Specific Gravity of Slip Calculator: Brongniart Formula with Deflocculant Estimator

Specific gravity is the ratio of a liquid’s weight to the weight of an equal volume of pure water. Water has SG 1.0 by definition. A ceramic casting slip at SG 1.75 weighs 1.75 times as much as the same volume of water. Potters measure it because it is the fastest, most reliable way to check whether a slip has the right ratio of dry clay to water for consistent casting results. Without measuring SG, two batches mixed on different days can behave completely differently even if they look and feel the same.

Alexandre Brongniart (1770-1847) was a French mineralogist who served as director of the Sevres Porcelain Manufactory from 1800 until his death. In managing the factory’s production of fine porcelain, he developed the mathematical relationship between the measured density of a clay slip and the amount of dry material it contains. The formula is derived from simple mass-volume relationships for a two-component system of solids and water. It has remained the standard ceramic industry tool for slip characterization for nearly 200 years because it is mathematically exact for uniform suspensions and requires only a density measurement and knowledge of the dry material’s density to apply.

The syringe method is the fastest approach in a working studio. Use a 10 mL plastic syringe with Luer slip tip, no needle. Stir your slip thoroughly. Pull 10 mL of slip into the syringe to the exact mark. Place the filled syringe on a digital scale accurate to 0.1 grams and weigh it. Subtract the known weight of the empty syringe (weigh it empty first). Divide the net slip weight by 10. That is your SG. If the net weight is 17.5 grams, SG = 1.75. This method was featured in Ceramics Monthly and is now used in studios across the United States because the syringe is easier to clean than a graduated cylinder and can be kept at the slip bucket for rapid daily checking.

The density of dry clay particles is typically around 2.58 to 2.62 g/mL for the kaolin and ball clay materials that make up most US casting slip recipes. US Patent 6,696,377 (a casting slip formulation patent in the USPTO database) specifies kaolin at 1.28 to 2.6 g/mL and feldspar at 2.56 to 2.63 g/mL. The industry standard default for Brongniart formula calculations is 2.60, which is widely cited in ceramic engineering references including the Glazy ceramics wiki. For typical studio clay bodies, this value is accurate enough for practical batch calculations. For high-density bodies containing significant whiting (2.71) or talc (2.78), the effective average density will be slightly higher, and the calculator allows you to adjust with a custom density value.

Not necessarily. SG and viscosity are two different properties. SG measures how dense the slip is, which reflects how much dry material is suspended per unit volume. Viscosity measures how easily the slip flows, which depends on particle interactions. A slip at SG 1.80 can have very different viscosities depending on whether it is deflocculated or not. A deflocculated slip at SG 1.80 flows like cream. The same amount of clay and water in a flocculated state (no deflocculant) at the same SG 1.80 would barely pour at all. If your SG is correct but the slip feels too thick, the issue is likely under-deflocculation rather than excess solids. Add a small amount of Darvan 7-N (starting at 0.1-0.2% of your dry weight) and stir thoroughly before re-testing.

You can estimate batch volume from the weight of your slip bucket. Weigh the full bucket, subtract the weight of the empty bucket, and divide by (SG times 1000) to get volume in liters. For example, a bucket containing 17.5 kg of slip at SG 1.75 contains 17,500 / 1,750 = 10 liters. If you have a bucket with volume markings, simply pour in enough to fill and read the level. Alternatively, if you know exactly how much water and dry material went into the original batch, use the Weights to SG mode first to calculate volume, then switch to Adjust mode with that volume.

Yes. The Brongniart formula applies to any suspension of dry ceramic materials in water, including glaze slurries. For glaze batches, the appropriate rho_s (material density) is approximately 2.60 for most base glazes, which is the same default as for clay bodies. Glaze slurries are typically applied at much lower specific gravities than casting slips, usually in the range of 1.40 to 1.60 for dipping and pouring application, and 1.40 to 1.50 for spray application. The SG meter in this calculator shows these as the “Thin / Engobe” range, which is the correct zone for most glaze application gravities.

ASTM C373 is the Standard Test Method for Relative Density (Specific Gravity), Absorption, and Porosity of Fired Whiteware Products, published by ASTM International and used in US ceramics quality control. While it primarily addresses the SG of fired ceramic bodies rather than slip, its fundamental methodology of measuring weight in air and weight in water (Archimedes principle) is the basis for pycnometer-style SG measurement of dry ceramic materials. Knowing the SG of your dry ceramic materials (rho_s in the Brongniart formula) to high precision requires this type of measurement. For standard studio work, the published density values for common materials provide sufficient accuracy.

Darvan 7-N is sold by most major US ceramic supply companies. As of 2025 and 2026, typical US retail pricing is approximately $10 to $15 for a pint (473 mL), $20 to $30 for a quart, and significantly less per unit for gallon quantities. Major US suppliers including Sheffield Pottery (888-774-2529), The Ceramic Shop, and Axner stock it. It is also available from Dick Blick Art Materials. The manufacturer, Vanderbilt Minerals LLC, can be contacted at vanderbiltminerals.com for industrial quantity inquiries. Since Darvan 7-N is used in very small quantities (0.2 to 1.0 percent of dry body weight), a single quart lasts most studio potters a very long time.

Two differences commonly cause this. First, viscosity: in a viscous deflocculated slip, the hydrometer experiences drag that prevents it from sinking to the correct depth, causing an artificially high reading. The syringe method is not affected by viscosity. Second, the hydrometer reading scale is calibrated for the surface of the liquid to meet the scale, but foam or slip buildup on the hydrometer stem from a thick slip can cause misreading. The syringe method directly measures the weight of a known volume of slip, making it more accurate for ceramic slips than most general-purpose hydrometers. If you use a hydrometer, choose one with a narrow SG range (for example, 1.5 to 2.0 rather than 1.0 to 2.0) for better precision, and stir the slip immediately before measuring.

Over-deflocculation produces a slip that is too fluid at a given SG, which sounds beneficial but causes serious casting problems. An over-deflocculated slip migrates water into the plaster mold so rapidly and unevenly that cast pieces dry with internal stress gradients, leading to cracking during drying. The cast wall can also absorb water from other areas of the mold inconsistently, producing pieces of uneven wall thickness. Additionally, over-deflocculation can cause the slip to become thixotropic, meaning it thickens more than expected on standing and then re-thins when stirred. If your slip is over-deflocculated, the only practical fix is to add a small amount of flocculant (magnesium sulfate or calcium chloride in very small amounts) or dilute with a fresh batch of under-deflocculated slip.

Yes, very slightly. As the plaster mold absorbs water from the slip during casting, the remaining slip in the mold becomes slightly more concentrated in dry material, meaning its effective SG would be marginally higher than the starting reading. In practice this effect is small enough to be negligible for the purposes of the Brongniart calculation. What matters more is that each new casting operation begins with slip at the correct SG. After a day of casting, the slip remaining in the drain bucket or the working bucket may have drifted up by 0.01 to 0.02 SG units from water absorption into the molds. Measure SG at the start of each casting session and use the Adjust My Slip mode to correct any drift before pouring the first mold of the day.

Yes, but use the Custom density option with a higher rho_s value if barium carbonate makes up a significant portion of your recipe. Barium carbonate has a density of approximately 4.29 g/mL, much higher than clay at 2.60. A slip containing 10 percent barium carbonate by dry weight would have an effective dry material density closer to 2.77 g/mL rather than the default 2.60. Using 2.60 in this case would cause the calculator to underestimate the dry material content for a given SG. To calculate the effective blended density of a multi-material recipe, take the weighted average of each ingredient’s density based on its percentage of the total dry weight. Also note that as flagged in our Glaze Batch Scaler tool, barium carbonate is EPA RCRA regulated as a hazardous material in ceramic glazes.

The standard studio test is the Ford cup or viscosity cup method: dip a standard Ford No. 4 cup into the stirred slip and time how long it takes for a continuous stream to fall after lifting. Different slip types have target flow times. A properly deflocculated casting slip typically flows for 30 to 60 seconds from a Ford No. 4 cup. Beyond viscosity, a well-deflocculated slip shows almost no separation on standing (clears only slightly at the top after several hours), pours cleanly without ropiness, and produces consistent wall thickness on multiple test casts in the same mold. If you do not have a Ford cup, a rough test is pouring the slip from a ladle: it should flow smoothly in a continuous stream, not in chunks or ropes.

The deflocculant gram amounts shown by this calculator are starting ranges calculated from the Vanderbilt Minerals published TDS specification of 0.2 to 1.0 percent of dry body weight for Darvan 7-N, applied to the dry material weight computed by the Brongniart formula for your specific SG and batch volume. These are starting points, not final amounts. The correct deflocculant level for your specific clay body, your specific plaster molds, and your target casting behavior requires bench-testing at the lower end of the range and adding more in small increments until you reach the desired viscosity. Record the final amount that works and use it consistently for every subsequent batch of the same body.

This calculator uses verified data from authoritative US sources to back its calculations. NIST (National Institute of Standards and Technology) is the US government body responsible for measurement standards, and their water property database establishes the precise reference density of water at 20 degrees Celsius (0.99820 g/mL) that underpins all specific gravity measurements. ASTM C373 is the American Society for Testing and Materials standard for specific gravity measurement in fired ceramics. Citing these standards in the PDF report provides documentation that the calculation methodology follows established US measurement conventions, which is useful for studios that maintain ISO or quality assurance documentation for their production processes.