Most flooring decisions start with a material category. Tile is different. It starts with a material category that then splits into a dozen distinct types — each with its own manufacturing process, absorption rate, surface hardness, and appropriate use case. Picking the wrong one does not just mean you dislike how your floor looks in two years. It means grout absorbing bacteria in a commercial kitchen, marble veining staining under acidic cleaners, or glazed ceramic cracking under heavy freight.
This guide covers every major type of tile flooring: what it is made from, how it behaves under real conditions, where it genuinely performs, and where it quietly fails. If you are comparing options for a specific room or project, the selection framework at the end will cut through the surface-level advice that fills most flooring guides.
What Makes Tile Flooring Distinct as a Category
Before breaking down individual types, it helps to understand what all tile flooring shares. Every tile is a rigid, hard-surface product installed in a grid pattern with or without grout joints. Unlike plank flooring, tile does not flex. Unlike carpet, it does not compress. Unlike most wood products, it does not react to humidity by expanding laterally across a room.
That rigidity is both its strength and its limitation. Tile resists moisture, bacteria, staining, and surface wear in ways no organic flooring material can. But it transfers heat poorly, feels hard underfoot, and cracks when a subfloor moves, flexes, or was never properly leveled before installation. Every tile type inherits these category-wide characteristics. What differentiates them is how their specific material properties, firing temperatures, and surface treatments shift the performance dial in one direction or another.
Understanding the comparison between tile and other flooring families — tile versus hardwood, for instance — often comes down to exactly these tradeoffs: hardness versus warmth, permanence versus repairability, moisture immunity versus acoustic comfort.
Ceramic Tile
Ceramic tile is the broadest category and, technically, the parent category for porcelain. In everyday use, however, “ceramic” refers specifically to non-porcelain tiles: products made from a natural clay body that is shaped, glazed on the surface, and fired at moderate kiln temperatures — generally between 1,800°F and 2,100°F.
The result is a tile with a water absorption rate typically between 3% and 7%. That sounds low, but it matters significantly in wet environments. The clay body itself is porous enough that sustained moisture exposure can cause issues over years if grout joints fail or the surface glaze chips. This is why ceramic performs well in bathrooms and kitchens but is less appropriate for shower floors, exterior applications, or any surface that sees standing water regularly.
The glaze applied before final firing is where ceramic tiles get their color, texture, and pattern. Modern digital printing technology means a ceramic tile can convincingly replicate stone veining, wood grain, fabric texture, or almost any surface look. The pattern exists only on that thin glaze layer, though. Where the tile edges are visible — at thresholds, on stairs, or where chips occur — the raw clay body shows through in a lighter, undecorated form.
Ceramic tiles are softer than porcelain, which makes them easier to cut with a standard wet saw. This reduces installation labor costs and makes them a better DIY option for straightforward layouts. The tradeoff is lower scratch resistance. High-traffic areas like entryways and commercial corridors will show wear on the glaze surface faster than porcelain equivalents.
Best for: Bathroom walls, kitchen backsplashes, laundry rooms, low-to-moderate traffic residential floors.
Avoid for: Outdoor installation, shower floors, commercial high-traffic spaces, anywhere requiring consistent freeze-thaw resistance.
Porcelain Tile
Porcelain is a subtype of ceramic, but the differences in manufacturing produce a material so distinct in performance that treating them as interchangeable is a costly mistake. Porcelain is made from finer, more purified kaolin clay mixed with feldspar and silica, then fired at significantly higher temperatures — typically 2,200°F to 2,400°F. The result is a tile with a water absorption rate at or below 0.5%, compared to ceramic’s 3–7%.
That single metric explains most of porcelain’s performance advantages. A tile that barely absorbs water resists staining, does not crack during freeze-thaw cycles, withstands high-humidity environments without degrading, and maintains structural integrity even in outdoor or commercial applications. Porcelain is also harder — typically rating higher on the Mohs hardness scale than standard ceramic — which translates to better scratch and abrasion resistance on floor surfaces.
There are two important subcategories within porcelain. Through-body (or full-body) porcelain has consistent color throughout the entire tile thickness. If it chips or scratches, the color below the surface matches the surface. Glazed porcelain, like ceramic, has color and pattern only in the surface glaze layer. Through-body porcelain is the better choice for high-traffic floors where surface wear is a long-term concern.
Porcelain’s density makes it harder to cut, requiring diamond-blade wet saws and more precise tooling. Installation costs are consistently higher than ceramic, both because the material itself costs more and because the labor is more demanding. Large-format porcelain tiles — 24×48 inches and beyond — are particularly challenging to install correctly, requiring exceptionally flat subfloors and specific large-format setting techniques.
The design range of modern porcelain is extraordinary. Wood-look porcelain that passes visual inspection from standing height, marble-look tiles that require a close look to distinguish from genuine stone, concrete-look finishes that give industrial aesthetics without porosity concerns — all of these use porcelain as the base material.
Best for: Shower floors and walls, kitchen floors, entryways, outdoor patios, commercial spaces, any area requiring high water resistance or freeze-thaw durability.
Avoid for: Situations where budget is the primary constraint and performance demands are low — standard ceramic will serve just as well at lower cost.
Ceramic vs. Porcelain: The Practical Decision Point
The ceramic versus porcelain decision comes down to three variables: moisture exposure, traffic level, and budget. For a guest bathroom floor that sees light use and no standing water, ceramic is entirely appropriate. For a master bathroom shower floor, an outdoor patio, or a commercial kitchen, porcelain is not a preference but a technical requirement. For everything in between, budget and tile thickness requirements drive the call.
Natural Stone Tile
Natural stone tile is quarried, cut, and finished rather than manufactured. Every piece is geologically unique. The variation in color, veining, and surface texture that makes stone so visually compelling also means no two installations look exactly the same — a feature some buyers prize and others find unpredictable.
Natural stone is the broadest sub-category in tile flooring and encompasses materials with dramatically different hardness ratings, porosity levels, and maintenance requirements. Understanding which stone type you are choosing matters far more than simply selecting “natural stone.”
Marble
Marble is a metamorphic limestone characterized by its signature veining patterns and ability to accept a high-gloss polish. It is calcium-carbonate based, which creates a fundamental vulnerability: acids etch the surface. Citrus juice, wine, vinegar-based cleaners, and even carbonated water can dull a polished marble surface permanently unless addressed immediately. Marble is also relatively porous — more so than granite — and requires sealing on installation and periodic resealing to resist staining.
On polished marble floors, the slipperiness when wet is a genuine safety concern. Honed marble — ground to a matte finish rather than polished — offers better traction and hides scratches more effectively, making it a more practical choice for floors as opposed to walls and decorative surfaces.
Despite these maintenance demands, marble remains one of the most visually impactful flooring choices available. The full pros and cons of marble tile make for important reading before committing to it in a kitchen or high-traffic entryway.
Granite
Granite is an igneous rock with a crystalline structure that makes it significantly harder and denser than marble. It resists scratches, chips, and staining far better than most natural stone options. Its porosity is lower than marble, meaning it requires less frequent sealing. Granite’s surface variation tends toward speckled or granular patterns rather than the linear veining of marble — a visual quality that works particularly well in entryways, kitchens, and any space where the floor needs to handle both heavy traffic and visual impact.
Cost per square foot for granite tile is typically lower than marble but higher than porcelain alternatives that mimic the same look. The installation weight is substantial, requiring verified structural support in upper-floor installations.
Travertine
Travertine is a sedimentary limestone formed by mineral spring deposits. Its characteristic appearance includes natural voids and pits across the surface — a result of carbon dioxide bubbles escaping during formation. These voids are typically filled with grout or resin during finishing, but that fill material can degrade over time, creating maintenance issues. Travertine is calcium-carbonate based like marble, sharing the same vulnerability to acidic cleaners and etching.
The warm, earthy tones of travertine — beige, walnut, cream, gold — make it one of the most popular natural stone choices for bathrooms, living rooms, and outdoor walkways where a Mediterranean or old-world aesthetic is desired. It requires consistent sealing and careful cleaning product selection throughout its lifespan. Detailed considerations around travertine’s specific strengths and limitations are worth reviewing before specifying it for wet areas.
Slate
Slate is a fine-grained metamorphic rock with a naturally cleft, textured surface that provides excellent slip resistance without requiring surface treatments. Its natural split-face texture is one of its primary functional advantages in wet areas, mudrooms, and exterior applications. Slate’s hardness is high, though it can be brittle along its natural cleavage planes — meaning it can flake or chip under impact in ways that granite would not.
Slate comes in darker, more dramatic color tones — charcoal, slate blue, deep green, rust — that perform well aesthetically in contemporary and industrial interior schemes. Its natural texture makes it harder to clean than polished surfaces, as dirt and cleaning products can accumulate in the surface relief. A penetrating sealer is recommended annually in high-traffic installations.
Limestone
Limestone is a sedimentary rock with a softer, more matte appearance than marble or granite. Its color range is primarily neutral — cream, beige, gray, taupe — which makes it versatile across many interior styles. Like marble and travertine, limestone contains calcium carbonate and is vulnerable to acidic substances. It is one of the softer natural stone flooring options, showing wear and scratching faster than granite or slate under comparable traffic conditions.
Limestone works well in low-to-moderate traffic residential areas where its subtle, elegant appearance outweighs its higher maintenance requirements. It is less appropriate for commercial spaces or family kitchens with daily heavy use.
Mosaic Tile
Mosaic tile is defined by format rather than material. Any tile measuring two inches or smaller — whether ceramic, porcelain, glass, natural stone, or metal — qualifies as mosaic. Most commercial mosaic tile today comes mounted on mesh backing in 12×12-inch or 12×24-inch sheets, which allows installers to set many small tiles simultaneously while maintaining consistent spacing.
The practical benefit of mosaic tile on floors goes beyond aesthetics. Smaller tiles mean more grout joints per square foot. More grout joints mean more surface texture and more coefficient-of-friction at the floor level. This makes mosaic tile one of the highest-traction tile options available — particularly valuable for shower floors, pool surrounds, and any wet area where slip resistance is a safety priority.
Design range in mosaic tile is effectively unlimited. Penny rounds, hexagons, arabesque shapes, basketweave patterns, subway-cut pieces — all are common mosaic formats. Glass mosaic tiles in particular are popular for backsplashes and accent areas because the material’s translucency allows color to shift depending on lighting conditions. Metal mosaic tiles add texture and industrial character to accent walls or kitchen features.
The maintenance consideration specific to mosaic is grout. More grout lines means more surface area for staining, discoloration, and mold growth over time. Epoxy grout — more stain and moisture resistant than cement-based grout — is the appropriate choice for mosaic installations in wet or high-traffic areas. The question of which grout formulation fits which installation has a significant impact on the long-term performance of any tiled surface, but particularly mosaic.
Large-Format Tile
Large-format tile is not a material category but a size category — generally defined as any tile with at least one dimension exceeding 15 inches, though contemporary usage often refers to 24×24 and larger. The defining characteristic is fewer grout lines across an installation, which creates a more seamless visual field and a perception of larger, more open space.
Large-format tiles are almost always porcelain or sintered stone. The material needs to be dimensionally stable, consistently thick, and dense enough to resist cracking across spans unsupported by grout joints. Thin large-format porcelain — sometimes as thin as 6mm — requires additional structural care during handling and installation but produces an exceptionally lightweight and visually clean result.
The installation requirements for large-format tile are substantially more demanding than smaller formats. The subfloor must achieve a flatness tolerance of no more than 1/8 inch over a 10-foot span — stricter than what most other tile formats require. Lippage — the height differential between adjacent tile edges — becomes visible and tactile at large formats in ways it rarely does at smaller sizes. Back-buttering (applying mortar to both the tile back and the substrate) is essentially mandatory. These installation demands are part of why tile flooring installation costs vary so significantly based on tile size and format.
Encaustic Tile
Encaustic tiles are cement-bodied tiles in which pattern and color are inlaid directly into the body of the tile during manufacturing rather than applied as a surface glaze afterward. This means the pattern runs through a significant portion of the tile’s thickness — typically 8mm or more of colored cement — rather than existing only as a thin glaze layer that can wear away over time. The color in an encaustic tile does not disappear when the surface is scratched or worn.
Traditional encaustic tiles were handcrafted using metal molds filled with layers of pigmented cement paste, a process that originated in Europe in the mid-19th century and became particularly associated with Victorian and Arts and Crafts interior design. Contemporary production largely preserves this manufacturing method, meaning encaustic tiles remain a craft product with intentional variation in color saturation, edge definition, and surface texture from piece to piece.
Encaustic tiles are porous by nature. They require sealing before grouting during installation and periodic resealing throughout their lifespan to prevent staining. They should not be used in wet areas without appropriate waterproofing and are not ideal for outdoor installations in climates that experience hard freezes. Acidic cleaners will damage the surface. Despite these maintenance requirements, encaustic tile is experiencing significant design-world momentum: its geometric patterns, Moroccan-influenced designs, and artisan character create a floor that no other tile type can genuinely replicate.
Best for: Entryways, living room accent areas, kitchen floors in moderate-traffic residential settings, bathroom floors (with proper sealing), heritage and character renovation projects.
Terrazzo Tile
Terrazzo is a composite material made by embedding chips of marble, granite, quartz, glass, or other aggregates into a cement or resin binder. The mixture is poured, allowed to cure, then ground and polished to reveal the aggregate chips in a smooth, speckled surface. Traditional poured-in-place terrazzo is a flooring system rather than a tile product. Terrazzo tile refers to pre-manufactured slabs cut to standard tile dimensions and polished in a factory setting.
Terrazzo’s durability is exceptional. Because the entire tile body consists of hard aggregate bound in a dense matrix, there is no surface layer that can wear away separately from the body below. The floor that exists after five years of heavy use is essentially the same material as the floor on day one. This makes terrazzo particularly suitable for high-traffic commercial spaces: airport terminals, school corridors, hospital lobbies, and retail environments all benefit from a material that looks better as it polishes with foot traffic rather than degrading.
Terrazzo tile requires sealing when cement-based (resin-based terrazzo is inherently non-porous and requires less maintenance). It is heavy, expensive per square foot compared to ceramic or basic porcelain, and difficult to repair if individual tiles crack or chip. The aesthetic — speckled, colorful, mid-century-influenced — has seen a major design revival, with contemporary colorways extending far beyond the beige-and-gray palette of institutional terrazzo from the 1960s and 1970s.
Quarry Tile
Quarry tile is a dense, unglazed ceramic tile made from shale, clay, and feldspar fired at temperatures up to 2,000°F. Despite the name, quarry tile has nothing to do with quarrying — the term derives from the French “carré,” meaning square. The manufacturing process produces a tile with extremely low water absorption, often between 0.5% and 3%, and an unglazed surface with natural slip resistance superior to most polished tile products.
Quarry tile’s performance properties make it the default specification for commercial food service environments. Restaurant kitchens, commercial food processing facilities, cafeteria floors, and similar spaces require a flooring material that resists grease, can be sanitized repeatedly with industrial cleaning products, handles foot traffic and equipment loads without damage, and maintains slip resistance in wet or contaminated conditions. Quarry tile meets all of these requirements at a lower cost point than porcelain in equivalent commercial grades.
In residential applications, quarry tile’s earthy red-brown coloration — a natural result of iron oxide in the clay — works well in rustic, farmhouse, and Mediterranean-influenced interiors. It does not offer the design versatility of ceramic or porcelain but performs exceptionally in utility spaces: mudrooms, laundry rooms, garages, and covered patios.
Glass Tile
Glass tile is made by coating the underside of glass pieces with pigment and firing them to create color. The material is non-porous, making it completely resistant to water, mold, and staining in ways that even porcelain does not fully match. Glass does not absorb liquids at any meaningful rate, which also means that glass tiles installed in wet environments stay visually clean without requiring sealing.
The limitation of glass tile as a floor material is its smooth surface. Polished glass has essentially no natural coefficient of friction, making it extremely slippery when wet. This is why glass tile is almost never specified as floor tile in standard sizes. It appears on floors primarily in mosaic format — where the density of grout lines provides traction — or as decorative accents within larger-format tile layouts. As wall tile, kitchen backsplash material, and pool liner tile, glass performs without these constraints and delivers a visual luminosity no other material provides.
Glass tile requires specific installation materials: standard Portland cement-based thinset mortars can telegraph color variations through transparent glass. White thinset mortar is the appropriate choice for most glass tile installations. Grouting requires care to avoid scratching the glass surface during the haze-removal stage.
Cement Tile (Separate from Encaustic)
Cement tile and encaustic tile are closely related but not identical. Both use cement as a body material. The distinction is primarily in surface design: encaustic tiles use inlaid pigmented cement to create multi-color patterns, while plain cement tile may be single-color or use a simpler inlay process. In practice, the terms are used interchangeably in most retail and design contexts. What they share is a porous, unsealed cement body that requires proper sealing and careful maintenance.
Cement tiles are produced without kiln firing — they are hydraulically pressed and air-cured, not heat-fired like ceramic or porcelain. This means they never achieve the near-zero porosity of fired tile products. It also means they are more susceptible to cracking under structural movement and should not be installed over substrates with significant flex or thermal movement without appropriate isolation membranes.
Sintered Stone Tile
Sintered stone is an engineered material produced by fusing natural mineral particles — silica, quartz, glass, and natural stone compounds — under extremely high pressure and heat. The process produces a tile that is non-porous, essentially scratch-proof under normal conditions, and chemically resistant to the full range of household cleaners including acids and solvents that would damage natural stone or cement-bodied tiles.
Sintered stone tiles are typically produced in very large formats — 60×120 inches and larger as slabs — that can be cut to standard tile dimensions or installed as fewer, larger pieces. The material’s non-porosity means no sealing is ever required. UV stability is excellent, making it appropriate for outdoor spaces with direct sun exposure. Sintered stone frequently mimics the appearance of marble, concrete, or wood with a realism that rivals or exceeds standard porcelain printing technology.
The category sits at a higher price point than most porcelain and significantly above ceramic. Installation requires the same demanding subfloor preparation as other large-format tile products, plus careful handling during transport and cutting to prevent edge chipping on such dense material. For renovation projects where performance and design expectations are both at the premium end, sintered stone represents the current ceiling of the tile flooring market.
How to Choose the Right Tile Type: A Framework
Every tile type performs well in certain conditions and poorly in others. The gap between a good and a bad tile decision usually comes down to matching material properties to actual installation conditions rather than simply choosing what looks appealing in a showroom.
Start with Water Exposure
Rank your installation’s water exposure honestly before evaluating aesthetics. A shower floor sees standing water and constant humidity. A residential kitchen floor occasionally sees spills. An entryway sees tracked-in rain moisture. An outdoor patio sees freeze-thaw cycles. Each of these environments requires a different minimum absorption rate threshold in your tile selection. Porcelain handles all of them. Ceramic handles the kitchen and entryway but not the shower floor or outdoor patio. Natural stone handles all of them with appropriate sealing, but the maintenance burden varies enormously by stone type.
Evaluate Actual Traffic Conditions
Tile carries a Porcelain Enamel Institute (PEI) wear rating for glazed tiles, running from 0 (wall use only) to 5 (heavy commercial and industrial use). For residential floors, PEI 3 is appropriate for standard rooms and PEI 4 covers entry areas and kitchens. PEI 5 is reserved for commercial installations with genuine heavy traffic. Through-body and unglazed tiles are not rated on the PEI scale because they have no glaze layer to wear through — their performance is determined by the hardness of the tile body itself.
The questions around which tile performs best in high-traffic areas consistently point toward through-body porcelain, quarry tile, and dense natural stone like granite — not because they are the most visually interesting options, but because their performance characteristics align with what high-traffic floors actually demand.
Account for Subfloor Conditions
All tile is rigid. Any movement or flex in the subfloor beneath it will eventually transmit to tile cracks, cracked grout joints, or loose tiles. Wood subfloors — plywood or OSB — flex under load in ways that concrete does not. Large-format tiles over wood subfloors require specific uncoupling membranes or properly engineered substrates. Concrete subfloors need moisture vapor testing before tile installation, since high vapor emission rates can fail tile adhesive bonds over time. Proper subfloor preparation for tile is where most long-term tile failures originate, regardless of which tile type was chosen.
Factor in Maintenance Willingness
There is a meaningful spectrum from essentially zero maintenance (porcelain, sintered stone) to ongoing maintenance requirements (natural stone, encaustic tile, terrazzo). Choosing marble for a family kitchen because it looks beautiful in a design magazine, then failing to reseal it twice annually and avoiding all acidic food contact, is a path to expensive disappointment. Choosing porcelain in a marble-look finish and never thinking about it again is a path to exactly the visual result you wanted at a fraction of the maintenance commitment.
The honest trade is between authenticity and convenience. Natural stone tile delivers something no engineered material can fully replicate: geological uniqueness, tactile depth, material history. But it asks for something in return. Decide whether that exchange makes sense in your specific context before committing to it in any room.
Align with Subfloor Heating Plans
Tile is one of the best flooring types for radiant underfloor heating systems because it conducts and radiates heat efficiently rather than insulating against it. But not all tile types respond identically to thermal cycling. Natural stone, particularly marble and limestone, can develop micro-cracks over years of repeated heating and cooling if the system runs at extreme temperatures or cycles rapidly. Porcelain is the most thermally stable tile type for underfloor heating applications. The relationship between tile type and underfloor heating compatibility is worth investigating carefully if that combination is part of your design plan.
Tile Type by Room: Quick Reference
Different rooms impose different requirements, and matching tile type to room function produces better results than choosing based on aesthetic alone.
In kitchens, porcelain is the strongest all-around performer — it handles spills, resists staining without sealing, and holds up under decades of high foot traffic. Ceramic works for lower-traffic kitchen floors in homes without young children or heavy cooking. Natural stone — granite in particular — works in kitchens where the aesthetic is the priority and maintenance habits are consistent.
In bathrooms, porcelain covers shower walls and floors most reliably. Mosaic tile on shower floors adds traction through grout line density. Marble on bathroom walls produces a luxurious visual impact but requires consistent cleaning with pH-neutral products. Ceramic is appropriate for bathroom floors and walls outside the direct wet zone of a shower.
In living rooms and bedrooms, the tile type priorities shift away from moisture resistance and toward surface feel, acoustic quality, and visual warmth. Large-format porcelain in wood or stone looks are popular for open-plan living areas. Encaustic or terrazzo tile creates statement floors in living rooms where design impact is the primary goal.
For outdoor applications, porcelain rated for exterior use and textured natural stone — granite, slate — are the appropriate choices. Any tile intended for outdoor use must carry a suitable frost resistance rating if the installation is in a climate with freeze-thaw cycles. More specific guidance on outdoor tile selection involves slip resistance ratings, frost resistance classifications, and drainage slope requirements that vary from indoor specifications.
What the Tile Type Does Not Tell You
Tile type is necessary context for a flooring decision but not sufficient by itself. The same porcelain tile can fail in five years or perform beautifully for fifty, depending entirely on what happens at installation. Setting material choice, substrate preparation, thinset selection, grout joint sizing, and grouting technique all contribute to real-world performance more than any comparison between tile types ever can.
The cost you see per square foot at a tile distributor does not reflect the full installed cost. Subfloor preparation, setting materials, labor rates for the specific tile format and installation complexity, and finish work at thresholds and transitions all add to the project total. Getting an accurate picture of tile flooring cost by type and format before committing to a specific material prevents budget surprises late in a project.
Tile type also does not tell you how a floor will feel to live with day-to-day. Tile is acoustically reflective — sound bounces off hard surfaces. It is thermally cold — without underfloor heating, bare tile floors in winter feel markedly colder than wood or carpet. Understanding these lived-experience characteristics is part of what makes certain tile types appropriate in some rooms and genuinely uncomfortable in others, regardless of how well they perform technically.
Summary: Tile Types at a Glance
Ceramic tile delivers affordability and design versatility for indoor, moderate-moisture applications. Porcelain extends those properties to wet areas, outdoor installations, and high-traffic commercial environments through its near-zero porosity and superior density. Natural stone — marble, granite, travertine, slate, limestone — provides genuine geological character at the cost of more involved maintenance and sealing requirements that vary significantly by stone type. Mosaic tile in any material adds traction through grout line density, making it the right call for shower floors and pool areas. Large-format tile reduces visual grout lines for a seamless appearance but requires exacting subfloor preparation. Encaustic and cement tiles deliver artisan-crafted pattern depth that manufactured tiles cannot replicate, at the cost of regular sealing. Terrazzo combines aggregate beauty with exceptional durability for high-traffic commercial and premium residential applications. Quarry tile dominates utility and food-service environments through unglazed slip resistance and industrial-grade durability. Glass tile provides non-porous, luminous surfaces suited to backsplashes, accents, and wet-area walls. Sintered stone sits at the performance ceiling of the category — non-porous, scratch-resistant, maintenance-free, and available in formats no other tile type matches.
The right tile type for any project is the one whose material properties align with the actual conditions it will face — not the one that photographs best in idealized conditions.
