Studio Pottery Glaze Test Tile & Firing Result Monthly Log

ContextTile thickness directly affects glaze behavior: thinner tiles heat and cool faster, which can exaggerate crawling tendency on aggressive firing ramps, while thicker tiles buffer temperature swings and may show better crater-healing in pinhole-prone glazes. Standardizing at 4 to 5 mm removes tile thickness as a variable between monthly batches. Wooden guide sticks, also called rolling guides or thickness slats, placed on either side of the clay slab prevent the roller from wandering below your target. A pasta machine set to setting 3 or 4 works well for high-volume tile production if your clay body is plastic enough to tolerate the nip rollers without tearing.

ContextUniform size keeps your tile rack slots, binder pockets, and photo compositions consistent month over month. The notch at the top right corner solves two persistent small problems: it orients any tile instantly without flipping it over to read the back code, and if you display tiles on a hanging reference rod, the notch keeps every tile facing the same reading direction. For tiles intended for a wall-mounted hanging rod system, punch a 5 mm hole 1 cm from the top center while the clay is still leather-hard—the bone-dry stage is more brittle and risks cracking the tile during hole-punching.

ContextThis is the single most important discipline in test tile work. An unstamped tile becomes anonymous after firing, and anonymous tiles are effectively useless as documentation. Use a rubber stamp with alphanumeric characters or carve the code directly with a loop tool. Your system should encode at minimum the recipe identifier, the clay body abbreviation, and the month and year. A code placed on the tile face may disappear entirely under an opaque glaze; the reverse is the reliable location. Develop a coding convention and commit to it—mixing ad-hoc notation styles across months makes database searches unreliable within the first year of logging.

ContextPaint wax resist into two 2 x 2 cm squares near the tile's lower edge before dipping or brushing any glaze. After firing, the waxed areas remain bare and show you exactly what the clay body looks like at that cone and atmosphere—an essential reference for color contrast and for observing how the clay and glaze interact at the interface. Running one patch under a normal single application and a second patch under double application on the same tile lets you compare single versus double coat behavior directly, halving the number of separate tiles needed to assess a glaze's thickness sensitivity.

ContextSkipping bisque and applying raw glaze directly to greenware introduces too many simultaneous variables for systematic testing: raw clay absorbs moisture differently than bisqueware, and moisture content in greenware changes glaze pickup thickness unpredictably even with a consistent specific gravity reading. Bisque at the cone matching your standard production workflow—typically cone 06 to 04 for mid-fire stoneware, cone 08 to 04 for high-fire work. If you are specifically testing a glaze formulated for single-fire production, log that as an explicitly separate test category, because results from once-fired tiles are not directly comparable to bisque-fired tiles of the same recipe and should never be merged in the same database entry.

ContextPercentages are the portable, scale-independent format for ceramic glaze recipes—they remain correct regardless of batch size and allow anyone to reproduce the formula at any quantity. Record the material brand and source where possible alongside the formula: Custer feldspar from one supplier is not chemically identical to G-200 HP, and this discrepancy becomes critical when troubleshooting color shifts across years of testing. If you are testing a variation of a base recipe—for example, swapping nepheline syenite for potash feldspar—log both the parent recipe and the variation as separate entries with an explicit cross-reference field linking them to each other.

ContextSpecific gravity is the most reliable application-consistency control number in studio practice. For most dipping glazes, the usable working range is 1.40 to 1.50 g/mL, though your individual studio optimum will emerge from your own log data over six to twelve months of testing. To measure with a graduated cylinder: fill to the 100 mL mark, weigh the filled cylinder, subtract the tare weight of the empty cylinder, then divide the result by 100. A hydrometer gives a direct reading more quickly if available. Record this figure without exception—it is often the fastest explanation for why a glaze that performed perfectly in one month is behaving differently in another, particularly after seasonal humidity changes or extended undisturbed storage.

ContextUse a pin tool to gauge dry application thickness immediately after the glaze surface loses its wet sheen but before it has gone fully bone-dry—it is more fragile at the bone-dry stage. For most stoneware glazes, the optimal thickness target is 0.8 to 1.2 mm; shino-style glazes and wood-fire ash glazes often perform best at 2 to 3 mm. Note whether you dipped and for how long (a 2-second dip versus a 5-second dip on absorbent bisqueware produces measurably different pickup), poured, brushed, or sprayed. Wax-resist layering, terra sigillata undercoats, and slip backgrounds should all be logged as part of the full application stack—any one of these layers can significantly alter the behavior of the glaze above.

ContextOne overhead photograph of the complete tile batch, with a handwritten card showing the batch code and date visible in frame, takes thirty seconds and becomes invaluable reference material later. Raw glaze color carries real predictive information: iron-rich glazes that appear green-gray or brown-gray in the raw state typically fire to amber or warm brown tones, and titanium-opacified glazes often reveal in their wet appearance how uniformly the opacifier has been dispersed through the slop. This photograph is your pre-firing baseline and your only proof of what was actually applied if any result is later questioned.

ContextKiln position is one of the most consistently underestimated variables in systematic glaze testing. The upper zone of a typical top-loading electric kiln runs 15 to 25 degrees Fahrenheit hotter than the bottom during the final 200 degrees of a firing, even with well-maintained and balanced elements. In gas kilns, reduction atmosphere is inherently uneven—tiles positioned near the bag wall see very different flame chemistry than tiles at the shelf center. A simple hand-drawn sketch with tile codes written in their actual positions, dated and filed with the batch, takes under two minutes. After six months of such diagrams, position-performance patterns become unmistakable within the accumulated log data.

ContextA pyrometer tells you the temperature at the thermocouple location; a witness cone tells you what heat-work actually occurred where your test tiles are sitting, which can differ by 30 to 50 degrees Fahrenheit from the thermocouple reading. Large Orton cones on a cone pad placed directly on the ware shelf are the ceramic industry standard for precisely this reason. For test tile firings, photograph each witness cone set immediately after the kiln opens and file the image in the digital batch folder. A slightly under-bent target cone on the bottom shelf explains far more about a matte glaze that failed to develop its intended surface texture than any other single data point in the log.

ContextFor electric kilns with programmable controllers, record the program number and every segment—degrees per hour and hold duration at each step. For manual or gas kilns, record time and temperature at 30-minute intervals, and explicitly mark transition points: start of the glaze-melt zone, quartz inversion at 573 degrees Celsius, start and end of body reduction if applicable, and the final peak temperature. Total firing duration is valuable for normalizing comparisons across kilns and across months: a glaze that crawls on a fast 5-hour firing may behave perfectly on a standard 8-hour schedule, and without firing-time records in the log, that connection will never be made.

ContextFor gas and wood kilns, log the reduction start point expressed as temperature or elapsed time, the intensity based on your damper position and burner settings (light, medium, or heavy), and when you returned to neutral or oxidizing before the end. A brief body-reduction window followed by light flame reduction to cone 10 produces entirely different results than sustained heavy reduction throughout—and a log without this detail cannot explain color or surface differences between otherwise identical recipe batches. For electric kilns, noting oxidation firing explicitly still has value because it contextualizes color chemistry when you later compare electric results against future gas or wood tests of the same recipe.

ContextSet up a permanent shooting station with a medium gray cardboard background (18% gray is ideal for neutral color rendering), a 5500K daylight LED panel positioned at 45 degrees to the tile surface, and a fixed camera-to-tile distance. Photograph the full grid first, then individual tiles for close-up documentation of interesting or problematic surfaces. The 45-degree light angle reveals the undulations of a crystalline formation, the roughness of a high-silica matte, and the mirror quality of a well-fluxed gloss in ways that direct overhead light completely flattens. Using the same setup, angle, and background every single month makes your archive a reliable cross-month color reference rather than a collection of incomparable images.

ContextUse a consistent rating scale: 0 means absent, 1 means minor (visible only on close inspection or affecting a small area), 2 means moderate (clearly visible at arm's length or covering a significant portion of the surface), and 3 means severe (dominant feature of the tile, unacceptable for any use). For crazing specifically, a score of 1 means cracks visible only under a 10x loupe; a score of 3 means cracks audible during or after the cooling cycle. A numerical scale, even a coarse one, converts qualitative impressions into trackable data points. Writing only 'some pinholing' means you will not know six months from now whether today's result was better or worse than any previous batch.

ContextDraw the tip of a pocketknife or dental pick firmly across a tile edge that was in contact with the kiln shelf—not a decorative face surface. A properly matured glaze should not scratch or flake; the mark should show either no trace or only a silver metal streak from the knife blade itself. Soft or chalky surfaces that scratch readily indicate underfiring or a glaze chemistry deficient in alumina and silica relative to its flux content. Shivering flakes—thin, sharp slivers that pop off during handling or in the days following cooling—should be flagged immediately as a safety concern for any functional ware and documented explicitly for chemistry review, not treated as a minor aesthetic note.

ContextAdopt a consistent framework: for color, describe the dominant hue, the value (light, medium, or dark), and the chroma (muted, medium, or saturated). For surface character, select from a defined short list: high gloss, satin, satin-matte, matte, dry matte, crawled, or intentionally textured. A description like 'beautiful rich blue-green with a lovely surface' is unmeasurable and cannot be compared to any past entry. The phrase 'medium-dark teal, medium chroma, satin finish, defect score 0 across all categories' can be placed directly beside any previous month's entry and evaluated objectively against it.

ContextFor glazes destined for mugs, bowls, plates, or other functional ware, pool undiluted lemon juice on a section of the fired tile surface and leave it covered for 24 hours, then wipe clean and inspect carefully under good light. Look for etching—a dull or matte patch where the surface was previously glossy—or any visible color change in the glaze layer. Etching indicates acid solubility that makes the glaze unsuitable for food contact regardless of aesthetic quality. This test does not replace formal lead and cadmium leaching analysis for commercial production, but it reliably catches severely underfired glazes or inherently acid-soluble formulations before they proceed into a production run. Record the outcome explicitly as pass, minor etch, or fail.

ContextA batch code is the thread connecting the physical and digital documentation of your studio. A clear format such as studio-initials, year, month, and batch letter—for example WP-2026-06-A—scales across years and accommodates multiple firings per month by incrementing the letter. This code goes on the physical tile rack slot label, the digital photo folder name, the kiln diagram, and the database record for every recipe included in that batch. The goal is that any result—physical tile in hand or digital database open on screen six months later—can be cross-referenced in under three seconds.

ContextPhysical tiles cannot be fully replaced by photographs. Surface texture, translucency, the depth of glaze layers, and the way a surface responds to raking or oblique light are three-dimensional qualities that even high-quality macro photography cannot fully convey. A wall-mounted wooden rack with labeled slots is the studio standard for accessibility; binder systems with clear plastic pocket pages—standard trading-card collector pages work well—offer portability and allow tiles to lie flat for side-by-side comparisons. Whatever system you adopt, ensure every label includes the full batch code and the month and year, not just the glaze name—glaze names get repurposed and reused across years in most active studios.

ContextYour master database is the cumulative intelligence of your studio practice. At minimum it should contain: recipe name and code, full dry-weight formula, clay body and cone tested, firing atmosphere, defect scores from the 0-to-3 rating scale, color and surface description using your structured vocabulary, food-safe test result where applicable, and the date last tested. Software options include spreadsheets, Notion databases, or dedicated ceramics programs such as Insight or Hyperglaze—the tool matters less than the discipline of updating it every single test cycle. A glaze that crazed at cone 6 in oxidation but fired clean at cone 6 with a 30-minute hold should receive its own updated entry with a note explaining the exact change, not a margin annotation.

ContextPromote means the glaze is ready for consideration in a full production run: it passed the acid test where applicable, showed no significant defects, and its color and surface meet your production standards consistently. Retest means it showed potential but requires a specific isolated variable changed next cycle—name that variable explicitly in the notes field, not just 'retest.' Hold means it functions but you have not identified a clear production context for it yet. Retire means it consistently underperforms, duplicates a superior glaze already in your library, or has defects that repeated testing has not resolved. This four-status discipline prevents library bloat: most active studios accumulate dozens of mediocre glazes that never get formally decommissioned, consuming bucket space, shelf space, and mental overhead indefinitely.

ContextGeneric file names such as IMG_4821.jpg become unretrievable within weeks. Rename files immediately on import: use the format batch-code_tile-code_raw.jpg for pre-firing shots, batch-code_tile-code_fired.jpg for post-firing shots, and batch-code_overview_raw.jpg for full batch grid photographs. Your operating system's native batch rename function or free tools like Bulk Rename Utility on Windows make this a two-minute task for a full month's batch. Consistent naming means the archive folder, sorted alphabetically, automatically organizes by month and recipe code without any further manual effort—which matters considerably when searching for a specific tile two years from now.

ContextWhen a February glaze is retested in June, place the February fired tile and the June fired tile side by side in a single composite image and label each with its batch code, cone, and atmosphere. A phone collage app, Canva, or any basic image editor takes only a few minutes. This comparison image is the clearest possible visual evidence of whether a recipe change, application adjustment, or firing modification improved, worsened, or had no effect. Without it, you are relying on written descriptions and memory to compare color and surface quality across months, which introduces retrospective bias—the tendency to perceive improvement in a result you hoped would get better.

ContextThis does not need to be formal—it is written for you six to twelve months from now, not for an audience. Note what you were trying to achieve this cycle, what surprised you positively or negatively, what failed in an instructive or unexpected way, and what specific changes you plan to make in the next round. A paragraph written within 48 hours of the kiln opening is more honest and more specific than notes reconstructed from tiles alone a week later. The cumulative record of these monthly narratives becomes, over years, an informal history of your studio's material thinking—exactly the kind of documentation that strengthens grant applications, residency proposals, and teaching portfolios in a way that tile racks alone cannot.

ContextNot every month yields a production candidate, and logging 'no promotion this cycle' is an active positive decision rather than a gap. When a candidate does emerge, note explicitly which qualities qualify it: consistency across different kiln zones, passing the acid test, visual distinctiveness relative to your existing production palette, or demonstrated compatibility with your standard clay body at varying application thicknesses. Scaling from a 100 g test batch to a 2 kg production batch typically requires recalibrating application method and specific gravity target, and those adjustments should be documented as their own firing event in the following month's log rather than assumed to transfer automatically.

ContextA busy test month can consume colorants and specialty materials faster than expected. Check stock of the most frequently depleted materials: zinc oxide, rutile, iron oxide, cobalt carbonate, tin oxide, and your primary feldspar. Calculate how many full test batches your current stock supports. Materials with long lead times—specialty frits, certain rare-earth colorants, or orders from international suppliers—should be ordered at least three weeks before you will need them. Note minimum order quantities for each supplier, since many have thresholds that make small top-up orders uneconomical compared to a slightly larger quantity that covers the next two or three test months.