Thursday, December 10, 2009

21_Supports and plastering

We painted on Hardibacker cement board, industrial foamboard, and ceramic tiles. However our frescoes still tended to crack more often than not, regardless of the support, perhaps because we applied just one coat of plaster. For a sounder fresco, we should probably apply more coats, and follow a more successful preparation procedure.

First fresco, done very casually. Overly wet plaster on bone dry Hardibacker cement board:

We are still looking for the perfect rigid, rough surface, to paint frescoes on; one that absorbs a bit, but is not too thirsty. The board should be slightly moist, to accept a plaster paste that is almost dry, in order to keep the fresco from cracking. According to Olle Nordmark, the top intonaco plaster should be stiff enough to offend a professional plasterer, who is accustomed to a wetter stucco that spreads fluidly onto walls. Thus the wrong, dry support will quickly wick out the precious water from the stiffer plaster, and lead to broken masterpieces. When the plaster dries too fast, it shatters into the ragged mosaic pattern of a dry desert lake bed. Even if the board and plaster are a "near match," the small moisture disparities can still promote hairline cracks. Our frescoes usually suffered something in between, kin to tiny canyons that broke up the painting into unpredictable compositions, which either endured an "aging effect" or lost to an "unhappy accident." Ultimately, the goal is to keep the lime/sand plaster from drying too fast and cracking -- by using "any support necessary."

The solution is probably to use at least 2 coats of fresco plaster, to keep the final fresco from cracking, as shown in this NOVA demonstration on how Michelangelo prepared the fresco in the Sistine Chapel. I have some fresco surfaces that dried before we could paint on them, and could easily add another coat of plaster on top (after wetting the underlying surface first). However, I have not done that yet, and thus do not know how such a second underlying plaster coat would help us:

We would surely suffer less cracking by following the example of Frederico Vigil, and apply 5 coats of fresco plaster, in the tradition of Stephen Pope Dimitroff and Lucienne Bloch, assistants to Diego Rivera. This process is detailed by Ilia Anossov as he explains how he prepared the Albuquerque Fresco. The problem with 5 layers of plaster is that it adds a lot of extra weight to fresco panels, turning them into sculptures.

If we stick to the "one-layer plaster-on-panel" method, we might think "thicker is better," and hope that a fatter wet plaster will keep our frescoes from cracking.


Hardibacker Cement Board

We prefer plastering and painting on 1/4 inch Hardibacker cement board, ever since Gonzalo suggested this support. It is light, cheap, and easily found at Home Depot.

One can score the Hardibacker board with a box cutter, or carbide tipped scoring knife, to break the board down into manageable tile-sized pieces. Or cut it with a power saw, after wetting the board first, to keep from raising hazardous dust. Mikee uses a water saw to cut Hardibacker cement board:

Before painting, we first soak the board underwater for at least 30 minutes, in distilled water:

Bubbles form after the cement board soaks for a while. If the board soaks long enough, the bubbles disappear:

The glistening wet board has to dry matte before we can plaster onto it:

When matte, we trowel one layer of plaster (intonaco) onto the grid side of the Hardibacker board, over the squares raised in relief. Likewise, the glistening plaster has to dry matte, before the artist can paint on a firm ground:

When we plaster after soaking the Hardibacker for only 30 minutes, the board may continue to outgas (if that is what is happening) and form bubbles in the plaster, separating the plaster from the surface. We re-trowel over the imperfections:

When the masterpiece is finished, we seal the Hardibacker fresco in a plastic box for 3 days. The object is to keep the surrounding air humid, in order to slow down the drying, and prevent cracking. This fresco did not suffer cracks:

Unfortunately, most of our other Hardibacker frescoes cracked. Either the cement board was too dry, or the plaster too wet. The exception was when those 2 felonies offset each other, and a small fresco tile would dry slowly enough to avoid fractures. The key seems to be matching the level of water saturation of the Hardibacker board with the correct consistency of wet plaster, an inexact science.

The larger frescoes probably cracked because the supports were too dry, and wicked the water out of the wet plaster too fast. I should have soaked the Hardibacker much longer than an hour:

Our smaller tile-sized frescoes cracked, I suspect, because the plaster mix was too wet. The final intonaco plaster has go on like cream cheese; and ours was closer to cottage cheese, bloated with water. When the applied plaster has too much water, the excess evaporates too quickly, causing the leftover volume to shrink traumatically. Shrunken pieces pull away from each other before drying, and can leave unsightly cracks in the final painting.

The basic lime putty (which we slaked over 6 months) was probably too moist, even though I reached deeply into the bucket to scoop out the most solid portions. Ironically, when adding sand, the mixture turned coarse and dry immediately:

However, after further mixing the "1-to-1" lime/sand mud, by working it with a trowel on a plastic cutting board, our final plaster turned wet again. It reluctantly clung onto the trowel when lifted over the mixing tub:

I emphasized the cracking damage, with an experiment that went wrong. I thought I could both retard the drying, and extend the painting time, if I soaked the Hardibacker tile overnight, instead of just 30 minutes. Thus I applied our water-bloated fresco plaster onto a thoroughly saturated cement board. The experiment worked too well, as the surface refused to dry matte. So I had to "cook" the wet tile on the dashboard of my car, in the harsh Tucson sun, in order that Rebecca could paint a fresco on it before the model left the 3 hour session:

The Hardibacker tile soon dried soon enough to paint on, but at the expense of large cracks:

During the UofA demonstration, I set other Hardibacker tiles in the sun, , trying to dry them to a matte surface. I only soaked those tiles for about 30 minutes. The plaster dried and shrunk so quickly, that it lifted itself off the underlying support:

I thought about squeezing the plaster though a nylon mesh cloth, to remove some of the excess water from the mix. However, I have not yet tried that:

We mixed our lime putty with 30 grit Superior Marble sand (lime and cement sand). A decade earlier in Albuquerque, I used silica sand. Perhaps the plaster mix would be drier if we used a finer, 70 grit sand:


Marble Dust and Sinopia Pit Lime

I experimented with a stiffer plaster, on a saturated Hardibacker cement board, which I soaked overnight. Then I mixed some pit lime with extra fine marble dust, both of which I mail ordered from Sinopia:

This plaster was a lot drier, and applied smoothly like cream cheese:

I troweled the smooth Sinopia plaster onto the saturated Hardibacker board, painted a color chart on it, and left it to dry in a covered plastic box. This fresco dried to a strange, mottled surface:

This surface did crack, but not as badly as those made with our homemade lime putty and 30 grit sand:

The first time we experimented with the Sinopia pit lime and marble dust plaster, I only soaked the Hardibacker cement board for about 30 minutes. The board was surely too dry, because this fresco started cracking immediately, while Gonzalo was painting it:

I am very partial to employing cement board as a fresco support. Likewise, Stephen Bennett in Corrales uses Durorock, a cement board similar to Hardibacker. However, marrying the Hardibacker board, with the correct consistancy of fresco plaster, is a tricky process which we have not yet mastered. Again, we can probably avoid cracking if we add additional plaster undercoat(s) to the cement board.



I bought some industrial foamboard at Home Depot -- Easyboard -- because I heard about artists using foamboard as a fresco support:

I expected a foamboard fresco to dry more slowly, figuring that the plastic filler would resist water, rather than wick it away from the wet plaster. Thus I did not dry my foamboard fresco in a sealed plastic box overnight, as a precaution, but let it dry in the air instead. The fresco did develop some cracks, but not as many as I might have expected:

I worried that the fresco would fall off the foamboard in time, because the industrial foamboard surface was smoothish, and did not give the plaster much to hold onto. Therefore, I primed another foamboard surface with an acrylic gesso first (Golden Sandable Hard Gesso):

The next day I troweled some fresco plaster onto the gessoed foamboards, to paint color charts:

The foamboard support seems to work well enough. It is worth pursuing, if only because it is very light, and would easily hang on a wall.


Fresco on Canvas

Reid showed me a flawless fresco he bought in Germany, which now hangs on his wall. It seems to be plastered over a piece of burlap, which was pulled tightly over stretcher bars:

You can see where the plaster coating did not quite come down to the edges:


Fresco on Ceramic Tiles

Jose Galindo painted on Saltillo tiles, which he sprayed well with water before plastering, when he was teaching the fresco class at the Mission Cultural Center in the 1990s, in San Francisco. The low fire tiles were porous enough to accept some water, but when wet, did not wick too much moisture out of the fresco plaster to cause major cracking. Jose also used sand and Sinopia pit lime, I believe.

I repeated the same process with JP in Albuquerque, painting on the Saltillo tiles I bought in Old Town. We slaked a sack of lime we bought at a local tortilla factory, and mixed it with silica sand. To my distress, I later saw the fresco separate from the tile, and break up in pieces. Of course, it did not help that that fresco tile had been vibrating about for a while, in the back of JP's car.

I wanted to rough up the surface of the Saltillo tiles, in order to give the fresco plaster something more substantial to hold onto. Therefore, when we started this fresco project years later, in Tucson, I asked Gonzalo to help me make some custom tiles, with textured surfaces, which we could use as fresco supports.

We used a clay with high sand content, which Gonzalo cut with a wire:

He beat the clay down flat, into a tortilla:

He ran the clay through the slab roller at 1/2 inch height setting:

Jeremy welded a cookie cutter for punching out square tiles. The tiles were just short of 12 x12 inches, so that they could fit in an airtight plastic box, the Masterson Palette Seal:

Peeling excess off the wet tile:

Texturing the clay surface with a metal bristle brush:

We placed the wet tiles on 12 x 12 inch sheetrock squares, and covered them with plastic to dry. Any tiles that warped the next day were still wet enough to press down flat:

The tiles were still not dry 4 days later, cold to the touch:

Thus we set the tiles out in the sun to dry faster:

Placed the tiles into the kiln, to be fired at cone 04:

Gonzalo turned the kiln on, but cracked it open for an hour first, to further dry the tiles:

A smoother commerical Satillo tile to the left of one of our fired, textured homemade tiles:

This fresco was painted over the homemade textured tile. Being just our second effort, the plaster was way too wet to apply, almost runny. I wrote the experiment off as a total disaster, but was pleasantly surprised when Gonzalo managed to paint on it the next day. Apparently fresco is more forgiving than I imagined:



Adobe bricks at the Presido in Tucson:

Perhaps a fresco should best be painted on a wall, as is traditional. In the Southwest, an artist might paint frescoes on the walls of old adobe homes -- or on their contemporary counterpart, the earth bag houses, made from Cal-Earth bags. Some of these earth houses have been white washed inside with lime, which repells the insects (a nice windfall). Why not go one step further, by lime plastering the walls and painting fresco on them?



We applied the fresco lime with the variety of trowels shown below, and also a putty knife. Mikee started with the instruments at the right, the large stainless steel trowel and floater. He finished with the round trowel at the left, because it leaves no ridgids in the middle of the plaster:

Stephen Bennett in Corrales recommends small trapazoidal trowels.


  1. Great work!

    we use a regular ceramic tiles for our small 16X16 frescoes. if applied properly they will never crack, will be paintable in 10-15 min and will stay paintable for ovr 3 hours. We also have a step-by-step real time dvd on how to do it.

    Fresco School

  2. chavezcesar9860@yahoo.comFebruary 9, 2012 at 3:17 AM

    Dude do four layers of plaster, I worked vigil and the sand u using is not right. I know the books call for marble sand but that's not what we used. The way u doing it is going to keep cracking

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