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Building a Pebble Mosaic Stepping Stone

Pebbles are tightly fit together so that the mortar doesn't show
Gold and Black alternating bands of Mexican Beach Pebbles in Los Angeles
The most frequently asked question I get in regards to my mosaic work seems to be "Where do you get your stones?"  After that comes the question "Do you ever teach workshops?"  I have on occasion taught them but it has been a long time.  The reason I quit doing them is that they are truly exhausting.  I have to assemble close to two tons of material to teach 20 people my technique.  Each person needs a wooden form to build their mosaic in, and then I have to procure the 20 bags of mortar (1,600 pounds) and another ton of sorted pebbles, and we haven't even started the class yet.

Reed College ? Mosaic
The last one I gave was at Reed College in Portland for the Hardy Plant Society of Oregon.  The mosaics were in situ in a parking strip garden in front of the Theater Arts Building off campus.  We made a series of theater masks in the themes of Comedy, Tragedy, and Theater of the Absurd.  The last title seemed to make the most sense as the mosaics were supposed to be pads for maintenance workers to step on to keep from compacting the soil in the beds.  So most of the laboriously constructed mosaics are now buried in vegetation and impossible to see except maybe in winter.  I still don't know if anybody in the workshop ever went on to build their own mosaics afterwards.  Such is the reward of such endeavors.

What I do now is tell people that if they want to try to build a pebble mosaic that they should start with a simple stepping stone.  I have built a number of them for a project as part of a trade I am doing with a man who is slowly fabricating wonderful steel structures for my garden.  I made 26 18x18" square mosaics for his garden in a simple box made of 2x4's and a square sheet of plywood screwed together with a cordless drill and 2 1/2 inch long screws.  The box has two longer pieces of 2x4 on two sides so that I can unscrew the box and tap the boards free after the step stone has cured, which takes about two or three days.  I use a framing square to make sure the angles are 90 degrees, and drill shorter screws through the 2x4's in to the plywood to give it rigidity.  If you want rounded corners or curves in your shape, I like to use the rolled flexible lawn edging that is normally used between grass and planting beds.  This can be used inside the wood form with some blocking if you need it to maintain the desired shape.

Wood form, lawn edging and beach rock mosaic
Once the box is built, you can mock up a mosaic inside of it.  The trickiest part of building a good mosaic is finding the right stones.  Most people are too lazy to really do a good job of this and they often ask me if I could sell kits where the stones are already sorted.  Sounds like a lot of fun to me, spending hundreds of hours sorting pebbles for other people so that they don't have to.  Pay me $100 a pound and I might consider it, but other wise you are on your own.  That means going to the beach, or a river, or a stone yard.  You can buy bagged pebbles although the selection is usually limited.  The most commonly available pebbles are Mexican Beach Pebbles from Baja California, which are gathered by small operations from beaches there.  I am amazed that the supply needed to stock stone yards all over the US are still available after decades of sales, but they still seem to come up with them.  They used to be only black, in a few sizes, but now you can sometimes find buff gold pebbles and dark red.  The black and gold seem to have the most usable shapes though.  Dont buy the ones larger than 2 inches as there will be very few usable shapes.
Bagged Pebbles at a stone yard in Seattle

Montana Rainbow Mix Pebbles
I also sort through piles of pebbles in stone yards, a rather joyless and tedious task that requires a meditative will, as it usually takes 3 or 4 hours to sort a couple of 5 gallon buckets worth.  You need to wet the pebbles to see the colors since they are usually dirty and all look brown when they are dry.  One of my favorite selections is called 'Montana Rainbow Mix', or 'Pame', which I assume comes from Montana.  The colors are a blend of pastels, with red and pink being the predominant shades.  The Miro mosaics that I wrote a blog about a while back are made mostly from this mix, sorted by color.  I also did a Brazilian Carnaval inspired mosaic parking strip for a client that features outlined areas of the various colors.

Carnaval Mosaic Panels
If you collect from the wild, you should do so with some discretion.  No picking in State or National Parks!  Not just any shape will work, and the pebbles cant be laid flat like pancakes so that you get more coverage as they will pop out over time.  They have to be set on edge, or if they are large, have enough thickness to stay imbedded in the mortar.  A great way to proceed is to put some temporary sand in the bottom of your form box, and practice placing the pebbles in the sand.  They ideally should be set on edge, so you need pebbles that have a flat top surface and straight sides so they fit tightly together.  You will get the hang of it if you do it in sand as you can see what it might look like when you actually set it in mortar.  You can even compose the entire mosaic in the sand and then take it apart.  If you do this you will know that you have the proper quantity of pebbles to do the job.  Chances are your first mosaic is going to be a little funky, and you will learn from experience, so don't do the entrance to your house first thing.

An 18x18 inch mosaic that is 3 1/2 inches thick is very heavy and takes two people to move.  I like to insert two pieces of 3/8th or 1/2 inch rebar in to the mortar for reinforcement because it sucks when a stepping stone you spent so much time on breaks when you take it out of the form.  You can get these already cut at Home Depot, or cut them to length with a hack saw.  It takes about 1 and 1/3 80 pound bags of Type S Mortar to fill a box of this size.  One bag should work on a 14 x 14 inch step stone and it will be easier to move when it is done.  Adjust the quantity if you use 60 pound bags.  Do not use concrete mix!  It dries out very fast and the stones wont stick.  It has to be mortar, which is usually used to set stone or lay bricks.  You will need another pieces of plywood a little bigger than the form to lay on top and step on to flatten the work once it is done.  The tools needed are pretty low tech.

Mixing Mortar in a wheel barrow with a hoe
You mix the mortar in a plastic mortar pan or better yet a contractors wheel barrow using a garden hoe.  I wear a dust mask when mixing to avoid breathing the dust.  Use a squeeze handle spray nozzle on the end of your garden hose to wet the mix.  Put some water in the pan or wheel barrow first and then open the bag on one end and dump the contents out, pulling up on the bag.  You add more water as you mix until it reaches a stiff pudding like consistency.  Dig down to the bottom of the wheel barrow so you don't have pockets of dry mix.  Be thorough.  If you can stick your finger in the mortar and it maintains the indentation without  being too liquid, then you are probably there.  If it cracks and looks dry, add a little more water, but be careful as it doesn't take much extra water to make the mortar soupy, and you cant build a good mosaic in a soupy mix.  Once it is ready, I scoop it in to the form, wearing rubber gloves, as the mortar is not good to get on your skin.  Working bare handed will lead to cracked dried out fingers.  You should work in an area that you don't mind making a mess in as you need to hose off the excess mortar when you are finished setting and flattening the mosaic.  You can work on a strong table if you want to sit or stand up.  I sit on a little plastic stool while I work.  Stand up and stretch from time to time or you will become a hunchback.  If you have extra mortar you can put it in a bucket and let it dry and then knock it out later, or even make another little mosaic top of that.  I have several of these little round bucket mosaics in my garden.

Keep your sorted pebbles in plastic nursery pots and dump them out to pick through them.  It is very important that your pebbles be sorted in advance.  Once you are working with the wet mortar you need to push on through, no going to pee or answer the phone.  Work in the shade unless it is a cloudy day as it will give you more time to work.  The mortar will set up a lot quicker on a hot day.  You need protection from rain if it is a wet day as it will make soup of your mortar.  I push the two pieces of rebar in to the mortar about a half inch from the bottom sheet of plywood.  The top level of the mortar should be about a half inch below the top of the form as the pebbles will displace some of the mortar as you set them in.  Make sure that none of your pebbles are thicker than the depth of the form itself.

Finishing a stepstone at the O'Byrne Garden in
Eugene, Oregon
You can draw your design in to the mortar with a stick.  I like to keep a flat screw driver on hand for this, and to pry out pebbles that didn't go in the way you want them to.  Look at each pebble as you put it in so the the best face is forward.  Each pebble should be wet (spray them with the nozzle), and imbedded at least an inch in to the mortar or it may pop out later.  This can take years to happen.  Well imbedded pebbles should stay put for decades.  The pebbles along the edge should ideally be set perpendicular to the form so that they don't come off if you step on the edge of the stepping stone later.  I like to select shapes for the corners that are squarish and deeply set so they dont break off later.  The more conciencious you are building your mosaic, the better it will look.  Just squishing in a bunch of rocks in the mortar will make for a very funky looking piece of work.  I've seen plenty of this.  If that is OK with you then chances are so are Motel paintings in your living room.  Try to think of it as an art form rather than a craft project, and shoot for beautiful results.
The pebbles should be fit tightly together, considering their composition and placement.  If I am doing a design with lines, I will set up the lines first after I have done the border, and then fill in the outlined spaces with carefully selected stones.  On a cool day you can work for as much as 45 minutes before the mortar is too dry to work with.  A 5 gallon bucket with some water in it is great for wetting individual stones and for rinsing your gloves if they get covered with mortar.

Lotus Stepstones and Wood Form
Once all the pebbles are placed, hopefully as flat as possible and not sticking up all over, you can put the sheet of plywood on top and step on it.  This forces the pebbles down to the same level and pushes mortar up in between them.  Don't do it too hard at first.  Lift the panel and check how it looks.  Then use a light spray to wash off the displaced mortar and flatten it again.  I usually do this a few times or more if I have every thing set just right.  You will get in to trouble if you had too much mortar in the form to begin with and it is oozing out all over and the pebbles are moving from their intended place.  Don't spray too hard so that you blast out your work either.

When you feel like it is flat and presentable and washed to properly expose the finished work, just let it sit for a few days to cure, getting it wet from time to time so that it does so slowly.  Then you can unscrew the form and take a screw driver to scrape off the slaggy mortar edge along where the forms were.  A well done mosaic doesn't show much mortar, just the lovely pebbles you so painstakingly selected.  It might be a pain to do this kind of work, but I have been doing it for many years, and I think it is one of the loveliest forms of pavement imaginable.  You could make a series of pavers and then put them together to make a pad, or space them and make a path.   If you are ambitious, you can do mosaic work all over your garden.  If you do them in place you don't have to move it in to place.  If you do this you should put drop cloths over plants and walls as the mortar splatters some when hosing the work off.    You will get better at it as you gain experience.  After the mosaic has cured for about 25 days you can clean it with Muriatic Acid diluted with 4 parts water.  Pour it on and scrub with a nylon bristle brush.    It will remove any dulling mortar film left on the pebbles.  Be careful using this stuff.  Wear rubber gloves and cover your skin, and do not breath the fumes.  You can buy it at the local hardware store.  Use a flathead screw driver to trim out any unwanted globs of mortar and to smooth the joints.

Lotus Step Stone Path in Portland, Oregon
 I have stood premade panels on end, mortaring them in to walls, and have inbedded flexible copper tubing normally used for ice machines and the like to hook up creating simple fountains.  I've built the panels in situ many times in parking strips, and as large stepping pads for wider paths, but I recommend you start small and manageable and moveable until you get the hang of it.

Paths made with 14x14" step stones make a lovely small path.  One set I built are lotus blossoms, alluding to a Buddhist Jataka Tale about Sidhartha Gautama being born, and having lotus blossoms spring from his footprints.  Another is simple alternating bands of black and gold Mexican Beach Pebbles.

If you precompose the design you are probably going to make a better stepping stone so I cannot emphasize enough the benefit of knowing what you are about to create.  Fit the pebbles tightly together and try to be the best artist you can.

Hand selected beach stones fit in to gaps in a mortar set stone parking strip pad

Simple bands of colored pebbles in round stepping stones match the mosaic path of a rattle snake between my two houses.  The smaller rounds were made in the bottom of a 5 gallon bucket.

The limits of what you can do with pebble mosaic are constrained only by your creativity.  Good luck to you if you ever decide to try it.  You might just impress yourself with your new found ability, and having created something beautiful.

3 Round Medallion mosaics mounted in a fountain wall, Portland

Plants encroach on 'The River of Life',  an early in situ mosaic in my career
Round step stone with marbles, Indonesian Turquoise, Red Montana Rainbow, Black Mexican Beach and center beach stone
sho fia

sho fia

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