Clay at Fishers Island

by Jane Ahrens

By Liz McCarthy, The Lighthouse Works Alumnae Fellow ’18
February 2021

This document is a brief overview of my experimentation with clay at Fishers Island (FI) during a 2018 residency with The Lighthouse Works. This will hopefully be expanded more someday. This includes some history, workability, and firing details for the material. 


During my time at Fishers, I did some research in the Henry Ferguson Museum. There is a lot of documentation there of clay used by the colonial groups of the 19th century, and some information on the indigenous use. There are some lovely artifacts of pre-colonial artifacts that are definitely worth looking at in person. The forefront of history focuses on brick manufacturing. Many parts of Brooklyn are built with the “F.I.” brick from Fishers Island. Brick production ended when the clay pit began filling with water, which is the state of the pit today. 

Here is documentation of some c1990 articles found at the HLF Museum

There are some images and info about the indigenous use and firing methods of the material at the museum. The indigenous communities seem to have been coming over in boats from the mainland to collect clay, and then bringing it back to form and fire. There are artifacts on the island as well. The indigenous communities fired the material in pits (which would be easy to replicate on the beaches of Fishers). This is a lower fire process. I found more information on clay and indigenous use through the Smithsonian, but it was hard to access files because I didn’t have JSTOR at the time. The museum has a lot of artifacts made from Fishers Island clay from the region. 

The clay material at Fishers is a deposit from the Wisconsinan Glacial phenomenon. I am Chicago-based, and the materials in my area are from the same phenomenon but have a pretty different mineral makeup.

The Geology of Fishers Island
c/o HLF Museum

The first detailed study of the geology of Fishers Island was undertaken nearly 100 years ago by Myron Fuller, a geologist, entrepreneur and philanthropist from Brocton Massachusetts. Fuller did extensive fieldwork on Fishers Island and nearby areas, and also made use of the log of a deep water well (the “Ferguson Well”) drilled on the island. Remarkably, Fuller’s most significant conclusions, including the glacial origin of the island, are still accepted today.

A Geological Inventory

Fishers Island is composed of four main rock and sedimentary units; a granitic basement, layered coastal plain sediments, glacial deposits, and modern marine deposits. Only the glacial deposits and modern marine deposits are exposed on the island. The glacial and marine deposits are very young and are mostly unlithified, meaning they have not undergone the geologic processes that turn sediments into rocks.


The structural basement beneath Fishers Island is a light gray granite encountered at a depth of 280ft in the Ferguson Well. It is continuous with the 260 million-year-old Narragansett Pier Granite which is actively quarried in coastal Connecticut and Rhode Island. Much of southern New England is underlain by this granite and a series of associated rocks. This region, including Fishers Island, is part of what geologists call the “Avalonian Terrane”. Avalonian rocks were once part of North Africa, but were united with North America during the formation of the super-continent Pangea.

Coastal Plain Sediments

Overlying the granitic basement is a 20ft thick layer of clay, known as the ìblue clayî because of its unusual color. This 65 to 100 million-year-old clay is a part of the large, westward-tapering, wedge of coastal plain sediments that extends along much of the eastern seaboard. The middle of the Cretaceous Period (about 100 million years ago) saw a dramatic rise in sea level to more than 300ft above the present level, resulting in the development of a vast network of shallow seas upon the continents, where coastal plain sediments were deposited. These shallow seas were the home to large marine reptiles and sharks. Fossils of these now extinct animals are common in coastal plain sediments.

Glacial Deposits

The uppermost sedimentary unit at Fishers Island is a 200-300ft thick sequence of gravel, sand, clay and boulders. These deposits record the advance and retreat of ice sheets over North America beginning at least 1.5 million years ago during the Pleistocene Epoch, culminating with the Wisconsinan ice sheet which formed the Fishers Island moraine by about 19,000 years ago. The internal structure of the Pleistocene deposits on Fishers Island is quite complex, with tight folding observed at some locations. A distinctive feature of the glacial deposits is the large boulders found all over the island. These boulders are known as “erratics” because they have been transported by the glacier to places where they otherwise would not be found. Erratics on Fishers Island are mostly volcanic and metamorphic rocks which the glacier plucked from outcrops to the north, in Connecticut and Rhode Island, as it moved southward.

Modern Marine Deposits

The harbors and shorelines of Fishers Island are underlain by very recent deposits of sand, gravel and mud. Most of the sand on the islandís beaches came via rivers entering Long Island Sound along the Connecticut shore line, and has been transported to the island by tidal currents. Accumulations of gravel and larger rocks, like those at Isabella Beach, are probably “lag deposits”, meaning that they are an erosional remnant. The cobbles that now rest on Isabella Beach were originally part of the Pleistocene glacial deposits that make up the bluffs above the beach. Large storms have eroded back the bluffs, and removed the fine-grained material within which the cobbles were once contained.


Here is a map of the clay pits, and this pen points to where The Lighthouse Works ____ Tryn Collins and I were pulling clay. Tryn has a good bit of knowledge of the digging process. You have to walk about a .25 mile into the spot (I don’t totally know where this is, Tryn took me there). Basically, you just wade into a shallow area and pull sheets of clay up from the bottom. It is helpful to wear water shoes or boots so you don’t hit rocks with your feet. You should bring shovels to pull up sections, and then you can just grab them with your hands. Hopefully, there is not a lot of rain when you go so it is shallow. If it is warmer… mosquito city, bring DEET. As you are pulling you will feel sections that have lots of rock/sand and others that are really nice pure clay. The stuff in sheets that just pulls right up is the more refined clay.


  • Buckets/bags
  • Good water boots/shoes (I wore Chacos)
  • Sturdy shovels
  • A cart or wagon or backpack for hulling back

Location of clay pit visit

Pulling Clay from the lagoon with Tryn photo documentation by Jane Ahrens


The clay at Fishers Island is what you call a “Secondary” material. This means the clay material is a slurry of many types of minerals that have deteriorated over thousands of years into fine particulate. Clay/silicate material has an electrostatic charge, so it attracts the other fine mineral particulate through magnetism – hence the plastic sheets that are building up at the bottom of the lagoon. The water aids in this electrostatic reaction. SO! This clay is very different than purified kaolin or silica deposit. The FI clay has a lot of flux material such as iron and magnesium that influence the character of its melt and vitrifying process in the kiln (will talk more about in firing). 

According to the historical documents, this is a general chemical composition of the material in the NY region (Not specific to FI but probably about right):

Clays of New York
Silica 59.05
Alumina 22.11
Peroxid of iron6.54
Lime 2.19
Magnesia 2.64
Alkalis 6.22

As you are digging the material you will find there are those pockets of very smooth fine material and more rocky sandy stuff. You can use both, it just depends on what you are after for your final result (coarse or fine). In brick manufacturing, the more fine material was pulled and then sand (coarse silica) was added to temper for strength. The indigenous communities often tempered with ground shells which is a Calcium carbonate, this would give the material more structural stability while working with it wet, and during the firing process.  Here I have two ways of processing the material. 

Slurry method (fast):

  • Soak material in a bucket of clean sink water for 24 hours
  • Pour off the organic water and material on the top, use the middle material (clay)
  • Dump the bottom quarter of the material – this is the heavier less useful stuff
  • Sieve the middle bucket material through a screen or sieve (I was using window screens from lighthouse works)
  • For best results let dry on plaster and wedge OR let dry in the sun on cardboard OR let the material hang wrapped in cloth so water drips off. Always wedge a lot before using
Processing the slurry method outside studios

Longer drying sieve process:

  • Let material completely dry out in the sun or dry area. 
  • With a hammer and hands break the material up into fine material and pull out the stones and organic material. WEAR A PARTICULATE MASK!
  • Sieve – this depends on how much dense silica material you want – a 40 mesh is going to leave larger sand/silicate particles. If you do something like a 100 mesh sieve you will get a really fine delicate material (might be harder to work with). The 100 mesh powder is good for making glaze (will talk about it later). WEAR A PARTICULATE MASK!
  • Let powder soak in water for a few hours
  • Pour off the water once the material has settled (organic material will float to the top)
  • Dry the clay slip on a plaster slab till it becomes workable and then wedge

As I mentioned above, people that have used this material in the past have often used tempering techniques to make the material more usable to the form they were after. The material is very elastic, it sort of melts slightly while you are using it. I found that when I added a little sand from the beach it held its form better, but wasn’t as pretty when it fired. If you want to use the more refined material for sculpting I would suggest coiling methods where you let it harden or carve a harder chunk. I was making pretty small stuff. 

To increase the plasticity (holding its form), you can add different materials that will affect the firing. If you do this I suggest working in small batches and calculating the percentage (ex. 100 grams and add 1-10 grams additive). Bentonite is probably the best. It is a highly magnetic clay that is often used as a flocculant (suspension agent). You could also add EPK (might lower melting temp) or OM4 Ball Clay (Might Raise temp). All of these additives will probably reduce the beautiful red color

This material would also probably be good for turning into a cone 6 glaze. I didn’t now do this at FI, but I worked with some found clay and transformed it into a nice greenish celadon in Michigan. The clay in Chicago turns more into a thick brown glaze. In order to do that I have added different flux materials to 100 mesh powder at 5%. Some Fluxes: Wood Ash (you can make this on the island), Gerstley Borate (usually gets good results and glassy melt), Soda Ash, Potash. (I will include some resources below on this)

During my time at Fishers I didn’t experiment as in-depth with local clay, but I have done more research and experimentation with midwestern clays, and clay from the Driftless region of Wisconsin. Generally this secondary martial is OK for working with, and you can shift it easily into glazes, slips, and terra sigillata. 


According to the documentation, FI Bricks were fired anywhere from 2000 – 2300 degrees Fahrenheit. Bricks, because they are so thick, need a longer and hotter firing than a more thin-walled vessel. The general firing range to get the nice red color was 2000-2100 degrees Fahrenheit, this is in keeping with what these historical documents outline. The earlier indigenous communities were probably firing between 1700-1900 degrees Fahrenheit in a below-ground kiln. According to the documents, the material becomes viscous (liquid) at 2400 degrees Fahrenheit. You could test this with a melt test. 

Scove Brick kilns

These were kilns basically made of the same material they were firing and circulating the fired bricks. They were wood-fueled. Image courtesy of HLF Museum

Pit Fire

This is a very early style of firing where the pieces are put in a low hole, piled with wood, and then burned for 2-6 hours. This is how pre-colonial communities would fire pieces.
Using the electric kiln at Lighthouse Works I did firing at three different temperatures.
Cone 06 (1860 degrees Fahrenheit) Far left: straight from the pit not sieved. Middle: with sand, Far right: sieved through a window screen.
Cone 04 (1970 degrees Fahrenheit) Far left: straight from the pit not sieved. Middle: with sand, Far right: sieved through the window screen.
Cone 1 (2100 degrees Fahrenheit) Far left: straight from the pit not sieved. Middle: with sand, Far right: sieved through the window screen.
Students making whistles out of Fishers Island clay at Fishers Island School.

Video of Fishers Island School kids playing whistles conducted by Yixuan Pan:


During my time at Fishers Island, I was working in FI clay and commercial clay, working on a variety of different projects, mostly not related to the native clay. Outside of my time at FI I have experimented with more regional clays from Michigan and Illinois to get good workable material and convert it into glaze. Based on my initial research at FI I taught a class called “Buried Histories” at Oxbow School of Art, and “Clay Ecology, Clay Knowledge” at the School of the Art Institute.

When I left FI, I left a lot of the pieces I made out of the clay on the island. It felt like the material should stay with the people that live there. I wish I had more FI material to play with at my own studio and further my research there, BUT I am excited that other artists can continue working with this material, and use it to learn about the human and non-human histories of the island. 

Many Thanks to Pierce Rafferty, Nate Malinowski, Tryn Collins, Jane Ahrens, and the Fishers Island community.

Additional resources on Clay Processing and Clay Material:

  • Glazes from Natural Sources: A Working Handbook for Potters, 2nd edition
  • Sutherland, Brian
Artwork by Bridget Mullen made from FI Clay

Featured Photo

Beach Rocks sun gaze, 12/10/20 Photo by Richard Breining

A Fishers Island Community Center Program and the accompanying Fog Horn eNewsletter serve as the communications resource for the Fishers Island community. The content – news, calendar, links and photos, milestones, ads, and more create a clear image of Fishers to those on and off the island.

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