Jim Hingst explains what equipment and raw materials you will need to make gesso for encaustic wax painting. This article provides detailed instruction for preparing a substrate and then gessoing the painting support.
By Jim Hingst @hingst_jim
For oil and acrylic painting, the easiest way to gesso a support is to use a ready-made product. That way you eliminate the measuring, the mixing and the mess. Gesso in a jar, however, may not be absorbent enough for encaustic painting. Here’s why. Gesso in a jar is usually different from the real thing. While commercial encaustic gesso is on the market, in many cases, what is marketed as gesso is just acrylic paint.
At the very least, an artist
should know how to make gesso from scratch. There are many different recipes
that you can follow. A popular recipe is one that the fifteenth century Italian
artist Cennino Cennini describes in his book, The Craftsman’s Handbook (Il
libro dell’arte).
The recipe that I describe in
this story is similar to Cennini’s. It is one that I learned from Jill London of the Society of Gilders.
Equipment and Supplies Needed to Make Gesso
In making gesso, you will
need the equipment and supplies that listed below:
Hog bristle brushes –
Available in a variety of sizes and shapes, such as fitches, filberts and
flats, bristle brushes are used to coat
the gesso and bole onto the wood support.
Double boiler or crock pot –
Used to heat rabbit skin glue and gesso mixture.
Digital measuring scale –
When you are following a recipe, pay close attention to whether the ingredients
should be measured by weight or my volume. Digital scales are inexpensive, and
are worth the money for accurately measuring solid ingredients by weight.
Kitchen measuring cups and
spoons – You will need an assortment of kitchen measuring cups and spoons to
measure ingredients by volume. Liquids are usually measured by volume. Dry
ingredients are usually (but not always) measured by weight, requiring a scale.
Flour sifter or metal sieve
– Have you made gravy before? Nothing’s worse than lumpy gravy. The same holds
true for gesso. To prevent lumps in your gesso, add the whiting using a sifter
or a metal sieve.
Raw Materials Needed for Gesso
 |
| Rabbit Skin Glue Granules |
Rabbit Skin Glue (RSG) –
Rabbit skin glue has the longest straight molecular chain of any glue. This property makes it very elastic. Because
of its flexibility, rabbit skin glue is tremendously strong. It bends but doesn’t break. Available in pellets, granules, and sheet,
the RSG is mixed with water and heated to a temperature between 98.6° F to 120°
F. Don’t overheat the glue mixture or
you will ruin its adhesive properties.
(NOTE: Animal hide glue is not the same thing as RSG. Hide glue is too
brittle and will crack as the wood expands and contracts.)
Rabbit Skin Glue is very
flexible and strong. It is not the same thing as animal hide glue.
Whiting – In making gesso,
whiting, which is ground up limestone or chalk, is mixed with rabbit skin glue.
Distilled water – When
mixing rabbit skin glue, always use distilled
water.
Sieves & Paint Strainers
– After mixing and heating the gesso, it is important to strain the mixture to
remove any lumps. Paint strainers from your local hardware store are OK. Fine sieves, which you can buy at a culinary
store, work better.
Surface Preparation
If you
are applying gesso to a wood panel, proper prior preparation is important. A plywood
support should be perfect for encaustic painting. There are many different
grades of plywood at your local lumberyard. You will want to buy a sheet of the
best quality available. The highest level of quality is the “A” grade. I buy A-C plywood. The A side of
the sheet is sanded smooth. This is the side that you want to gesso. The C side, which has imperfections, is the
back of your panel. It does not get gessoed.
Suggestion: If you are buying a large sheet of plywood, which will be cut into several panels, have the lumberyard cut it for you. This will save you the time required to do the work and the cuts will be square and accurate.
Selecting the type of wood
is important, because a smooth, even grained wood serves as a good base or
footing for the foundation of gesso, which is applied over it. Plywood is not
the only option when painting on wood panels. It might be your best choice. It is
generally lighter, stronger and less expensive than solid wood panels. Other woods
which could be suitable for painting include basswood panels. This is a very
soft wood, with a very fine grain providing a smooth finished surface, after it
is sanded with a fine grit paper.
Typically basswood also does
not contain oils, which can bleed through the layers of gesso and bole,
discoloring the gild. Woods, such as
pine, contain oils, which are very problematic and, for all practical purposes,
make them unsuitable. Oils and sap in
the wood can bloom to the surface
causing the gesso to delaminate from the wood.
Durham’s Rock Hard Putty
Prior to the application of gesso, you should fill any nail holes and dings in the wood. One of the best products to use is Durham’s Rock Hard Water Putty. The putty, which comes in powdered form, is mixed with water to form a paste. Don’t mix the wood putty too thin. It should have some body to it. An important feature of the putty is that it dries fast – really fast. The advantage is that you don’t have to wait very long after applying it to sand it.
The saying is that you can’t judge a book by its cover. The same holds true for judging a product by its packaging. The design on the can of Durham’s Rock Hard Water Putty is flat out cheesy and out of date. But what’s in the can has stood the test of time – since the Depression.
Durham's Water Putty bonds well to wood, dries rock hard and does not shrink, even
years after it has been applied.
Durham’s Water Putty is a
gypsum-based powder, which is mixed with water and used as a wood filler. When
the water putty hardens, it becomes, as the name on the packaging suggests,
“rock hard”. Work fast when using this
product, because it cures in short order.
One reason that craftsmen
swear by this product, is that bonds to wood like white on rice and doesn’t
shrink, even years after it has been applied.
When you work with Durham’s
Water Putty, only mix a small amount that you use within five to ten
minutes. Since the water putty will
harden rock hard, you may want to mix it in a clean tin can that you can toss
in the garbage when you are done.
The directions on the can
indicate that you should add one part of distilled water at room temperature to
three parts of the powder. This will
produce a putty with the consistency of bread dough. You can mix the water putty so that it is a
little juicier – to the consistency of sour cream.
Whatever you do, don’t use
hot water to mix up a batch of putty.
The heat of the water will accelerate the curing time, resulting in the
putty hardening, before you have a chance to use it.
Be sure to keep your greasy,
grubby paws off of the wood or you could contaminate the surface, compromising
the adhesion of the water putty to the substrate.
You can use a craft stick or
putty knife apply the putty, or you can use your fingers. After you press the
putty into the nail holes or cracks and crevices in the surface of the wood,
wait about twenty to forty minutes for it to set up. Small nail holes, should
be ready to sand within this time. If
you have filled a bigger hole with the putty, you will probably have to wait
anywhere from four hours to a day before sanding. Be sure to wear a dust mask and safety glasses
and work in a well-ventilated area, when sanding.
If you keep a tight lid on
the powder, Durham’s Water Putty will be good to use for years.
After filling in any
holes, sand the wood with a 150 grit
sandpaper and wipe the surface clean with a tack cloth. Sanding serves two
functions. Not only does it smooth the surface of the substrate, it also helps
to raise the grain of the wood, which gives the substrate some tooth for the
gesso to adhere to.
Preparing the Rabbit Skin Glue
In its dry state rabbit skin
glue is dehydrated glue. To rehydrate
the glue you must add water at a ratio of 10 parts of water to 1 part
glue. Below is the basic rabbit skin
glue recipe, that Jill London of the Society of Gilders recommends:
In a clean glass canning jar
with a lid add 28 grams of rabbit skin granules or pellets by weight. (Note: 28
grams equals one ounce.)
Add 10 ounces (or 1-¼ cups
by volume) of distilled water to the
glue, and cover the jar to prevent any contamination.
Wait at least 2 to 3 hours
for the rabbit skin glue to absorb the water.
Better yet, mix the RSG and water the day before you intend to use it and
allow it to soak overnight.
Before heating the glue,
strain the mixture using a kitchen strainer.
In a double boiler, heat the
glue/water mixture to a temperature between 98.6°F and 110° F. Carefully monitor the temperature using a
candy thermometer. You can also monitor the temperature of the water with your
hand. The water should feel warm. If the
water is too hot to put your hand in it, it is too hot for the glue. Excessively high temperatures above
120˚F will destroy the adhesive
properties of the glue. Continue to heat
the glue until all of the granules have dissolved and disappear. If you
overheat the glue, throw it away and start over.
Note: you can store the 10:1
RSG mixture in a refrigerator for up to two weeks. After that, the glue will
decompose and spoil.
Sealing or Sizing the Wood
After sanding, you need to brush
on a thinned coating of the rabbit skin glue, at a ratio of 20 parts of water
to 1 part RSG. Take one part of the 10:1
RSG and mix it with one part distilled water. This will give you a 20:1 RSG
mixture.
Before dipping your brush in
the RSG mixture, wet it and squeeze out the moisture between your thumb and
forefinger. Do not pull on the hairs of the brush, or you could pull them out.
Liberally coat the surface of the wood with the 20:1 mixture. This coating will raise the grain of the
wood, giving it some texture for the glue to grab onto. This is an important
step that ensure good adhesion of the gesso to the wood substrate. If any part
of the frame was not sized, you could
experience delamination of the gesso from the wood.
Some people have the
misconception that the glue forms a barrier preventing the transmission of
moisture between the wood and the layers of gesso and clay on top of it. Not at all. Instead the water-based coating
of sealer allows the wood to expand and contract as it gains or loses moisture.
Some people will brush on a
second thin layer of the 20:1 rabbit skin glue, applied about twenty minutes
after the first coating. The application of the second coating of glue helps
fill in some of the grain, if you are working with open grained woods, such as
oak and walnut.
The light layers of glue
also help bind the subsequent layers of gesso the wood. For smooth surfaces,
you can use a bristle brush, such as a flat or a fitch. Or you can use a sable brush.
Gesso
After you have sanded and
sealed the wood, keep your grubby paws off of the substrate as much as
possible. Oils from your skin can contaminate the surface, preventing good
adhesion of the gesso to the sealed wood.
As smooth as the wood
surface may be, it most likely has tiny imperfections. Left uncorrected, any imperfections,
regardless of how minute they may be, will be magnified, after highly polished
gold is applied. One function that the gesso serves is to fill in the chips,
cracks and other defects, providing a smooth surface for subsequent layers.
The main ingredient used in
preparing the gesso layer is calcium carbonate (CaCO3 ), which is found
naturally in limestone, chalk and ground up sea shells. If you have ever used whiting, that is
calcium carbonate. The calcium
carbonate is mixed with a binder, such as rabbit skin glue. When you are buying
whiting, caveat emptor (buyer beware).
Not all whiting is the same. The
grit of the product will vary from one part of the world to another.
To build up a perfectly
smooth surface, several layers of this mixture are liberally applied with a
brush.
In addition to the calcium
carbonate and rabbit hide glue, other components are added to the mixture. The addition of linseed oil, for example, can
improve the flow out of the gesso, yielding a smoother surface.
While gesso will fill in the
pores on the surface of the wood, it does not seal the wood. Rather, since the gesso is hygroscopic, it
allows the wood to absorb and release humidity as the atmosphere changes. This lets the wood expand and contract
naturally. Although the gesso expands and contracts at the same rate as the
wood, the gesso is not flexible material.
Gesso combines a filler,
which is the calcium carbonate, with a binder, which is the rabbit skin
glue. The strength of the gesso is
directly related to the ratio of the filler to the binder. That ratio is very important to how the gesso
performs. If too much filler comprises
the mixture, the binder cannot hold the mixture together and the gesso can
crumble when pressure is applied to it.
If the mixture has too much binder, the gesso becomes too hard. When
this happens the gesso can shrink and crack and can break away from the wood.
The ratio of rabbit skin
glue to whiting is critical in determining the hardness of the gesso. The more glue that you add to the mixture,
the harder the gesso will become. As you add more whiting, the gesso becomes
softer. If the gesso is too soft, it can crumble under pressure. The harder the gesso is, the more brilliant
the burnish can be. So why not mix the
strongest gesso you can? One problem is
that the harder the gesso is, the more difficult and time consuming it is to
sand. Even worse, if the gesso is too hard, it can become brittle and
potentially can crack.
The amount of water in the
gesso mixture does not affect its hardness.
It does, however, determine the viscosity of the gesso. Thin mixtures are easier to brush, and yield
a smoother finish surface with less brush marks. The disadvantage is that you
will need to apply more layers of these thin coatings to achieve the desired
thickness.
Thick mixtures require fewer
layers of gesso. If the gesso is too
thick, you will produce brush marks. If you see brush marks as you are coating
the gesso, you should thin the mixture with distilled water.
As mentioned earlier,
coatings are applied wet on wet. Subsequent layers are applied to previous ones
before the gesso dries. If the gesso dries before you coating the next layer,
you can rewet or dampen the surface with a wet sponge.
 |
| 6 to 8 coatings of gesso are necessary for a smooth surface |
Typically six to eight
coatings of gesso are applied to the wood.
After you apply the initial coating of gesso, you should apply each
subsequent coating before the previous coating is completely dry. This practice ensures good intercoat adhesion
between layers of coating.
During the very best of
conditions, it can take days for the gesso to completely dry. High relative humidity can prolong the
process extending the drying period to weeks.
Sanding the Gesso
After you
have coated the wood with gesso and have allowed to it dry thoroughly, you
should sand it. Any imperfections in the surface will often result in
imperfections in the subsequent layers of clay.
In turn, these imperfections will glaringly show up when you lay the
leaf.
In sanding the gesso, you
will typically use with 400-grit wet/dry sandpaper. In using the sandpaper dry, this very fine
grit will gradually level and smooth the surface of the gesso. Most beginners should sand dry. While
dry sanding can be a slow process, you are less likely to cut through the gesso
and sand right down to the wood. As you
sand the wood, don’t over handle the surface, because you could potentially
contaminate the gesso with skin oils. To
prevent contamination you could wear clean cotton gloves.
As you become more
experienced, you may want to try wet sanding.
This is a much faster process, but requires extra care so you don’t pull
the gesso off of the surface. After soaking
the wet/dry paper in water, lightly sand the surface.
When you are finished the
surface should be as smooth as silk. Carefully inspect the surface for any
imperfections. Because your sense of
touch is much more acute than your eyesight, rub your fingers over the sanded
gesso to find any rough spots. Also look
for any other defects, such as pinholes.
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About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.
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Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer. Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.
Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for publications, such as Signs Canada, SignCraft, Signs of the Times, Screen Printing, Sign and Digital Graphics and Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.
© 2015 Jim Hingst
The article is very informative, however I'm still not sure how much whitening to add. What are the actual or ideal ratios (binder to solids) for a good encaustic gesso or is it just a case of its consistency?
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