortar. I only use this mix when I'm re-pointing interior basement walls or wall that will have every joint re-pointed. If you're "spot re-pointing" (only re-pointing areas with bad mortar) it is very critical to get an analysis and be sure the new mortar matches the old.

Always do a test in an out of the way area to see if the mortar will match. If you hire the work done, be sure the mason does several test patches as well. If you want to see if your mix will match without putting mortar into the joint there is a way. I take several mixes and make sure I'm documenting them in my note book. Take some mortar from each mix and roll it into a cigar shape. Be sure you roll them very tight so there is not air inside the sample. Put the samples on a cookie tray and bake them for about 30 minutes at 450 degrees. Whatever the color is when you remove them will be very close to the normal air cured color of that sample.

Here are some sources:

David Arbogast

Architectural Conservator

Mortar, Stucco, Paint & Plaster Analysis

Davenport, Iowa

Ph- 573-355-1553

www.paintanalysis.biz

 

US Heritage Group

Mortar analysis & pre-mixed and ready to go supplier of lime putty mortar

Chicago, Illinois

1-773-286-2100 

www.usheritage.com

Mortar

HI Bob.  Good article.

For stone foundation pointing, type N mortar is great, and for below grade work (basements), I even suggest using an acrylic adative for greater resistance to water.  For repointing brickwork, however, I would suggest that you try type O mortar (and never use the acrylic addative).  It has more lime and is lower in strength than the type N.  Only thing is, it is a little more difficult to find.