The Ravenna Method
BY MICHAEL KRUZICH
This feature originally appeared in Groutline, Summer 2014, Volume 15 Number 3, the quarterly journal of SAMA, the Society of American Mosaic Artists. Reprinted with permission.
- Temporary Binder: lime paste or clay (regular water- based potter’s or sculptor’s clay, not oil-based clay like Plasticine)
- Heavy-gauge plastic
- Tracing paper
- Water-soluble marker or tracing inkSpray bottle
- Brush for glue
- Bone Glue Pearls : an animal glue made from bone and hide that’s very strong yet water soluble: has been used for centuries in the Ravenna method. Also used by oil painters, woodworkers, and museum restorationists. Any leftover can be poured into a container to cool; just re-melt to use for other projects. It lasts about a week or so. I buy bovine bone glue pearls here: highlandwoodworking.com/boneglue1lb.aspx
- Cheesecloth or scrim mesh: scrim is a natural woven-mesh fabric available at many fabric stores. It’s similar to cheesecloth. Whether you use cheesecloth or scrim it’s important to use a somewhat fine weave: not grade #40 or below (too open), but grade #50–#80
- Natural Oxides: available through Sherri Warner Hunter (she calls them concrete colorants) sherriwarnerhunter.com
I use and teach the ancient Ravenna method in much the same way it was taught to me at The Mosaic Art School di Luciana Notturni in Ravenna, Italy. The Ravenna method is a direct method achieved face-up (just like any other direct method), but in a temporary bed of lime or clay, rather than placing tesserae directly into thinset. The mosaic is then removed from the temporary binder and transferred to the permanent binder of thinset or cement.
Some advantages to using the Ravenna method: it allows you to make
changes easily, keep the piece upright and step away to view it from a distance, look at the face of the mosaic as you work, and leave it for a time and then resume work.
STEP 1: Tape or tack heavy-gauge plastic to work surface.
STEP 2: Temporary binder: create flat bed of clay or lime paste 1/4” to 3/8” thick onto plastic—a bit larger than size of mosaic.
Clay: Easy to find, provides crisp print of mosaic when transferring the tracing, trickier to remove from back of mosaic.
Lime paste: Traditional method used in Ravenna; in US, more difficult to find and more expensive. It must be worked and dried over some days before it can be printed on, but it comes off the back of a mosaic like a dream. Because of this, I use clay for most of my work, but prefer lime paste for large pieces. Clay works well with smaller tesserae;
it’s a bit harder to push larger tesserae into the clay as it gets firmer—not an issue with lime paste.
STEP 3: On tracing paper, trace the tesserae or lead lines of your mosaic in reversewith a water-based ink. Poke air holes in the tracing paper with skewer or other tool to help the air pass through so the paper makes better contact with the binder.
STEP 4: Transfer design—lay design, ink side down, on the clay and rub the tracing into the clay, making sure all ink has made contact. Then just peel off the paper; the ink will transfer more or less immediately but it has to make contact with the clay. It works the same way with lime paste, but you can’t rub the back of the tracing as firmly as the lime paste is much softer than the clay.
STEP 5: Lay tesserae into the binder no deeper than 1/8” (can be difficult to remove later if too deep). Keep the binder moist (but
not saturated) as you work. You may have to spritz it with water from time to time. When finished for the day, spritz the mosaic and cover with plastic, taking care to tuck in the edges so no air gets in. In this
way, the binder will remain moist so you can leave it as long as you need, occasionally returning to check and re-spray with water if temporary binder is getting too dry.
STEP 6: Melt bone glue and water in a double boiler in a ratio of 1:2.5 (one part glue pearls to two and a half parts water; a 12”x12” mosaic should take about 3/4 cup of the melted down glue).
STEP 7: Cut mesh a bit larger than mosaic; rinse in water to remove any sizing or starch that may be in the fabric. Place over mosaic.
STEP 8: Apply the glue to the mesh, working from center. Don’t lift the brush—stroke all the way off the edges, making sure the edges are glued well. With the same brush, scrub the still-wet glue into a froth so that the foam permeates all the holes of the mesh and makes contact with the tesserae.
Let dry uncovered 10–12 hours (out of direct sun). When dry, the glued surface should be a hard-dried shell; if it’s still moist, place in the sun to dry fully.
Release the plastic from the work surface.
Place plywood or other surface on top of the mosaic and flip it like a sheet cake so the glued top of the mosaic is face down on the new work surface.
STEP 9: Remove the plastic from the back of the binder. If using CLAY, it should be “leather dry” (feels like firm leather to the touch; no longer pliable, but not so dry that the clay has begun to lighten in color, buckle, or shrink too much). It’s important to remove the clay before it gets too dry. Use a utility knife to score the clay in long strips, cutting all the way down to the back of the tesserae. Peel off in long pieces. If the clay is too dry, cross-score the strips into smaller cubes and pry off with a spatula. If the clay is too wet, flip the mosaic and let it dry longer, glue side up.
If using LIME PASTE, you have leeway on the timing. If wet, the paste will easily peel off the back. It’s best to let it crack and dry completely so it falls off easily.
If tesserae are dislodged while removing the clay, put a drop of household glue on the face of the tessera and replace it face down onto the cheesecloth where it dislodged from.
STEP 10: Use picks to clean any leftover clay or lime paste. The backside of the tesserae must be clean so they can make good contact with the mortar. Trim excess mesh close to edge.
STEP 11: Brush the interstices with dry marble dust, Portland cement, or thinset to form a barrier so the mortar doesn’t squeeze through to the front of the mosaic. Marble dust is traditional. I prefer plain Portland cement—it’s a darker color and not as gritty as thinset.
Apply an even layer of thinset to the backside of the mosaic, 1/8”–1/4”
(no deeper than the longest tesserae), mixed to a creamy peanut butter consistency
STEP12: Apply a thin scratch coat of thinset to the surface of
Place the substrate onto the mosaic, thinset layers facing.
STEP13: Flip the mosaic “sandwich.” Remove work surface to expose mosaic, face up with the substrate on the bottom. Press the mosaic from the center, outward, to remove any air bubbles and to make sure all edges are in contact with the mortar.
Let dry 24–48 hours so the mortar has time to cure to good strength bond.
STEP 14: To remove the glue and mesh, saturate a cloth in warm water and place on top of glue/mesh/mosaic. Let sit 5–10 minutes to begin breaking down the glue.
Remove the cloth; if the glue is not released, repeat application. Or, use a scrub brush dipped in warm water to gently scrub the glue and mesh until it releases.
Use a wet scrub brush and toothbrush to remove remaining glue from interstices (it will foam, as when first applied to mosaic). Gently rinse and repeat until glue is removed.
STEP 15: As a finishing step, seal the stone, or use mineral pigments to antique the mosaic: mix natural oxides (tan or walnut color) with water to make a wash. Brush onto stone; let dry back to powder, then wipe away excess with damp sponge.
Michael Kruzich studied at the Mosaic Art School in Ravenna, Italy, under Maestra Luciana Notturni and colleagues. He’s based in San Francisco, California, where he maintains his studio practice creating fine art mosaics, reproductions, and commissions for private and commercial clients.
Michael teaches the classical Italian principles and methods of making mosaics.
Here is a wonderful video on the Conservation of the ancient mosaics at Zeugma. You will see in this video some very clear examples related to how the Ravenna method was developed to save the ancient and Byzantine mosaics that were threatened or having trouble with their original mortar beds. You will see the mosaics being covered with canvas mesh and glued to remove them from original beds. Later during the restoration you can see clay being added to the back then flipped so that they can remove the canvas from the front and replace and restore damaged sections in clay.