The Practice of
Tempera Painting - Carriers & Grounds (cont.)

Take the scraper in both hands, with the burred edge toward you, your thumbs [p. 30] planted firmly in the middle of that side, and the ground edge away from you. Hold the blade vertically, and draw it toward you. If the edge has a good burr, it will scrape a rectangular shape of white in the gray ground. Do not try to take off too much at each stroke. Low spots in the gesso will show as gray surrounded by white, and you must work down to them gradually. The way to do this is to work around them with a sort of crisscross, herringbone motion. Do not work over one place too long, but move on and come back to it.

For scraping the moldings, you will need some curved scrapers, but not so many shapes as you might suppose; for by turning the scrapers a little from side to side they can be made to adapt themselves to various curves, flatter or steeper than those that they fit in their normal position. An old saw blade makes good stock for these scrapers. Draw the temper, by heating it red hot and letting it cool gradually. Break it or cut it into pieces of various sizes, and grind these pieces on a grindstone to curves of likely shapes, sharpening them like a chisel, and making the edge perfectly smooth on an oilstone. Then put a slight burr on them, as before, and restore the temper by heating and plunging into water. As the hollows of the moldings will generally not be burnished, they need not be made so perfectly smooth as the tops. For very quick, rough work, the tops of the moldings may be smoothed down simply by rubbing with a [p. 31] wet cloth; but that is not a method which can be recommended when any refinement of form is a consideration. [pp. 3o-32]

Stoning Down a Flat
A plain panel without moldings can be smoothed down very quickly with a stone, and the result is not obviously inferior to scraping. Take a small whetstone, say two by three inches, and dip it into cold water. Wet part of your flat gessoed surface with cold water, and immediately grind the stone over it with a rotary motion until it slides freely over the panel. Wipe off the softened gesso with a sponge or damp cloth, and treat another part of the flat in the same way. Do not work too long over any one part, or the stone may cut way through the gesso as the moisture penetrates it. The panel will dry sufficiently in a few hours to be sandpapered lightly, and will come out perfectly smooth and even. The manipulation of the stone requires a little skill; and if the slightest grain of sand or grit finds its way under the stone, the gesso may be marked very injuriously. For all ordinary purposes, however, this method is quicker and easier than scraping, and quite good enough. [p. 32]

Beginners are often tempted to try to smooth the gesso down with sandpaper. If the gesso is as hard as it should be, this is extremely slow and laborious. Gesso can be made softer by the use of weaker size; and this softer surface can be sandpapered down easily enough. But it is not fit for gilding, and not so good for painting. Sandpaper is useful for smoothing up after grinding with the stone. It need not and should not be used after scraping. If you use a sandpaper at all, fold it around a block of cork or wood, and work over the surface of the panel rather lightly and always in straight lines. Twisted into a cone sandpaper is useful for reaching into corners and hollows which are hard to get at with the scrapers.

When the panel is all scraped and smoothed, moldings and all, you should take a soft sponge, squeezed out in cold water, and sponge over the whole surface quickly. This will remove all traces of loose dust, and leave the panel ready for drawing. If the work has been done well, it will be a pleasant sight, all pure, soft white, silkily smooth, as matte as cut ivory, and as crisp or melting in its contours as you have cared to make it. Chamfer the edges a little, to keep the gesso from being broken. [pp. 32-33]

Qualities of Good Gesso
Good gesso is perfectly even in color and texture, free from gloss, unblemished by scratches, pinholes, or flecks of any foreign matter. It is so hard that it can just be [p. 33] scratched with a fingernail. If your fingernail marks it without scratching, your gesso is harder than it need be; if it scratches it very easily, like so much chalk, the gesso is too soft for good gilding. Sandpapering will give good gesso a slight eggshell gloss. If it makes it really shiny, the gesso is too hard. If sandpapering makes the gesso dust away easily, it is to soft. If you have followed directions exactly, you will find that your gesso is exactly right. For good gilding, the ground must be perfect. For painting, a little leeway may be allowed; but if the gesso is not very nearly right, it should not be used for any serious painting. To take off a bad gesso, pile damp sawdust on the panel overnight, and the next day the gesso will be soft eough to scrape off with a spatula. [pp. 33-34]

Pastiglia
Any patterns which you wish to work out in low relief on the panel, either under the gold or under the painting, may be executed at this point in pastiglia. This useful technique consists in building up reliefs with the warm gesso mixture, applied with a brush. It is particularly useful under guilding, as it produces rich repoussÚ effects; but one may have occasion to model up figures or accessories in low relief on the ground in the painting proper. To do this pastiglia, draw whatever forms you want on the smooth gessoed panel, and apply liquid gesso with a brush, until you have a sufficient body of gesso in place. It will shrink somewhat in drawing; but there is generally a tendency to make the relief too high, so this is no disadvantage. When it is dry, it can be scraped and tooled to any desired degree of finish, and will give the effect of relief carving on the surface. It is a little easier to see what you are doing if you put a little bole or Indian red into the gesso mixture [p. 34] with which you work out these pastiglia ornaments. If you want to make an even line in pastiglia, it is best to make a thin core at first, of the necessary height, putting on a thin line at first, and building it up with repeated coats. As each coat sets, the next may be fed on from the brush. When the central core is high enough, gesso can be flowed in at the sides to make the required width and roundness. [pp. 34-35]

Gessoing Frames
A word about the preparation of frames is perhaps in order here. If the frames are separate from the panels, they should be sized and gessoed in the same way. They may be smoothed down quickly and easily with pumice templets, and these are best made before the gessoing is begun. Take a piece of natural pumice, and saw it and file it roughly into the shape required to fit over the molding of the frame. It is often necessary to cut two or three pieces, or even more, to fit the separate members of a complicated molding. Lay a piece of sandpaper, rough side up, on the bare wood of the frame, and rub the pumice over it until it fits smoothly and exactly. In applying the gesso, put on a brush coat first; let it dry dull; and then apply a tap coat, following it immediately with a brush coat. Then take your templets and "draw up" the moldings, from end to end, so as to make the gesso as smooth and even as possible as it is applied. When this has dried dull, put on another tap coat and brush coat, and draw it up again in the same way. This will usually be enough; and when the gesso is thoroughly dry, it may be stoned down with the pumice templets dipped in water, and finally smoothed off with sandpaper. Carved frames should, of course, be scraped with tools. A little carving is often an improvement to a stock molding; and as the gesso is fairly thick, the carving may be [p. 35] quite crude, and finished in the gesso with tools. Fine carving should be gessoed with gesso sottile, so as not to obscure the details. [pp. 35-36]

Cennino's Gesso
The process of gessoing which Cennino describes calls for different materials from the foregoing. The whiting which we have been using is a fine powder of chalk, calcium carbonate; Cennino uses calcium sulphate, which has quite different properties. Calcium sulphate is found in nature in the form of alabaster and gypsum, and from these plaster of Paris is made by burning the natural, crystalline minerals, and driving off part of the water upon which their crystalline structure depends. This roasted material, plaster of Paris, will crystallize again if it is mixed with water, and set to a familiar hard white mass. If it is mixed with size, it sets more slowly, but the resulting material is very much harder and less brittle than the ordinary plaster cast. Cennino uses this mixture of plaster of Paris and size as a basis for his final gesso, and it is an admirable material for his purpose. [p. 36]

Parchment Size
Cennino uses parchment size, not ready made gelatine. It is quite easy to make your own gelatine from parchment, exactly as he describes, by soaking parchment cuttings overnight, and boiling them the next day until the water in which they are boiled has cooked down to a third. As long as there are plenty of parchment cuttings, it does not seem to matter just how many there are. This makes a very fine and flexible size, but it is always somewhat uncertain as to strength. It should be much stronger than the gelatine solution used for the whiting gesso: one ounce of gelatine dissolved in ten ounces of water gives an equivalent solution of the right strength.

The panel is first sized with a mixture of equal parts of this one-to-ten gelatine solution and hot water. When dry, it is sized with the solution full strength, preferably two or three times, with intervals for drying. Linen is then applied in pieces with the full strength size, and allowed to dry for several days. This series of sizings is intended [p. 36] to penetrate the wood deeply, and to stop its pores entirely. The linen, its pores again entirely filled with size, provides a good tooth for the gesso, and insures against cracks in the wood. [pp. 36-37]

Gesso Grosso
Grind some pure, fine plaster of Paris with the one-to-ten gelatine solution to a stiffish paste, and apply this paste to the flat of the panel with a spatula. The best spatula for this purpose is a wooden slice, a chisel-edged slip of hard wood. Work the gesso well into the grain of the linen, and then spread a thick coat of it over the whole of the flat with the edge of the slice, leaving it as smooth and even as possible. Then warm some of the gesso, to liquefy it; and apply it to the moldings two or three times and pass two or three coats also over the flat which you have already done with the slice. Allow two or three days for this gesso to dry.

Cennino calls this "gesso grosso," thick gesso. It goes on thickly, makes a thick, hard, solid coating. It is rather coarse grained, and not particularly smooth, even when it is scraped down. Scrape it, however, when it is dry, and get the surface and the moldings as smooth and well shaped as possible; for the remaining operations of gessoing will not change them very much. [p. 37]

Gesso Sottile
The next step is to apply eight coats of what Cennino calls "gesso sottile," thin gesso. The basis of this mixture is slaked plaster of Paris. To prepare it, fill a wooden tub or cask with water, and sprinkle into it a pound of plaster of Paris for each gallon of water the container holds. Stir it as you add the plaster, and go on stirring it for fifteen minutes. Then stir it every quarter of an hour for two hours. By that time, the danger of its setting will be over. Cover the container, to keep out dust, and leave it for a month, stirring it every day and adding more water if it is needed. If the water which [p. 37] stands over the plaster is not perfectly clear and clean, pour it off, and add fresh every day until it becomes clean on standing. At the end of a month, pour off as much water as you can, dip some of the material out on a cloth, and wring it dry. Make it into little cakes in this way, and put them aside to dry. [pp. 37-38]

Mixing the Thin Gesso
When you are ready to mix the thin gesso, gesso sottile, soak some cakes of this slaked plaster again in water, and grind them fine on the slab with a muller. Put the ground gesso into a cloth as you grind it, and wring it as dry as you can. Take the gesso out of the cloth, and shave it up into fine slices with a knife, putting them into the top of a double boiler. Pour over them a little of the one-to-ten gelatine solution, and mix the size and gesso thoroughly together with your fingers. Add more of the size, and continue mixing. It is a sticky buisness, but there is no other way. Add size until the mixture reaches the consistency of batter. It will not be nearly so liquid as your whiting gesso. Try it with the brush as you mix it, and when it works smoothly and easily, it is properly mixed.

You must take the same precautions as before against overheating this gesso. As long as it is just barely liquid, it is warm enough. Put a coat all over the panel which you have prepared with the thick gesso and smoothed down, and rub it in with a circular motion of your hand. This coat will dry rapidly, and you may then proceed to put on, as Cennino says, "at least eight coats of it on the flats." You may do with less on the foliage ornaments and other reliefs" such as the moldings; 'but you cannot put too much of it on the flats. This is because of the scraping which comes next." You will find that this gesso is so thin that it is almost transparent as it goes on, and shows very little tendency to choke up fine detail. This thinness is a very great advantage in gessoing carvings or fine moldings, and also for executing delicate pastiglia later on; but it needs a carefully finished surface under it [which the first thick plaster provides]. On a perfectly smooth wooden panel or composition surface it is possible to dispense with the thick plaster; but ordinarily it will be needed. [p. 38] This thin gesso is, of course, too delicate to be ground down with the stone. It must be dusted with powdered charcoal, and scraped. [pp. 38-39]

The Perfection of the Method
This method produces the finest possible ground for painting of gilding. It is brilliantly white, far whiter than a whiting gesso, and exquisitely smooth and perfect. It is a slower method than that which we described at first, a good deal more laborious, and for a beginner much less certain to produce a good result. It should be learned, however, by any serious tempera painter, and used whenever time and cost permit for works of any pretension. It would be pleasant always to use it; but whiting grounds are very satisfactory and will probably continue to be used. As far as technical considerations go, they are about as good as these fine plaster grounds, except where the design calls for fine carving or delicate work in pastiglia. You can work pastiglia on a bold scale with whiting gesso; but fine work has to be tooled afterward, whereas with gesso sottile it can often be worked out perfectly with the brush, and not touched with a tool at all after it is dry. [p. 39]