Preparing and Laying Out Basement Plaster Project
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| Jail Basement Wall After Preparation Work |
One of the main plaster projects of the Fall semester was applying a traditional three –coat lime plaster finish to the basement wall in jail. Upon analysis of the existing plaster finish, we found that the plaster was a very lean mix meaning that there was much more sand than binder. This was primarily because the substrate was very porous bricks that cause a lot of suction and pulls the moisture from the mix. Hammers and chisels were used to demolish the existing plaster in order to prepare the surface for a new finish. The plaster was removed fairly easily because it had been turned into sand basically from capillary action stemming from moisture movement from the ground. After demolition was completed, the room was laid out to determine the face of finish for the new plaster. We started laying by measuring four feet inward from both the North and South ends of the room and found the overall length of the room. The next step was measuring horizontally from East to West and the average distance was taken to find the center line of the space, and this measurement was scribed into the floor. We then used a board and pencil as a compass to square the room and found the midpoint by using the center line method.
Gauges, Dots and Screeds
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| Plaster Dot |
In order to find the face of finish for the basement project, measurements were taken from the center line of the room to approximately 1/2” from away from the East and West walls at several points along the length of the room. This information provided the thickness of the scratch coat and the depth of the gauge to set the dots. We found that after averaging all of the measurements that a depth of 1” would be sufficient for both sides of the room. Once we had the depth, a thin board was marked with measurements to form our gauge. The gauge was placed on the floor and each measurement mark was held on the center line and pushed a mixture of natural cement and sand against the wall to form our dots.
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| Screeds Transferred Up the Wall |
The initial mix for the dots was 1:1 Vicat Prompt natural cement and sand. We found that this mix went off a little to quickly to work efficiently, so the mix was modified to 2 parts natural cement to 1 part lime to retard the setting. This mix worked much better but because the substrate had such a high suction signs of cracking were seen due to the addition of lime. Once the base dots were allowed to set, we used them to transfer additional dots vertically up the wall using a straightedge and level and a piece of particle board coated with Crisco was placed on each dot.
After the dots were set, the walls were soaked with water and the same mix was used to connect the dots vertically forming the screeds. The screeds were applied by hawk and trowel and pulled off using a straightedge and level resting on the previously placed dots. It is important to accurately set the dots and screeds because they will act as guides for applying the scratch, brown, and finish coats. The screeds were allowed to set and any jagged left by the rodding process were scrapped away with a margin trowel. We soon found that it was more efficient to rod the screeds evenly the first time to save time and have a more accurate guide rather than having to scrape away imperfections later.
Three –Coat Plastering of Jail Basement
Traditional three –coat plaster consists of the scratch, brown, and finish coats with scratch being no more than 1/2” thick. The scratch coat is primarily used to build up material and to provide keys, or points of attachment, for the following two coats to adhere. In the jail basement project, we took the data gathered during the previously mentioned mortar tests and designed a 3:1 sand to NHL mix by weight. This mix was converted into a volume ratio of 5 gallons sand to approximately 3 gallons NHL because it is more practical to use volume measurements during the production phase.
The scratch coat was applied at a thickness of 1/2” which left about 1/2” to 3/8” of space below the screeds. It is important to really apply to first coat very tightly to ensure that the plaster is forced into all crevices in the brick substrate and provide a strong key on which material can be built up. Once the material was applied to the pre –soak wall, rods or straightedges were used to pull away excess material and scratched using a scarifier.
After the scratch coat was allowed to set, there was minor cracking due the moisture being pulled from the lime by the porous bricks, but this did not present a much of a problem. The scratch was again soaked with water to provide suction, and the brown coat was applied at a thickness of approximately 3/8”, which provided about 1/8” of space below the screeds for the finish coat. We wanted to make the brown coat truly “brown,” so okra pigment was added to the mix to give us a yellowish –brown tint as seen in figure 2. The finish coat of lime plaster was applied tightly with a trowel at a thickness of 1/8” and pulled off using a featheredge. While the finish coat was still a little green, the surface was worked with a sponge float to give us a slightly textured, uniform plaster finish.
American Clay Finish on Jail Basement
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| American Clay Finish |
American Clay product was applied to the previously lime plastered walls in the jail basement which provided an earth tone finish. This product provides a natural, environmentally friendly alternative for interior finishes. Since the product is natural clay, it can easily be worked with a damp sponge float at pretty much anytime to repair any blemishes on the finish coat.
In our project, we used a mix consisting of one 50 lbs. bag of Loma clay to one 50 lbs. bag of Enjarre. We also added to one packet of Osage coloring pigment to produce a light grey finish. The clay was applied and leveled with a straightedge similarly to lime plaster. We noticed that the mix need to be fairly wet to prevent the clay from sticking and provide greater workability. It was very easy to let the mix dry out a bit too much which made it very difficult to work and produced a marred finish. Once the clay was applied to the wall, a felt brush was dipped in water and lightly run over the clay. This process was followed by a plastic trowel finishing trowel immediately after brushing. Teams of two with one person working the felt brush and the other the plaster trowel, were sometimes employed to provide a systematic approach to finishing the clay. The purpose of the plastic trowel is to tightly pack the clay and create a smooth, soft texture. It is easy to realize when the area is finished because the tone of the trowel will change from an almost grating sound to a low whistle.
Stuc Pierre Wall
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| Stuc Pierre Wall in the Stone Shop |
During the week of the Master of the Building Arts Festival, Patrick Webb instructed the trowel trade students on how to create a stuc pierre wall. Stuc Pierre is a gypsum plaster that is meant to simulate limestone or bricks. The brick wall in the stone shop at the jail was given a brown and scratch coat of various plasters that were available. After these coats set, screed points were applied to make a guide for leveling the remaining wall. The wall was then coated with a fairly thick layer of gypsum plaster. Once this coat was pulled off with a straightedge and allowed to set, a chalk –line was used to layout the ashlar stone pattern. These lines were scribed in, and using a straightedge as a guide, a plaster tool was used to cut 3/8” joints in the plaster wall. Molding plaster was applied to the joints with a putty knife or trowel and allowed to set. The entire surface was then shaved using a straight blade plaster tool. This process smoothed the plaster, allowed the joints to come to the surface, and gave the finished product a cut stone appearance. A sting course molding is to be installed later.
Running Plaster Moldings
Running traditional plaster moldings begins by designing and cutting out the knife profile. Zinc or aluminum are good choices for the knife and can be cut using a scribe, tin snips, or scroll saw. The scribe and straightedge method produces a very clean edge that can then be filed and sand for optimum results. Though, running is the only true test of whether the knife is top shape.
Once the profiles were cut and filed, granite counter tops were used for running the USG #1 gypsum molding plaster. It is important to use soapy water on the counter tops so that the plaster does not stick. The muffle is then attached to the running mold and builds the core.
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| Core that has been coated with shellac and release agent |
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| Zinc profile/knife |
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| Section of molding that was cut with the knife |
Pointing Plaster Moldings
While it would seem like a very simple process, pointing is an art in itself. Pieces were cut from lengths of molding made previously, and were attached to a section of drywall for pointing practice. The pieces were attached with Durabond as the binder, and they were leveled by using a straightedge to check for flatness. A joint of approximately 3/16" was left to accommodate expansion of the plaster as it sets. Once the pieces were bonded to the drywall, molding plaster was mixed in a flexible containers. Water that was retarded with citric acid was used to mix the plaster, as well as the option of rabbit size. Plaster is then placed in the joints, and worked with a rectangular piece of metal, half -moon piece of metal, and clay tools to get the required shape. A thinner mix was used to finish the pieces after all major building was done. Light sanding can remove small burs or imperfections after the plaster has dried.
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