INTERFACING A DRIED GESSO, ILLUMINATOR’S BOLE,  
                         JERRY TRESSER [email protected] www.jtresser 561 810 8956 


This process, called Interfacing comes from one of the chapters from The Technique of Raised Gilding. 1992, Michelle Jordan Publication. The author: Jerry Tresser

 How to add wet gesso to dry gesso, or how to add a second layer of Illuminator’s Bole to a dried bole size. This would probably work well with any dried size that needs to be increased in size or remedied because of an unsightly drying process.

 One of the problems when working with gesso that has been made as an illumination size which has slaked plaster is that for whatever reason (mostly related to humidity) the gesso often dries in different heights on the same item or its neighbor, being gessoed. This can be a problem if you are writing letters or creating different objects that contain the plaster-based gesso.

 The question becomes: how we keep everything at the same height once dried. More importantly, if the gesso dries and the heights are uneven how do we add more gesso to make sure the objects are raised if necessary and uniform in height? 

 One thing you should not do is add water to soften the gesso, or just add more gesso. With a slaked plaster base, you CANNOT add something wet to something dry. So just adding some more gesso to increase its height is out! Nor should you add any water to soften the gesso. That would be disastrous as the water would seep down the gesso and destroy or weaken the foundation of the plaster.

  The answer is rubbing alcohol. Commonly call Isopropyl Alcohol. The term is called Interfacing.  You may need to sand the surface of what is being raised with very fine sandpaper. Basically to eliminate any facial imperfections before applying the alcohol. When ready, just take a brush dampen the gesso lightly with the alcohol and immediately add the fresh prepared gesso

. Alcohol dries almost immediately on contact with the air. But allows enough wetness to allow attachment with something dry to something wet.  If additional layers are necessary, allow the gesso to dry completely and re apply the process again. This type of surgery is done in the application stage. Here is where we fix things. a WORD OF CAUTION

. Illumination gesso is like a turtle. A hard-outside shell but a soft inside body, so allow the gesso time to dry before adding any additional layers. Or you will kill it..   

 When using Illuminator’s Bole, the process is the same with one exception. Heat up the bole on hot plate to get it warm (not hot)  and then apply it  with a brush to the dry bole after swabbing it with alcohol. This is the proper way to use a raising preparation for all sizes… 
OUT OF PRINT  ONLY AVAILABLE IN A PDF FORMAT
.Let’s talk about Gesso. Some of the important factors when dealing with this product. First, when I say Gesso, I mean the real slaked plaster of Paris. What you have in the Liquid Gesso is pure reagent grade slaked plaster. Exactly as the formula called for in the 14th century, ours is 99% pure.  

The best way to work with Gesso when applying, is by stirring when being used. I mean if your using a brush, a nib, a ruling pen, always give the plaster a stir and then apply, re-stir and add more. This is critical. Why you ask?

By its very nature, slaked plaster wants to dry. The liquor in the mix that you receive or even in the original formula, which was reconstituted with Glair, will act the same way. It wants to dry. By NOT stirring, it will begin to harden. So constant stirring is a central part of the application process. The liquor helps preventing that hardening. Loose the liquor and the plaster will set. 

Once exposed to the air outside of the liquid, the Gesso begins the drying process to harden in 9 seconds. You can see what we are dealing with.
It is for this reason, one must be prepared, with tools in hand to work diligently in this application process. As I indicated, by the very nature of the plaster, it will begin to settle and dry even if your working on it. So, you need a plan of attack to complete whatever your project is as a one step process. 

What you do NOT need is to have the Gesso dry above if your working below on the same object. That could be a problem, especially if your attempting to write or apply some form of lettering. Dried Gesso can be added to Gesso if necessary, if you have to stop. Contact me on knowing how to interface plaster. 

True slaked Gesso is Calcium sulphate dihydrate. Unslaked is Calcium sulphate. What’s the difference? Aside from different crystal structures, slaked plaster dries slower than the unslaked plaster. That’s about it. 

For building walls, unslaked is a great product, but for illumination we need a bit more time, not to mention the amount we are using is minuscule in the application process. In both cases, the plaster does what its intended to do. You just have to be aware of its chemical characteristic.

       JERRY TRESSER   www.jtresser.com   [email protected]   561 810 8956

  UNSLAKED               SLAKED